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2. Section 8 : Entomology in Public Health Practice
154 Introduction to Entomology Rina Tilak 903
155 Principles of Vector Control Rina Tilak 906
156 Housefly Rina Tilak 927
157 Mosquitoes Rina Tilak 931
158 Fleas Rina Tilak 940
159 Human Lice Rina Tilak 944
160 Sand Flies Rina Tilak 946
161 Some Annoying Pests Rina Tilak 948
162 Envenomizing Pests Rina Tilak 952
163 Ticks and Mites Rina Tilak 955
164 Rodents Rina Tilak 960
166 Snakes Rina Tilak 966

3. • 903 •
154Introduction to Entomology
Rina Tilak
The word ‘Entomology’ is derived from the Greek words
‘ENTOMON’ meaning an insect and ‘LOGOS’ meaning science,
thus ideally making Entomology ‘the branch of science which
deals with the study of insects’; however, the scope of the
subject has been broadened to include study of all Arthropods.
Phylum Arthropoda constitutes all invertebrates with jointed
appendages and presence of chitinous exoskeleton besides
other features. The word Arthropoda is derived from two words
‘ARTHRON’ meaning jointed and ‘PODA’, which means legs or
appendages. The Phylum has many important classes of which
Class Insecta is the largest, constituting more than four million
insect species. The other important classes are Arachnida,
Crustacea and Myriapoda.
Vector borne diseases are one of the leading causes of morbidity
and mortality the world over and pose a major public health
challenge especially to the third world or developing countries.
One of the important measures to combat these diseases is
through control of vectors. To ensure effective vector control,
the knowledge about their lifecycle, habits, habitat and
diseases transmitted is essential. The chapters in this section
will guide the reader on these aspects so that sound vector
control strategies can be formulated, wherever and whenever,
vector control is desired.
Classification of Arthropods
Diversity of structures amongst arthropods necessitates the
sub-division of the phylum Arthropoda into a number of
classes, orders, families, genera and species. The following
classes include species of medical importance.
(a) Class Insecta : It comprises about 70% of all the known
species in the animal kingdom. The insects are characterized
by the presence of six legs, body divided into head, thorax and
abdomen and presence of antennae besides other features.
The head bears the mouthparts, eyes in the form of compound
or simple eyes or at times may have no eyes and a pair of
antennae. The thorax is subdivided into three segments with a
pair of legs in each called the pro, meso and metathoracic legs.
The class is further subdivided into 29 orders of which only 4
contain species of medical importance.
(i) Order Diptera : It possesses one pair of wings and a pair
of ‘halters’, which are vestigial wings. This is the largest order
comprising about seventy thousand described species with
world wide distribution. It contains such insects of medical
importance as mosquitoes, sandflies, simulium flies and
house-flies.
(ii) Order Anoplura : It comprises true or sucking lice, which
are parasitic on mammals and the chewing lice, which infest
birds as well as mammals. Lice of medical importance to man
are human lice.
(iii) Order Siphonaptera : It contains many species of fleas
including the rat flea, which is a vector of plague and endemic
typhus.
(iv) Order Hemiptera : It contains the bedbugs and some other
species like kissing or ‘assassin’ bugs, which act as vectors of
‘Chagas’ disease.
(b) Class Arachnida : It includes arthropods like ticks, mites,
spiders and scorpions. The class is characterized by the presence
of eight legs, body divided into two parts viz. cephalothorax
(head and thorax are fused together) and abdomen and
absence of antennae and wings. The cephalothorax bears six
pairs of appendages, the first two pairs function as mouthparts
(chelicerae and pedipalps) and last four pairs as walking legs.
(c) Class Crustacea : It includes lobsters, crabs, water fleas
and cyclops; some species of these are intermediate hosts of
certain human helminths, e.g. Cyclops as intermediate host of
guinea worm infestation.
(d) Class Myriapoda : It includes centipedes and millipedes.
Modes of Disease Transmission
Arthropods transmit diseases to man through specialized modes
of disease transmission. The modes of disease transmission by
arthropods can be classified as under :
Direct Contact : When two hosts are in direct contact with
other, the arthropod vector itself gets transferred from one host
to the other, e.g. pediculosis and scabies.
Mechanical transmission : In this mode of transmission,
the disease causing organism is transmitted on the outside
or inside the bodies of arthropods without undergoing any
development, propagation or any changes in the pathogenicity
e.g. diseases transmitted by houseflies - diarrhoea, dysentery,
cholera, hepatitis A & E etc.
Biological Transmission : The disease causing organism
undergoes certain biological changes inside the body of the
vector. Depending on the type of biological changes, biological
transmission has been further classified as :
Cyclo-developmental : In this mode of transmission, the
disease causing organism undergoes a part of its cycle in the
vector and simply develops or grows inside the body of the
vector, e.g. Wuchereria bancrofti (Filariasis) transmitted by
Culex female.
Cyclo-developmental-propagative or Cyclo-propagative :
In this mode of transmission, the disease causing organism
undergoes a part of its life cycle in the vector and also grows/
develops and multiplies inside the body of the vector. Example
is Plasmodium sp (Malaria) transmitted by Anopheles female.
Propagative : In this transmission mode, the disease organism
simply grows and multiplies in the body of the vector, e.g.
Yersinia pestis (Plague) transmitted by rat flea and all
bacterial, viral, spirochaetal or rickettsial diseases transmitted
by arthropods.
Other Specialized Modes of Disease Transmission : These
specialized modes of disease transmission are generally
encountered in ticks and mites.
Trans-ovarian : The disease organism is transmitted to eggs
through ovary of infected female, e.g. Orientia tsutsugamushi,
the agent for Scrub typhus transmitted by trombiculid mite-
Leptotrombidium deliense.

4. • 904 •
Table - 1 : Important arthropod borne diseases
Disease Vector Causal organism Reservoir
I Mosquito borne diseases
Malaria
Filariasis

Chikungunya
Dengue fever & DHF
Yellow fever
Japanese Encephalitis
Anopheles species
Culex quinquefasciatus
Aedes niveus group
Mansonoides species
Aedes species
Aedes species
Aedes species
Culex vishnui group
(C tritaeniorhynchus)
Plasmodium species
W bancrofti (nocturnal, periodic)
W. bancrofti (diurnal sub-periodic)
Brugia malayi
Arbovirus group A
Arbovirus group B
Arbovirus group B
Arbovirus group B
Man
Man
Man
Man, Primate
Man
Man
Man/ Monkeys
Mammals/ Birds
II Sandfly borne diseases
Leishmaniasis
Visceral (Kala azar)
Cutaneous (Oriental sore)
Espundia
Sandfly fever
Phlebotomus argentipes
P. papatasi
P. sergenti
P. sergenti, P. papatasi
Leishmania donovani
L. tropica
L. braziliensis
Virus
Man/Mammals
Man/Mammals
Man/Mammals
Man
III Fly borne diseases
Bacillary dysentery
Amoebic dysentery
Gastroenteritis
Typhoid
Paratyphoid
Cholera
Poliomyelitis
Viral hepatitis (Type A)
Trachoma
Yaws
M domestica
M domestica
M domestica
M domestica
M domestica
M domestica
M domestica
M domestica
M domestica
M domestica
Shigella
E. histolytica
Specific/Non specific organisms
Salmonella typhi
Paratyphoid A&B
Vibrio cholera
Virus
HAV
C trachomatis
T pertenue
Man
Man
Man/animals
Man
Man
Man
Man
Man
Man
Man
IV Flea borne diseases
Plague (Bubonic)
Endemic/Murine Typhus

Chiggerosis (Jigger)
Dipylidium caninum
Hymenolepis diminuta
H nana
Xenopsylla species
Xenopsylla species
Tunga penetrans (chigoe)
Ctenocephalides. felis/canis
X cheopis / N fasciatus
X cheopis / C canis/ Pulex irritans
Yersinia pestis
R. typhi
-
Dipylidium caninum
Hymenolepis diminuta
H nana
Rodents
Rodents/
Domestic animal
-
Dogs, cats, wild
Carnivores
Rats, mice
V Louse borne diseases
Epidemic typhus
Epidemic relapsing fever
Trench fever
Dermatitis
Pediculus humanus
Pediculus humanus
Pediculus humanus
Pediculus humanus/ capitis
R. prowazeki
Borrelia recurrentis
Bartonella quintana
Secondary organisms
Man
Man
Man/animals
Man
VI Tick borne diseases
Kyasanur Forest Disease (KFD)
Tick typhus
Tularaemia
Relapsing fever
Hard ticks species
Hard ticks species
Hard ticks species
Soft Tick
Arbovirus group B
R conorii
P tularensis
B duttoni
Monkeys/Birds
Dogs
Rabbits/ Rodents/ cattle
Rats
VII Mite borne diseases
Scrub typhus
Rickettsial pox
Scabies
L. deliense
Allodermanyssus sanguineus
1

S scabei
Orientia tsutsugamushi
R akari
-
Rodents
Rodents
Man
VIII Cyclops transmitted diseases
Dracontiasis
Fish tape worm
Cyclops species
Cyclops species
D medinensis
D. latum
Man
Fish
IX Reduviid bugs
Chagas disease Reduviid/Cone-nosed T cruzi Domestic animals/ man
X Tsetse flies
Trypanosomiasis Glossina species T gambiense and T rhodesiense Wild animals/ cattle/ man

5. • 905 •
Trans-stadial : The disease causing organism is transmitted
from one stage to another e.g. Tick typhus organism - Rickettsia
conorii transmitted from infected larva to nymph to adult.
Arthropod Borne Diseases
Arthropods are responsible for transmission of innumerable
diseases. Some of the important arthropod borne diseases is
listed in Table - 1 along with their vectors, causative organisms
and reservoir hosts.
Summary
Entomology is the branch of science which deals with the
study of insects, however, the scope of the subject has been
broadened to include the study of all Arthropods. Amongst
the many important classes of the phylum, Insecta is the
largest constituting more than 4 million species. Vector borne
diseases are one of the leading causes of morbidity & mortality
especially in the developing countries. These diseases can
be largely combated by effective vector control for which a
sound knowledge of the bionomics of the vector is needed. The
important classes of Phylum Arthropoda are Insecta, Arachnida,
Crustacea, Myriapoda. The class Insecta is characterized by
presence of six legs, body divided into head, thorax & abdomen.
The class has 4 orders of medical importance, Diptera, Anoplura,
Siphonaptera & Hemiptera. The order Diptera has 1 pair of
wings and contains insects such as mosquitoes, sandflies,
simulium flies and houseflies. The order Anoplura comprises
true or sucking lice. Order siphonoptera includes the rat flea;
the order hemiptera contains bed bugs. The class Arachnida
includes ticks, mites, spiders and scorpions; body is divided
into cephalothorax and abdomen and presence of eight legs.
The class crustacea includes lobsters, crabs, water fleas and
Cyclops. Myriapoda includes millipedes and centipedes.
Arthropods transmit diseases to man by different modes of
transmission, mainly direct, mechanical & biological. In direct
transmission, two hosts come in direct contact with each other
e.g. pediculosis & scabies, mechanical transmission where
disease causing organism is transmitted without undergoing
any change in pathogenecity or development e.g. diseases
transmitted by houseflies. Biological transmission is further
divided into cyclodevelopmental - the disease causing organism
undergoes part of its life cycle in the vector e.g. Wuchereria
bancrofti; cyclopropogative - the disease causing agent
undergoes part of the lifecycle in the vector & also multiplies
inside the vector e.g. malaria transmitted by Anopheles female.
The other mode is propogative where disease causing organism
simply grows & multiplies in the body of vector e.g. Yersinia
pestis (plague) transmitted by rat flea, other specialized modes
are transstadial & transovarian.
Study Exercises
Short Notes : Modes of disease transmission.
MCQs & Exercises
1) Class Insecta is characterised by presence of how many
legs? : (a) 4 (b) 5 (c) 6 (d) 8
2) Mosquito belongs to the order a) Diptera (b) Anoplura
(c) Siphonoptera (d) Hemiptera.
3) Mode of transmission of malaria by Anopheles is :
(a) Cyclo - developmental (b) Cyclo - propogative
(c) Propagative (d) Cyclo - developmental propagative
4) Orientia tsugtsugamushi is the causal organism for the
diseases (a) Scrub typhus (b) Epidemic typhus (c) Endemic
typhus (d) Tick typhus.
5) Which of the following is not a mosquito borne disease :
(a) Dengue (b) Filaria (c) Leishmaniasis (d) Yellow fever
6) Aedes species cannot transmit : (a) Chickungunya
(b) Dengue haemorrhagic fever (c) Japanese Encephalitis
(d) Yellow fever
7) Causal organism for bubonic plague : (a) R typhi
(b) Y pestis (c) H nana (d) H diminuta
8) Murine Typhus is transmitted by : (a) Hard tick species
(b) Soft tick species (c) Xenopsylla species (d) Trombiculid
Mite species
9) Causal organism for epidemic Typhus : (a) B quintana
(b) R prowazeki (c) Borrelia recurrentis (d) R typhi
10) KFD is transmitted by : (a) Soft Tick (b) Mite (c) Hard Tick
(d) Lice
11) ___________ is the largest class of Phylum Arthropoda
constituting more than __________ insect species
12) Direct mode of transmission is seen in _______ & ______
13) Entomology is derived from the Greek word ENTOMON
meaning _________ & LOGOS meaning __________
14) Arthropoda is derived from two words ARTHRON meaning
__________ & PODA meaning __________
15) Leptotrombidium deliense is vector for __________
Answers : (1) c; (2) a; (3) b; (4) a; (5) c; (6) c; (7) b; (8) c;
(9) b; (10) c; (11) Insecta ; 4 Million; (12) Pediculosis;
Scabies; (13) Insect, Science; (14) Jointed, Appendages;
(15) Scrub Typhus.
Further Suggested Reading
Gordon C cook, Alimuddin Zumla, Manson’s Tropical diseases, 21st Ed, 1.
2003, Chapter 50, Rickettsial infections, p 900.
Service Mike. Medical Entomology for students. 3rd ed. Cambridge : 2.
Cambridge University Press, 2004.
Kettle DS. Medical and Veterinary Entomology. 2nd ed. CAB International, 3.
1995.
Hati AK. Medical Entomology. Allied Book Agency, Kolkata, 2001.4.

6. • 906 •
155Principles of Vector Control
Rina Tilak
Control of arthropods is one of the key strategies in the
management of vector borne diseases. A strategist should
have sound knowledge of the bionomics, distribution, seasonal
prevalence, vectorial capacity, insecticide susceptibility status
and role of arthropods in diseases transmission coupled with
the knowledge of identification features of the incriminated
vectors for formulating effective control strategies. Once armed
with this knowledge, the choice of effective vector management
tools may be exercised; the range and sophistication of control
methods is impressive. The various control options available
are as follows :
Environmental Control●●
Chemical Control●●
Biological Control●●
Personal Protective measures●●
Mechanical control●●
Physical control●●
Genetic control●●
Legislative control●●
Environmental Control
The important environmental control measures which are
increasingly being used are described below :
Environmental Management
This has been defined as “The planning, organization, carrying
out and monitoring of activities for the modification and/or
manipulation of environmental factors or their interaction with
man with a view to prevent or minimize vector propagation and
reducing man-vector-pathogen contact.” This is a naturalistic
approach which attempts to extend and intensify natural
factors which limit vector breeding, survival and contact with
man.
(a) Environmental Modification : It is defined as “A form
of environmental management consisting of any physical
transformation that is permanent or long-
lasting of land, water and vegetation
aimed at preventing, eliminating or
reducing the habitats of vectors without
causing unduly adverse effects on the
quality of the human environment”.
Environmental modification includes
drainage, filling, velocity alteration,
land levelling and transformation of
impoundment margins.
(b) Environmental Manipulation : It
is defined as “A form of environmental
management consisting of any planned
recurrent activity aimed at producing
temporary conditions unfavourable to
the breeding of vectors in their habitats”.
Examples of environmental manipulation
activities are water salinity changes,
stream flushing and regulation of the water level in reservoirs,
vegetation removal, shading and exposure to sunlight.
(c) Modification or Manipulation of Human Habitation or
Behaviour : This means “A form of environmental management
that reduces man-vector - pathogen contact”. Examples of this
approach are siting of settlements away from vector sources,
mosquito/rodent proofing, personal protection and hygienic
measures against vectors and provision of mechanical barriers,
providing facilities for water supply, disposal of waste water
and excreta, laundry, bathing and recreation to prevent or
discourage human contact with infested water.
Chemical Control
The new era of control of vector borne disease began with the
discovery of the insecticidal value of Dichloro Diphenyl Trichloro
ethane (DDT). DDT was first synthesized by Othmar Zeidler in
1874 at Strasbourg, Germany. In 1939, Paul Muller of the Geigy
Company in Basle, Switzerland, discovered its remarkably long
residual insecticidal property, earning him the Nobel Prize in
Medicine. The availability of several effective, safe and low
cost pesticides, coupled with improvements in the techniques
of their application, made it possible for many governments in
the developed as well as developing countries to embark upon
extensive countrywide programmes for the control or eradication
of vector borne diseases. However, development of resistance
amongst vectors to insecticides has necessitated reassessment
of the place of pesticides in vector control programmes. Besides
the technical and financial difficulties, there is a growing
concern about the environmental contamination resulting from
the persistent use of insecticides.
Classification of Insecticides
Pesticides may be classified in many ways based on mode of
entry, target stage, chemical composition and mode of action.
However, the most common classification used is based on
chemical composition. According to this classification, the
insecticides are classified in the following categories as
presented in Fig. - 1.
Fig. - 1 : Classification of insecticides as per chemical composition
Insecticides
Natural Synthetic
PlantBased
Pyrethru m
Azadirachtin
MineralO ils
Inorganic
ParisGreen
Organic
Organochlorines
DDT
Organophosphates
Malathion
Temephos
Fenthion
Dichlorv os
Fenitrothion
Pirimiphosmethyl
Carbamates
Propoxur
Bendiocarb
Carbary l
Synthetic
Pyrethroids
Deltamethrin
Cyfluthrin
Bifenthrin
Lambda-Cyhalothrin
Permethrin

7. • 907 •
Natural Insecticides
Plant based
Pyrethrum : Pyrethrum extract is obtained from the dried
heads of the flower Chrysanthemum cinerariafolium and
contains the active ingredients pyrethrins I and II, constituting
1 to 2% of the total weight of the raw pyrethrum. Pyrethrum is
characterized by rapid knockdown action on arthropods even
when used in very low dilution. It is very unstable in light and
air and has practically no residual effect. This makes repeated
applications necessary. Pyrethrum is available as 2% extract,
which needs 20 times dilution to make it 0.1% solution, which
is actually used for spraying. Using a 0.4 mm or lower calibre
nozzle, 50 to 100 ml of pyrethrum solution in kerosene oil is
sprayed per 100 m
3
of space. Addition of an Organophosphorus
insecticide to pyrethrum formulation is a common commercial
practice for obtaining a better effect. It is one of the main
insecticidal constituents in aerosol dispensers and also an
insecticide of choice for ULV sprays. Pyrethrum is perhaps the
most acceptable insecticide for use in cook houses, dining halls
and other food preparation areas.
Azadirachtin : The active ingredient Azadirachtin is obtained
from the seed kernels of neem plant Azadirachta indica.
Azadirachtin has insecticidal, fungicidal, bactericidal, viricidal
properties including insect growth regulating qualities besides
deterrent, anti-ovipositional, anti-feedant, fecundity and
fitness-reducing properties on insects. It has been variously
formulated for mosquito larval and adult control in the form
of liquid and cream formulations. Neem products contain up
to 3% Azadiractin.
Mineral Oils : Kerosene oil, diesel oil, petrol and crude engine
oil have been successfully used as mosquito larvicides. The
oil film cuts off the air supply, enters and blocks the trachea,
may act as a stomach poison, and also lowers surface tension
which prevents larvae from floating. Malariol is the best and
easiest larvicidal oil to use for Anopheline and Culicine larvae.
Malariol is used as it is @ 10 litres per 500 linear meters.
Synthetic Insecticides : These can be organic or inorganic.
The only inorganic compound used in vector control (mosquito
larvicide) was Paris green (Copper-aceto-arsenite). It acted as
stomach poison when ingested by mosquito larvae. However, it
is not used any more for mosquito control.
The Organic Insecticides fall into four major groups viz.
Organochlorines, Organophosphates, Carbamates and Synthetic
pyrethroids.
Organochlorine Compounds : These compounds are contact
poisons and act on the nervous system. These or their
degradation products are more or less stored in body fat and
may be demonstrated in milk, urine or sweat. They all have
variable residual action for variable periods and are toxic to
man and animals. The most important and the only member of
this group used in Public Health is DDT.
Dichloro-diphenyl-trichloroethane (DDT) : Currently DDT is
being used for indoor residual spray in the North Eastern states
of India. A deposit of 1 g of DDT/m
2
of surface area of walls
and ceiling upto a height of 3.5 m in all dwellings applied at 8
weeks’ interval, effectively controls majority of the mosquitoes
and also other arthropods resting on the treated wall (Refer to
Chapter on Mosquitoes for further details on use). As per Govt.
of India Gazette notification number S.O. 378(E) dt 26
th
May
1989, the use of DDT in Agriculture has been withdrawn and
restricted to 10000 MT/ annum for Public Health programme
except in case of North East as Insecticidal Residual Spray
(IRS).
Organophosphorus Compounds : These insecticides are
derivatives of phosphoric acid and act by inhibiting the
activity of cholinesterase. Many of the insects, which have
become resistant to Organochlorines are still susceptible to
the members of this group. However, due to their extensive
use in agricultural as well as public health field, more and
more insects are developing resistance to Organophosphorus
compounds. Some of the common compounds are Malathion,
Temephos, Fenthion, Dichlorovos (DDVP) and Fenitrothion.
Malathion : It is one of the least toxic Organophosphorus
compounds. Malathion is a broad spectrum insecticide, with
efficacy against a large number of pests ranging from mosquitoes,
houseflies, cockroaches, bedbugs, lice etc. It is available as
Malathion Technical (95%) for use as space spray, 50% Water
Dispersible Powder (WDP) and Emulsifiable Concentrate (EC)
for residual control and 90% dust for use against fleas and lice.
Malathion under the National Vector Borne Diseases Control
Programme of India is being used as Indoor Residual Spray
against mosquitoes in areas where the vectors have become
resistant to DDT. The dosage of its application is 2 g/m
2
(Refer
to Chapter on Mosquitoes for further details). As ULV spray it
has been very widely used during outbreaks of Dengue and JE
as an anti adult mosquito measure. However, development of
resistance has been reported in a large number of vectors to
Malathion.
Temephos : It is available as 50% EC. It is the only insecticide
approved for use in potable water. Because of its low toxicity,
it has been successfully used for the control of Anopheles
stephensi breeding in wells and domestic containers at a
dosage of 1ppm (Refer to Chapter on Mosquitoes for details).
Sand impregnated with Temephos in 1% concentration has been
used in some countries against Aedes aegypti which breeds in
containers of clean and potable water. It has proved to be very
successful in Guinea worm eradication programme in India.
Fenthion : It is formulated as 82.5% EC and as granules
containing 2% toxicant. It is a good mosquito larvicide but can
not be used in potable water bodies. It is highly effective as a
larvicide against Culex quinquefasciatus or any other vector
found breeding in non potable water bodies at a dosage of 1
ppm (Refer to Chapter on Mosquitoes for further details). It
can also be used for housefly control as a larvicide (Refer to
Chapter on Houseflies for details).
Pirimiphos methyl : This insecticide is being considered as an
alternative insecticide for Indoor residual spray. It is available
as 25% WP; 2 kg is mixed in 10 litres of water and sprayed @ 10
litres/ 250 sq m area to give a deposit of 2g/sqm. Three rounds
of spray are recommended as is followed in case of Malathion.
Dichloro-dimethyl-dichlorvinyl-phosphate (DDVP or
Dichlorvos) : It differs from other organophosphorus
compounds in that it possesses a much greater vapour pressure
at ordinary temperature which produces fatal insecticidal

8. • 908 •
vapour. It is available as 72.6% EC. It can be combined with solid
substances like wax and used as tablets or bricks thus allowing
it to evaporate slowly. It is one of the common insecticides used
for disinsecting aircraft. It is an effective housefly larvicide.
Fenitrothion : It is available as Fenitrothion 40% water
dispersible powder (WDP). The insecticide has shown promise
as an effective insecticide for control of bedbugs; however
toxicity constraints have limited its widespread use.
Carbamates : These compounds are derivatives of carbamic
acid and resemble Organophosphorus compounds in their
mode of action. Some of the preparations produce a rapid
knockdown effect like that of pyrethrum. The inhibition of
Acetylcholine esterase is reversible with Carbamates and hence
these compounds are less toxic. Some of the compounds in
common use are Propoxur, Carbaryl and Bendiocarb.
Propoxur : It is formulated as WDP as well as EC. It is considered
as the least toxic Carbamate compound for man and domestic
animals. It has a flushing out effect and therefore is commonly
used for cockroach and bedbug control. It is also used in bait
formulations against houseflies and cockroaches.
Bendiocarb : Bendiocarb is an alternative insecticide for Indoor
residual Spraying. It is available as 80% WP. For indoor residual
spraying, it is recommended @ 200 mg/sqm. Two rounds of
spray are recommended for effective control against malaria.
Synthetic Pyrethroids : These are synthetic derivatives or
analogues of natural Pyrethrum. These are broad spectrum,
highly potent with quick knock down action and long residual
life. Synthetic pyrethroids are many times more effective than
the previously available insecticides. Their relative safety
to man and higher animals, their efficient biodegradability
together with their higher target specific toxicity makes
them very attractive materials for integrated vector control.
The commonly available products are Permethrin, Allethrin,
Phenothrin, Cypermethrin, Cyfluthrin, Deltamethrin and
Bifenthrin. The Synthetic pyrethroids are formulated as WDP,
EC, SC, Flow, EW and ULV formulations. Being broad spectrum,
these insecticides are being used for vector control as residual
spray, space spray and topical application as well as for
treatment of clothing.
Deltamethrin : It is one of the most widely used Synthetic
pyrethroid molecule in the field of vector control. It is available
in many formulations for various vector control strategies viz.
SC 2.5% (Flow) formulation for treatment of bednet and routine
household pest control activity; 2.5% WP formulation for Indoor
Residual spray in Malathion resistant areas and 1.25 ULV for
space spraying. The target dose (for Indoor Residual Spray) is
generally 20 mg of a.i. (active ingredient) per sq m of surface
area.
Cyfluthrin : Besides Deltamethrin, this is the next most widely
used molecule. It is available as 0.5% EW formulation for
treatment of bednets; 5% EC for household use and 10% WP for
use as indoor residual spray in Malathion resistant areas.
Permethrin : Widely used for control of lice, scabies and for
treatment of clothing and bednets. The product is formulated in
varying concentration as Shampoo formulation for use as anti-
lice treatment and 5% cream for use in scabies treatment. Bed
nets treated with Permethrin at the manufacturing stage itself
are available as Pretreated or Long Lasting Nets (LLNs).
Other Synthetic pyrethroids used in Public health : There
is a large range of molecules used in the field of Public health
besides the ones listed above. These molecules are Allethrin,
Resmethrin, Phenothrin, Cypermethrin, Imiprothrin, Bifenthrin,
Cyhalothrin, Cyphenothrin etc. These are all available as WP, EC
or Aerosol formulations for use against pests like cockroaches,
houseflies and mosquitoes.
Newer Group of Insecticides
Phenyl pyrazoles : Fipronil is the only member of this class of
insecticide. Fipronil acts by antagonizing the effect of GABA.
It is available as 0.3% Gel for use against cockroach as a crack
and crevice treatment. It is a systemic material with contact
and stomach activity. It has a unique action called ‘cascade
effect’ which is evident due to necrophagy seen in cockroaches.
When cockroaches consume the insecticide bait, they are killed;
these dead cockroaches when consumed by other cockroaches
bring about the death of these cockroaches and this goes on for
about two months or so, thus obviating the need to retreat the
area at lesser intervals.
Neo Nicotinoids : Imidacloprid is the sole member from this
class. It acts by causing irreversible blockage of postsynaptic
acetylcholine receptors. Imidacloprid is a systemic insecticide,
having notable contact and stomach action. Imidacloprid is
available as 2.15% Gel for use against cockroaches and as Bait
for use against houseflies, where it is formulated with housefly
pheromone - Muscalure.
Biorational Insecticides : ‘Biorational’ means any substance
of natural origin that has a detrimental or lethal effect on
specific target pest, e.g. insects. These insecticides are non-
toxic to man, plants and animals and have little or no adverse
effects on the environment. An overview of the biorational
insecticides is presented in Fig. - 2.
Fig. - 2 : Biorational Insecticides Used in Vector Control
InsectGrowth
Regulators
BiorationalInsecticides
Pheromones
Muscalure
Ovipositionattractant

Biocides
Bacillus
thuringiensis
varisraelensis
Bacillus
sphaericus
ChitinSynthesis
Inhibitors



ufenuron
Diflubenzuron
L
Novaluron
JuvenileHormone
Mimics



Methoprene
Pyriproxyfen
Fenoxycarb
Triflumuron
Insect Growth Regulators : A new approach to vector control
is the use of substances that adversely affect insect growth
and development. The enzymes and hormones that regulate
developmental processes within an insect’s body can sometimes
be exploited as chemical control weapons. These compounds,

9. • 909 •
often known as Insect Growth Regulators (IGRs) can be used
to stimulate development at inappropriate times or inhibit it
at other times. They are quite selective in their mode of action
and potentially act only on target species. Most of the IGRs that
have shown effectiveness against insect pests, cause the rapid
death of the insect through failure of a key regulatory process
to operate or function. IGRs generally control insects either
through inhibition of chitin synthesis or interference with
metamorphosis by mimicking the action of juvenile hormone.
The major groups of IGR compounds include :
Chitin Synthesis Inhibitors : These chemicals inhibit the
moulting process by blocking the activity of chitin synthetase,
an enzyme needed by epidermal cells when constructing
a new exoskeleton. Because of this mode of action, Chitin
Synthesis Inhibitors (CSIs) are highly specific to arthropods.
They act rather slowly (2-5 days), but eventually disrupt any
process that involves construction of new cuticle (e.g., molting,
hatching, pupation). They are most effective when used against
the immature stages of a vector. Diflubenzuron, is used for
controlling mosquitoes, houseflies etc. It is available as 25%
EC, WP & 0.5% Granules and is used @ 1.0 g/ acre of surface
water as mosquito larvicide. Lufenuron, is a systemic CSI and
is especially effective for flea and tick infestation control on
animals. Novaluron is a recent addition to the list, which has
been found effective against the mosquitoes. It is a contact
larvicide and is available as 10% EC. It is used @ 20 μg a.i./ l
and the efficacy lasts up to 3 months.
Juvenile Hormone Analogues or Mimics : Juvenile hormone
analogues or Juvenile Hormone Mimics (JHM’s) act by inhibiting
the developmental changes associated with embryogenesis,
morphogenesis, and reproduction. During normal development,
JH levels are elevated in larvae (or nymphs) and decrease
prior to pupation (or adult eclosion). Contact exposure to JH
analogues during the egg stage or after the last larval molt
can inhibit development, delay maturation, and eventually
result in death. Since the onset of mortality is usually quite
slow (days to weeks), these chemicals are not used during
epidemics; however, these chemicals are much in demand for
routine vector control due to their specificity and safety to
non-target organisms. Several compounds (e.g., Methoprene,
Pyriproxyfen, Fenoxycarb, Triflumuron) have been successfully
incorporated into vector management programmes especially
Dengue and Malaria and in products used for controlling ants,
fleas, and other household pests. Pyriproxyfen can cause
sterilization and inhibition of growth of adult insects; it has a
residual effect up to 3 to 6 month indoors and 30 days outdoors.
It is widely used against mosquitoes @ 2gm a.i./ sq m.
Pheromones : Pheromones are semiochemicals (chemicals
which mediate interactions between organisms) secreted by
an organism which provokes specific reaction in receiving
organisms of the same species. These chemicals may further
be classified based on the type of interaction mediated e.g. sex
pheromone (muscalure secreted by houseflies), oviposition
attractant (mosquito larvae), aggregation pheromone
(cockroach, bedbugs), alarm pheromones, trail pheromones
etc. The scope of pheromones in vector control is promising,
however so far only two pheromones viz. Muscalure (in
combination with Imidacloprid insecticide as baits against
houseflies) and oviposition attractant (Aedes control) have
been exploited in the field of vector management.
Biocides : The development of insecticide resistance amongst
the major pests and vectors coupled with the non target toxicity,
necessitated development of safer alternatives to insecticides.
This led to the screening, promotion and use of a large number
of biorational products of which biocides are one of the most
important control options. The two biocides used in the field
of vector control are Bacillus thuringiensis var israelensis and
Bacillus sphaericus. Both these products are widely used as
larvicides in Mosquito control programmes and act as stomach
poison.
Bacillus thuringiensis var israelensis (Bti) : It was discovered
in 1976 and has been found to be effective as mosquito larvicide.
It is a gram positive spore forming bacteria. Bti produces toxins
which are present in parasporal body called the ‘protein crystal’.
It primarily kills by the action of delta-endotoxin. When the
mosquito larva ingests the protein crystal (inactive protoxin),
it is activated inside its midgut by the action of proteases into
active protoxin; these bind to the cell receptors present on
midgut epithelium and cause disturbance in osmoregulatory
mechanism which leads to swelling and eventual bursting of
the epithelium and finally death of the larvae. The product is
available as WP, Granules, AS & Briquette. Bti 12 AS is used @
20ml/m
2
and has been found to be effective up to 15 days (for
details refer chapter on mosquito). Bti however, suffers from
the disadvantage that it can not be used in polluted waters
or where particulate matter is more; it also cannot recycle in
nature. It is used in non potable water bodies.
Bacillus sphaericus : A naturally occurring bacterium used
against mosquito larvae. It is more effective in polluted water
and can recycle and persist in nature. It is available in various
formulations like Bti viz. pellets, briquettes, granules & WP. It
is used @ 20ml/m
2
and has been found to be effective up to
three weeks.
Fumigants : Some of the fumigants used as pesticides are
carbon tetrachloride, methyl bromide, ethylene dibromide,
chloropicrin, carbon disulphide and dichlorvos (DDVP).
Application Techniques
Control of arthropods in different habitats necessitates the use
of different types of spraying equipment as well as a variety of
formulations such as liquids, granules and dusts. For example
control may involve treatment of small domestic or peridomestic
water collections which are ideal breeding places for Aedes
mosquitoes; applications to stagnant waters in cesspools,
ditches and drains where Culex mosquitoes breed, large bodies
of standing water where certain Anopheles mosquito species
may be breeding; or aerosol spraying of extensive areas to halt
epidemics. To meet with diverse situations, significant progress
has been made in improving the spraying equipment. The Ultra
Low Volume (ULV) equipment for ground and aerial spray to
control mosquitoes and other haematophagous arthropods has
resulted in not only the elimination of several impediments
like frequent mixing and reloading but helped in increasing
the speed of application and reducing the dosages and costs. It
is specially recommended for control of an outbreak of vector
borne disease.

10. • 910 •
Formulations
Manufacturers combine pesticides with other materials
to make usable concentrations called formulations. These
formulations are designed to kill insects readily without
causing undue hazards to non-target organisms when diluted
and applied correctly. Factors influencing application and
efficacy of an insecticide are its toxicity, size and shape of
its particles, concentration in formulation; type of solvent
used, type of surface to be sprayed, atmospheric temperature
and humidity, type of sprayer and its nozzle, training of the
spraying personnel, the bionomics, morphology and physiology
of the particular arthropod. No single preparation can meet the
requirement of vector control in all spheres of human ecology.
Solutions, emulsions, suspensions, water dispersible powders,
dusts and granules to suit different conditions and problems
are therefore, prepared and used.
Technical Grade Pesticide
This is the basic toxic agent in its purest commercial form.
Some technical grade pesticides are liquids; others occur in
solid form. Technical grade Malathion is used in ultra low
volume space applications.
Types of Formulations
Formulations essentially are of three types : Solid or dry, liquid
and gaseous formulations.
Solid or Dry formulations
Dusts : Dusts are normally ready-to-use formulations with a
low percentage of active ingredient (usually 1 - 10%) plus a
very fine inert carrier such as talc, chalk, diatomaceous earth,
clay or volcanic ash. These materials are usually low in cost,
easy to apply, non-staining and non-toxic to vegetation. Dusts
are always used dry and can easily drift into non-target areas
if they are not applied carefully. For this reason, outdoor
applications should be made only when the wind is calm. A
common use for dusts is in crack and crevice or spot treatments
indoors in out-of-sight areas (behind equipment, in wall voids
and so on) which remain dry. The residual pesticidal activity
of dust is normally fairly long, provided the dust stays dry, but
quickly loses it toxicity in the presence of moisture. They don’t
adhere well to vertical surfaces.
Dusts are used on people during mass delousing operations to
control outbreaks of lice borne diseases. Dusts are also used
for flea control during plague outbreak. Dusts aren’t generally
absorbed through the skin, but may be dangerous if inhaled
into the respiratory tract.
Granules : These are basically the same as dust formulation,
except the carrier particles are larger and thus don’t stick to
leaves allowing penetration in dense foliage. This is a real
advantage when the pesticide must reach the water surface for
mosquito control in vegetated swamps, or if it must get to the
ground surface through trees and shrubs for chigger control.
Granules are also available in timed-release formulations that
release a dosage of the pesticide over an extended period of
time. Other advantages of using granules are that they provide
longer lasting effects and their use results in less drift than
generally occurs with liquids or dusts.
The percentage of insecticide in granules and pellets varies
from 1 to 5%. These can be used in irrigation channels,
irrigated or flooded lands, paddy fields and particularly where
there is vegetation on the water surface. After sinking, these
formulations disintegrate slowly releasing small particles of
insecticides. These can be effectively used also in small water
collections such as ornamental tanks and earthen pots, tree
holes and other domestic or peridomestic breeding places of
Aedes mosquitoes.
Wettable Powder : This formulation consists of the technical
grade pesticide, an inert carrier and a wetting agent (usually a
synthetic detergent) that helps it mix with water. These usually
contain 50 to 75% of the toxicant. Most of these can be put
directly into water and require only slight agitation to make
suspension; others may require mixing with a small amount of
water to form a paste or slurry. The required volume of water
is then added to paste or slurry followed by thorough agitation
of the mixture.
When water is added to a wettable powder it makes a suspension;
this enables the pesticide to stay on porous surfaces like
concrete, plaster or unpainted wood. Water penetrates these
surfaces, leaving the carrier and the maximum amount of the
pesticide on the surface available to kill pests. Suspensions have
other advantages, too. They have no solvent odour, and they
don’t tend to irritate or penetrate skin. However, they generally
need agitation to keep pesticidal particles from settling out.
Also, they tend to clog sprayer nozzles and strainers, especially
when the wettable powder is stored for long periods in humid
areas or when a high concentration is used.
Liquid formulations
Emulsifiable Concentrates : Emulsifiable concentrates consist
of the technical grade pesticide (typically 45% to 75%), a solvent,
and an emulsifying agent, usually a synthetic detergent. This
agent is used to allow the concentrate to be diluted in water,
resulting in an emulsion.
Emulsifiable concentrates are usually clear but emulsions look
similar to milk. Finished sprays are emulsions or solutions
diluted to field strength. Unlike solutions, most emulsions
need a little periodic agitation to keep the concentrate from
separating out of the water. Emulsions are used for residual
treatments. Pests that contact these surfaces are killed by
the pesticidal residue. Emulsions may damage aluminium,
varnish, and painted surfaces due to the action of solvents such
as Xylene. Emulsions may also be corrosive to metal sprayers
and their fittings and hence sprayers made of stainless steel,
aluminium or other non-corrosive materials should be used.
Oil Solutions : These formulations consist of a technical grade
pesticide dissolved in a solvent such as kerosene or diesel
oil. Solutions are available as ready-to-use formulations (for
example ordinary household fly and mosquito sprays with a
low percentage of pesticide) and as solution concentrates. These
concentrates contain a high percentage of insecticide and must
ordinarily be diluted in oil or another suitable solvent. Some
concentrates are used without dilution in Ultra Low Volume
(ULV) applications. Oil solutions applied as finished sprays
often kill insects on contact, since the oil helps the pesticide
penetrate the insect’s waxy body wall.
Ultra-Low Volume (ULV) : While most items of ULV pesticide

11. • 911 •
dispersal equipment use the readily available solutions or
technical grade formulations, there are special ULV formulations
available for e.g. Deltamethrin 1.25 ULV etc.
Gaseous formulations : Gases are primarily used in
fumigation operations. They may be prepared as liquefied
gases and packaged in pressure containers or in a material
form that reacts with the moisture in the air to form a gas.
The gas molecules can penetrate cracks, crevices and tightly
packed material. Gases are the most dangerous pesticides used
and hence special safety equipment and training are necessary
when using gases and must never be attempted except by
trained pest management personnel operating in pairs. One
of the common gaseous formulations viz. Calcium cyanide
(powder) and Aluminium phosphide (tablet) are used for rodent
control.
Special formulations
Resin Strips : Pesticide-impregnated resin strips release
vapours as they are heated or exposed to normal room
temperatures. The use of resin strips in rooms occupied by
the young, the elderly or in food preparation and food serving
areas is strictly prohibited.
Baits : Baits are commonly used to manage scavenging pests
such as rodents, ants, flies, and cockroaches, which are
particularly difficult to manage with standard techniques.
Baits consist of the toxicant mixed with a food attractive to the
target pest or with water. For this reason, baits made with local
foods are normally more effective than premixed formulations.
Recent development is the use of pheromone Muscalure with
Imidacloprid as bait for houseflies.
Gel : One of the special formulations developed for use against
cockroaches is Gel formulation. Gels comprise some food
attractant mixed with the toxicant and some stabilizing agents.
Examples are Fipronil and Imidacloprid Gels marketed against
cockroaches.
Shampoo : This formulation has been specially developed
for use against head lice infestation. Permethrin is the most
common ingredient of the commercially available anti-lice
Shampoo formulations worldwide.
Beads / Pellets / Briquettes : Small floating beads, pellets or
briquettes incorporating biocides - Bti and B sphaericus have
been developed against Anopheline larvae. These formulations
can be made as controlled release formulations as well.
Paints and Lacquers : These can be used for incorporation
of insecticides especially for control of pests on ships. These
preparations remain effective for long periods. The new
insecticide, Imidacloprid is also available as a paint formulation
against houseflies.
Mats / Coils : These are special formulations which have been
developed as controlled release formulation for indoor use
against mosquitoes. These have synthetic pyrethroids such as
Allethrin, which acts as toxicant to knock down the mosquitoes
when used indoors.
Aerosols : Aerosols are pressurized cans containing a small
amount of pesticide driven through a small nozzle. They’re
commonly used as space sprays for flying insects viz.
mosquitoes and houseflies and as residual sprays for Mites/
Ticks depending on the formulation. Care should be taken
while handling aerosol cans since they can explode if punctured
or overheated, even after the pesticide has been dispensed.
Common insecticides used as aerosols are Pyrethroids,
Malathion, DDVP and repellents like DEET and DEPA. These
are used for disinfesting aircrafts, tents, rooms, other small
enclosures, uniforms and for topical application. An emission
of nearly 15 seconds is enough for a 100 m
3
space.
Equipment
Equipment used for vector control can be broadly classified as
ground equipment and equipment used for aerial applications.
(a) Ground Equipment
(i) Sprays for production of fine or coarse spray which may be
either manually operated or power operated.
(ii) Sprayers for the production of mist which may be either
with gaseous energy nozzles (manual operated or power
operated) or with centrifugal nozzles.
(iii) Devises for the production of aerosols which may
be mechanical, thermal or gaseous energy aerosol
generators.
(iv) Dusting equipment which may be manually operated or
power operated.
(v) Applicators for granules and pellets, manually or power
operated.
(b) Aerial Equipment
Equipment for aerial sprays is essentially the same but with
certain modifications. The equipment in common use is the
boom and nozzle system.
Sprayers
The equipment commonly used for spraying various insecticidal
formulations are the hand operated sprayers, power operated
sprayers, aerosol dispensers, fog generators and dusters.
(a) Hand Operated
These are hand sprayers, knapsack sprayer and compression
sprayer.
(i) Hand Sprayer : The hand sprayer is used for space spraying
of small apartments. It is provided with a small can for holding
½ to 1½ litres of spray fluid and a cylindrical plunger type air
pump. The nozzle size is less than 0.4 mm in order to produce a
fine spray. The simplest form is the familiar ‘flit gun’ producing
intermittent spray. A number of other light hand sprayers have
been designed, which can be pressurized in the manner of
compression sprayers and are used to produce a mist or fine
droplet spray.
(ii) Knapsack Sprayer : This is designed to fit on to the back
of the operator and usually has a capacity of 15 to 20 litres.
It incorporates a light but powerful diaphragm pump actuated
by a lever carried forward to the operator’s hand where it is
worked by an up-and-down movement. These sprayers are used
both for larviciding and residual spraying. The nozzle size used
for residual spraying varies between 0.78 to 1.0 mm so as to
produce a coarse spray.
(iii) Compression Pneumatic Sprayer : This is the commonest
type of equipment used in National Vector Borne Diseases
Control Programme for the application of insecticides. It has

12. • 912 •
a hand operated pump incorporated to build up adequate
pressure. When the pressure is released by a trigger on the
lance, the liquid is forced out from the tank to the nozzle by
the compressed air and a continuous spray of the insecticide
formulation is produced. It is slung over the shoulder with
one strap or may be carried on the back with two straps. It is
operated by one person.
(b) Power Operated Sprayers
These are useful for application of insecticides over large areas.
These are hydraulic sprayers in which the spray liquid is expelled
to the nozzle by positive displacement by the plunger pump.
Insecticide tanks built into a truck or mounted over a hand
trolley are connected directly to a power operated compressor.
By means of a long hose the spraying fluid is conveyed under
pressure through the lance to the nozzle.
(c) Insecticidal Fog Generators
Several types of fog generators are now available for the
production of insecticidal fogs in the open on a large scale.
In these fogging machines the oily solution of the insecticide
is finely atomized by the powerful blast of hot exhaust gases
from a petrol engine.
(d) Aerosol Dispensers
These are used for disinfestations of aircrafts, tents, rooms
and similar small enclosures. It contains insecticide and a
propellant. Common aerosols contain Synthetic pyrethroids
or their combination, which are routinely used for mosquito,
cockroaches and fly control.
(e) Dust Gun
Insecticidal dusts are applied against lice and fleas in rat
burrows or on water surfaces as dry powders diluted with inert
dusts. Small light weight guns are used for mass delousing of
infested people.
Residual Spraying
This is the application of insecticides to surfaces so that the
insecticide particles remain on the surface in the form, size and
quantity suitable for insects to pick up on contact and sufficient
to exert a lethal effect over a long period. Organochlorine,
Organophosphorus, Synthetic pyrethroids and Carbamate
compounds can thus be applied on the inside walls of houses
and also on thick bushes in forests. The type of surface to
which an insecticide is applied influences its toxicity against
insects and its persistence. Solutions and emulsions quickly
get soaked in the absorbent surfaces of soft bricks and mud
walls which take in a large portion of insecticidal material
deposited on them; but when suspended in water it remains
over the surface after the water evaporates or gets absorbed. The
nozzles of sprayers used for residual spraying must conform to
the need of having a droplet size which is neither too large nor
too small. Similarly, safety precautions should be observed, as
follows, while spraying as per standard WHO guidelines :
(a) Do not eat, drink or smoke while working.
(b) Wash your hands and face with soap and water after
spraying and before eating, smoking or drinking.
(c) Shower or bathe at the end of every day’s work and change
into clean clothes.
(d) Wash your overalls and other protective clothing at the
end of each working day in soap and water and keep them
separate from the rest of the family’s clothes.
(e) If the insecticide gets on your skin, wash off immediately
with soap and water.
(f) Change your clothes immediately if they become
contaminated with insecticides.
(g) Inform your supervisor immediately, if you do not feel
well.
(h) Wear protective clothing (Fig. - 3) :
- Broad rim hat (protects head, face and neck from spray
droplets).
- Goggles or face shield (protects face and eyes against
spray fall-out).
- Face mask (protects nose and mouth from airborne
particles of the spray fall-out).
- Long sleeved overalls (Keep overalls outside of boots).
- Rubber gloves.
- Boots.
Preparations
The Household : Inform the householder of the spraying
schedule and the purpose of spraying, giving them time to
prepare and vacate the house. Occupants MUST leave houses
before spraying. Rooms occupied by sick people who cannot
be moved must NOT be sprayed. Remove all household items,
including water, food, cooking utensils and toys from the house.
Move and cover, or take out the furniture to allow easy access
for spraying walls. Items that can not be removed should be
well covered.
Equipment : Indoor residual spraying of insecticides is
normally done using hand-operated compression sprayers.
Before starting a spray operation, the equipment must be
checked. Faulty sprayers may result in poor control or over-
treatment. Examine the sprayer visually to ensure that all parts
are present, assembled correctly and are in good condition
(Fig. - 4).
Fig. - 3 : Protective
clothing for spraying
Fig. - 4 : Hand-operated
compression sprayer

A:Hat
B:Goggles
C:Mask
D:Long
Sleeved
Overalls
E:Rubber
Gloves
F:Boots
A.Sprayertank
B.
Shoulder
strap
C.Lid
D.Pump(handle)
E.Pressuregauge
F.Lance
G.Strainer
H.Hose
I.Nozzle
J.Trigger
K.Footrest
Nozzlebody
Nozzletip
Nozzlecap

13. • 913 •
A. Sprayer tank
B. Shoulder strap
C. Lid
D. Pump (handle)
E. Pressure gauge
F. Lance
G. Strainer
H. Hose
I. Nozzle - check correct type of nozzle is fitted and is not
damaged or worn (flat fan nozzle with 55º to 60º swath
and 0.75 l/min flow rate at 700 g/ sq cm).
J. Trigger on/off valve. Is the strainer inside valve handle
clean?
K. Footrest
Before using an insecticide, use clean water to ensure that the
equipment operates properly and does not leak. Wear protective
clothing. To check, follow the steps below :
(a) Pour clean water into the tank (never fill tank more than
3/4 full) (Fig. - 5).
Fig. - 5
(b) Fit the lid. Turn the handle to lock the lid in position
(Fig. - 6).
Fig. - 6
(c) Operate the pump using both hands and with foot on
the footrest. Pump to the working pressure of 55 psi
(Fig. - 7).
Fig. - 7
(d) Check tank is holding pressure. Listen for hissing sound of
escaping air (Fig. - 8).
Fig. - 8
(e) Check to make sure there are no leaks along lance and
hose, especially where hose joins tank and trigger on/off
valve (Fig. - 9).
Fig. - 9

14. • 914 •
(f) Operate trigger on/off valve to make sure that spray is
emitted from the nozzle (Fig. - 10).
Fig. - 10
(g) Check the spray pattern from the nozzle by spraying a
dry wall surface. Look to see that the pattern is even and
without streaks. Ensure nozzle does not drip when trigger
on-off valve is released (Fig. - 11).
Fig. - 11
(h) Calibrate the nozzle with water in the tank. Pump to 55
psi (700 g/ sq cm). Open the trigger on-off valve for one
minute, collect the discharge and measure the amount in a
measuring jug. Empty the jug. Discharge for a further one
minute and measure the amount. Repeat again and calculate
the average of the three, one-minute measurements.
With the above procedure, the average discharge of an 8002
nozzle is about 750 ml per minute. If the discharge is incorrect,
check the nozzle and the screen filters to ensure they are not
clogged. If necessary replace nozzle. Repeat the calibration.
If the nozzle is clogged : The opening in a nozzle is very small
and must not be damaged. Clogged nozzles should be put in a
container with water for several hours before the blockage is
removed by a very soft toothbrush. NEVER clean nozzle with
a hard pin or piece of wire and NEVER put a nozzle to your
mouth to blow through it.
Mixing, Handling and Spray Techniques
Prepare the insecticide spray according to the manufacturer’s
instructions. The insecticide may be mixed separately in a
bucket and poured into the sprayer. Water soluble sachets,
tablets and insecticides granules are added directly to the
water filled tank. These formulations mix readily with water
and reduce the hazards associated with handling and mixing
in a separate container. When the sprayer has been filled with
water to the maximum level indicated on the tank, the lid of
the tank is fitted and the sprayer pumped to a pressure of 55
psi by pumping 55 times (700 g/sq cm). The contents of the
tank should be thoroughly mixed by shaking the tank before
starting to spray (Fig. - 12).
Fig. - 12
Spraying in a room should commence from the backside of a
door clockwise completing the plain surfaces of walls. Then the
crevices on the walls and inside portion of windows etc. should
be sprayed. Thereafter the pillars, under surfaces of furniture
and lastly the ceilings should be taken for spray.
Spray is done from roof to floor, using downward motion, to
complete one swath; then stepping sideways and spraying
upwards from floor to roof. Spray is applied in vertical swaths
52-56 cm wide. Swaths should overlap by 5 cm and spraying
should be undertaken as shown in Fig. - 13. Normal swath
coverage will take 2.7 sec if height of wall is assumed to be
3 meters and hence in one minute 22-23 swaths will cover a
wall of 10-11 metres length and 3 metres height i.e. 30 - 33 sq
m. It takes about 5 minutes to spray a house with an average
surface area of 150 sq m.

15. • 915 •
Fig. - 13
To ensure the correct swath width, keep the spray tip about
45 cm from the wall. Lean forwards as you spray from top of
the wall and move back as you bring the nozzle downwards
(Fig. - 14).
Fig. - 14
The flow of liquid from the nozzle tip at 700 g/sq cm pressure
is 750 ml/minute. Hence 30 sq m surface will be covered with
750 ml of the insecticide solution.
Time your spray speed to cover one meter every 2.2 seconds,
i.e. 4.5 seconds for a 2 m high wall. Timing may be aided by
mentally counting “one thousand and one - one thousand and
two - one thousand and three -…”. Adjust the mental counting
procedure according to the local language (Fig. - 15). If spray
stops due to a blockage in nozzle, unscrew the nozzle cap,
remove blocked nozzle and replace with a new one. The blocked
nozzle should be cleaned as explained above. Do not let spray
drip on the floor. Re-pressurize the tank when the pressure
falls.
Fig. - 15
Procedures after Spraying
(a) Advise the occupants to stay outside until the spray is
dry.
(b) Instruct the householder to sweep or mop the floor before
children or pets are allowed to re-enter.
(c) Instruct the householder not to clean the sprayed
surfaces.
Disposal of Remains of Insecticides and Empty Packaging:
At the end of the day’s work, put the washings from the sprayer
into pit latrines, if available, or into pits dug especially for
this purpose and away from sources of drinking water. Dilute
any insecticide with more water before putting into pits. It
is advisable to prepare only sufficient insecticidal solution
to avoid disposal of remaining insecticidal solution. Never
pour the remaining insecticide into rivers, pools or drinking
water sources. All empty packaging should be returned to the
supervisor for SAFE disposal. Never re-use empty insecticide
containers. Empty insecticide containers should NOT be burned
or buried.
Maintenance of Equipment : After completing the day’s work,
de-pressurize the tank and empty any remaining insecticide,
following the instructions given in the previous section. Clean
the tank as explained below :
De-pressurize the tank.●●
Fill the tank half-full with clean water.●●
Replace the lid.●●

16. • 916 •
Shake the tank so all inside surfaces are washed ●●
(Fig. - 16).
Fig. - 16
Pump up to 700 g/ sq cm pressure. Spray water through ●●
nozzle (Fig. - 17).
Fig. - 17
De-pressurize the tank and pour out any remaining water ●●
into pit latrines or into a pit away from sources of water.
Unscrew trigger on/off valve handle and check and clean ●●
the strainer.
Reassemble the trigger on/off valve (Fig. - 18).●●
Fig. - 18
Remove the nozzle tip and wash●●
Refit the nozzle●●
Clean the outside of tank.●●
Fig. - 19
With lid open, turn tank upside down, open the on/off valve ●●
and let all the water drain out of the hose and lance.
Ensure the lance is parked to protect nozzle when not in ●●
use.
When storing the sprayer for a long period, hang it upside ●●
down with lid open to allow air circulation. Allow lance to
hang from D-ring on the tank with the trigger valve kept
open (Fig. - 20).
Fig. - 20
Space Spraying
It is an ideal method for bringing about rapid control of vectors
in emergency or epidemic situations and may also be used for
seasonal control of flying insect pests or vectors. An additional
objective may be to reduce or interrupt the transmission cycle
of insect-borne diseases. However, it may not be ideal for all
vectors or situations and as such may not be an economical
method of control. Among the disease vectors affecting public
health, the most important and widespread are mosqui­ toes,
houseflies, sandflies and other biting flies; some of these may
be tar­ geted for space treatment.
Immediate killing of actively flying insects requires a cloud
of insecticide droplets that they will encounter in flight. To

17. • 917 •
be cost-effective and obtain good biological efficacy, space
spraying requires :
(a) Knowledge of the behaviour and biology of the target
species - to understand where and when space treatments
will be effective;
(b) Knowledge of insecticides and formulations most suitable
for space spraying;
(c) Knowledge of pesticide application technology - to know
which equip­ ment is needed and how to use it; and
(d) Monitoring and surveillance of the target species and
vector-borne disease problem to evaluate the efficacy of
the programme.
A space spray - technically a fog (sometimes referred to as an
aerosol) is a liquid insecticide dispersed into the air in the form
of hundreds of millions of tiny droplets less than 50 μm in
diameter with a view to cause by contact, immediate knock
down of the flying or resting insects in confined spaces. Space
sprays, even when they settle on surfaces do not have much
residual action. It is only effective while the droplets remain
airborne. Therefore, they have to be repeated at frequent
intervals. Space sprays are applied mainly as thermal fogs or
cold fogs.
Thermal Fog
Thermal fog is produced by special devices known as thermal
foggers that use heat to break up the chemical into very small
droplets (usually in 5-30 µm diameter range) which then
disperse in the air. When the chemical (usually diluted with
oil-based carrier) is heated, it is vaporized in a combustion
chamber and then expelled via an outlet tube to form a dense
fog cloud when it condenses on contact with cool ambient air.
The insecticide used in thermal fogs is diluted in a carrier
liquid, which is usually oil-based. Hot gas is used to heat the
pesticide spray, decreasing the viscosity of the oil carrier and
vaporizing it. When it leaves the nozzle, the vapour hits colder
air and condenses to form a dense white cloud of fog. Most of
the droplets are smaller than 20 µm. The droplet size is affected
by the interaction between the formulation, the flow rate and
the temperature at the nozzle (usually > 500°C). The volume
of spray mix­ ture applied in vector control is usually 5-10 litres
per hectare, with an absolute maximum of 50 litres per hectare.
The hot emission gas is obtained from engine exhaust, friction
plate/engine exhaust or from a pulse jet engine.
Advantages
Easily visible fog, so dispersal and penetration can be ●●
readily observed and monitored;
Good public relations in some circumstances as people can ●●
see some­ thing being done about the problem; and
Low concentration of active ingredient in the spray mixture ●●
and reduced operator exposure.
Disadvantages
Large volumes of organic solvents are used as diluents, ●●
which may have bad odour and result in staining;
High cost of diluents and spray application;●●
Householders may object and obstruct penetration of fog ●●
into houses by closing windows and doors;
Fire risk from machinery operating at very high ●●
temperatures with flam­ mable solvents; and
Can cause traffic hazards in urban areas.●●
Cold Fog
The cold fog is produced by a special device (cold fogger) that
breaks up the chemical into microscopic droplets by mechanical
means, basically with a high-pressure pump and an extremely
fine nozzle. The spray droplets are generated without any
external heat. With cold fogs, the volume of spray is kept to
a minimum. Ultra-low-volume insecticide formulations are
commonly used for such applications. The cold fogger may
dispense formulations in a very concentrated form and generate
the droplets (usually in the 5-30 micron diameter range) in a
precise manner. However, its ability to penetrate dense foliage
or obstacles is not as good as that of the thermal fogging. Cold
fogging is sometimes called Ultra Low Volume (ULV) treatment
as it allows the utilization of only a very small amount of
chemical for coverage of a large area.
Like thermal fogging, cold fogging also does not have lasting
residual effects. It is, therefore, essential to carry out fogging at
the time when the vectors are most active to hit them directly.
Advantages
The amount of diluents is kept to a minimum, resulting in ●●
lower appli­ cation cost and increased acceptability. Some
formulations are ready to use, thereby reducing operator
exposure
Mostly water-based and water-diluted formulations ●●
are used which pose a low fire hazard and are more
environmental friendly
Application is more efficient because a lower volume of ●●
liquid is applied
No traffic hazard as the spray cloud is nearly invisible●●
Disadvantages
Dispersal of the spray cloud is difficult to observe●●
Higher technical skills and regular calibration are required ●●
for efficient operation of equipment.
Space Spray Equipment
Selection of appropriate equipment for space spraying depends
on the size and accessibility of the target area as well as the
human resources and operational capacity of the programme.
Sometimes smaller machines may be needed in conjunction
with vehicle-mounted equipment to treat narrow pathways and
other areas inaccessible to vehicles or sheltered from prevailing
air movements. Cold fog equipment is recommended where
thermal fogs may cause a traffic hazard. Aerial application
of space sprays may be justified where access with ground
equipment is difficult and/or extensive areas need to be treated
very quickly.
Equipment for Thermal Fogging
Hand-carried Thermal Foggers : These are used for treating
houses and certain outdoor areas of limited size or accessibility,
e.g. markets, hotel grounds and parks. There are two types of
hand-carried thermal foggers; pulse jet and fric­ tion plate.
Vehicle-mounted Thermal Foggers : Large thermal fog
generators use an air-cooled motor to run an air blower, fuel
pump and insecticide pump. Air from the “roots type air blower”
is delivered into the combustion chamber. There it is mixed with
gasoline vapour and ignited so that temperatures reach 426-
648°C. The diluted insecticide liquid is pumped via a simple flow
delivery valve and injected into a cup in the fog head or directly

18. • 918 •
into the nozzle. The insecticide liquid is vaporized by the blast
of hot gases. Despite this high tempera­ ture, insecticides show
very little degradation of active ingredient. This is because the
time spent at that temperature is only a fraction of a second,
which is not long enough to cause serious degradation. The
hot gases then pass out of the machine. As the hot oil vapour
is discharged through a relatively large nozzle into the cooler
outside air, it condenses to form very small droplets of thick
white fog. Delivery rates of up to 10 litres per minute can be
achieved with larger machines.
Aircraft Application of Thermal Fogs : For aircraft application
of thermal fogs, the diluted insecticide formula­ tion is fed into
the aircraft exhaust. The exhaust is adapted with vanes to swirl
the fog droplets as they are formed. The application of thermal
fogs by aircraft is very limited.
Equipment for Cold Fog Application
Hand-carried Cold Foggers : Most of these machines have
gasoline engine or electric operated which drives a blower
unit to discharge air through the nozzle. Air may also slightly
pressurize the insecticide formulation tank so that the liquid
is fed via a restrictor to the nozzle. However, negative pressure
generated by the air flow passing through the nozzle allows
liquid to flow from the tank. In addition to hand-carried units,
knapsack cold fogging units are also available as are several
electrically driven models.
Vehicle-mounted Cold Foggers : A high volume air blower
forces air at a rate of approximately 6 m
3
per minute at low
pressure to nozzle. The pesticide container may be pressurized
to force the formulation to the nozzle, or positive-displacement
pumps may be used.
Alternatively a high-pressure, low-volume air source is used
with an air compressor, rather than a blower. On these machines,
nozzles ranging from the standard industry “paint gun nozzle”
to proprietary nozzles that atomize well up to a flow rate of 0.5
litres per minute are available. Another design uses a rotary
nozzle coupled with an electric motor which operates at a very
high speed.
Aircraft Application of Cold Fogs : Both fixed-wing aircraft
and helicopters have been used to apply cold fogs. Conventional
low-volume nozzles (e.g. flat fan) have been used on fixed wing
aircraft to create fine sprays, using moderate or high pressures.
However, the droplet spectrum is generally poor so preference
is given to the use of rotary atomizers or very-high-pressure
systems.
Insecticide Products for Space Spraying
Space-spraying formulations are generally oil-based. The
oil carrier inhibits evaporation of small fog droplets. Only
insecticide products with high flash points are used for thermal
fogging. Diesel is used as a carrier for thermal fogging, but
creates a thick smoke and oily deposits, which may lead to
public rejection. For environmental reasons, water-based
formulations have been made available in recent years. These
formulations may also contain substances that prevent rapid
evaporation. Table - 3 lists selected insecticides suitable for
space spraying against mosquitoes. These insecticides may also
be used against other insect pests and vectors, but different
dosages may be required.
Space Spray Treatments - General Considerations
Optimum Droplet Size : Space treatments are only effective
while the droplets remain airborne. Droplets fall by gravity and
some are deposited on horizontal surfaces while the majority is
lost to the atmosphere especially in outdoor spraying. Droplets
bigger than 30 µm in diameter are less effective as they do not
remain airborne for sufficient time. Droplets smaller than 5 µm
in diam­ eter do not readily come in contact with flying insects,
as the movement of the smallest droplets is affected by the air
turbulence created by the insect’s flight. It is generally accepted
that droplets should be generated in the range of 10-30 µm
so that even with some evaporation and after some time, they
remain in the correct range for optimal airborne suspension
and insect impact. The optimum droplet size for space spraying
against mosquitoes is 10-20 µm.
Spray Concentration : For a flying insect to be killed, it
must acquire a lethal dose of insecticide in the droplets that
impact on it. The lower the concentration of active ingredient,
the larger the number of droplets of a given size required to
achieve a lethal dose. Ultra-low-volume spraying aims, largely
for operational reasons, to minimize the total volume of diluted
insecticide applied (usually < 2 litres per hectare).
Wind Speed : Wind speed has a profound effect on droplet
distribution and impingement on insects. Spraying should not
take place when wind speed exceeds 15 km/hour. The type of
terrain and vegetation affects air movement and hence the
distribution of the droplets. In open terrain with relatively
sparse vegetation, wider effective swaths can be obtained than
in urban areas where the obstruction of buildings alters the
Table - 3 : Insecticides Used for Space Spraying
S No. Insecticide Dilution Equipment required Remarks
1 Pyrethrum 2% Extract1 litre in 19 litre
Diesel (0.1%)
Flit gun/ Thermal
Fogging machine
For space spray indoors (Keep rooms
closed for 30 min) or outdoors
2 Pyrethrum 2% EC 1 litre in 19 litre WaterULV Fogging machineFor space spray indoors
3 Malathion Tech 5 litres in 95 litre
Diesel (5%)
Thermal Fogging
machine
For space spray outdoors
4 Deltamethrin 1.25 ULV1 litre in 199
litre Diesel
Thermal Fogging
machine
For space spray outdoors
5 Deltamethrin 1.25 ULV1 litre in 19 litre DieselULV Fogging machineFor space spray outdoors
6 Deltamethrin 1.25 ULV1 litre in 19 litre WaterULV Fogging machineFor space spray indoors

19. • 919 •
flow of air.
Wind direction : With vehicle-mounted and aerial spraying
the spray route must take account of the wind direction to
maximize the distribution of the spray throughout the target
area. Fig. - 21 illustrates the spray application route relative to
wind direction.
Fig. - 21 : Spray Application Route
Winddirection Spraydrift
Pathofspraying
Temperature Effects : In direct sunlight the ground is heated.
This causes air to rise. In the middle of the day, outdoor space
spraying will largely be wasted as the spray droplets will tend to
rise upwards rather than drift horizontally. Ideally an inversion
is needed, i.e. colder air closer to the ground. This generally
occurs early in the morning after the ground temperature has
fallen during the night, but can also occur in the evening when
the sun has set and ground temperatures begin to fall.
Time of Treatment : Knowledge of the time of peak flight
activity of the target species is crucial to ensure that space
treatments are planned to coincide, as far as possible, with
these times. Fortunately, peak flight activity of many vectors
is around dusk and/or dawn, when weather conditions are
often favourable for space treatment. Aedes aegypti and Aedes
albopictus, mosquito vectors of Dengue fever and Chikungunya
are active during daytime with peak flight activity in the
morning and afternoon. With these species, a compromise is
usually made outdoors by spraying in the early morning or late
afternoon. The timing is less important if indoor spraying is
conducted.
Indoor Fogging
Personnel conducting this work require training on the safety
measures to be followed. Several rules apply :
Protect all water containers and foodstuffs.●●
Remove fish or cover fish tanks.●●
Ensure all occupants and animals remain outside the ●●
house during spraying and stay outside for 30 minutes
after spraying. Ensure that the building is ventilated before
reoccupation.
Close all doors and windows before spraying and keep them ●●
closed for 30 minutes after spraying to ensure maximum
efficacy.
Spray operators should work backwards and away from ●●
the fog to minimize exposure.
For small single-storey houses, the spray can be delivered ●●
from the front door or through an open window without
having to enter every room of the house, provided that
adequate dispersal of the insecticide droplets can be
achieved.
For large single-storey buildings, it may be necessary to ●●
apply the spray room by room, beginning at the back of the
building and working towards the front.
For multi-storey buildings, spraying is carried out from top ●●
floor to the ground floor and from the back of the building
to the front. This ensures that the operator has good
visibility at all times.
Outdoor Ground Fogging
Advanced route planning should precede outdoor ground
fogging operations and may require a combination of vehicle-
mounted and hand carried or knapsack equipment in areas
with difficult or limited vehicle access. Consideration must also
be given to the following :
Spraying should not be undertaken when it is raining, ●●
when winds exceed 15 km/hour, or in the heat of the day.
Doors and windows of houses and other buildings should ●●
be open to allow penetration of the spray cloud for improved
efficacy.
For vehicle-mounted equipment, in areas where the roads ●●
are narrow and the houses are close to the roadside, the
spray should be directed backwards from the vehicle. In
areas where the roads are wide, with buildings far from the
roadside, the vehicle should be driven close to the roadside
and the spray should be directed at an angle (downwind)
to the road rather than directly behind the vehicle.
The nozzle of vehicle-mounted cold fog machines may ●●
be directed upwards at an angle when there are barriers
that impede airflow, e.g. boundary walls and fences; for
vehicle-mounted thermal foggers, the nozzle should be
directed horizontally.
A track spacing of 50 metres is generally recommended, ●●
with the vehicle moving upwind so that the fog drifts
downwind away from it and the operators.
Aerial Application of Fogs : Suppression of vector populations
over large areas can be carried out using space sprays released
from aircraft, especially over areas where access with ground
equipment is difficult and extensive areas need to be treated
very rapidly.
Evaluation
Evaluation of the efficacy of spray operations is carried out
using techniques that are largely specific to the target insect.
Space sprays are transient and only insects flying at the time
of the application are affected.
Area Spraying
This is carried out for treatment of land against mites and ticks
and also as an anti-larval measure over vast water surfaces.

20. • 920 •
Against mites and ticks, suspensions are used on land and
vegetation; WDP is used for anti-larval treatment of lakes and
swamps. Aerial spraying is resorted to for agricultural purposes
and sometimes for veterinary and rarely medical purposes.
Dusts are applied to manure yards and dry refuse yards to
control flies and other pests. For all such uses, power driven
sprayers and dust guns are used. The larvicidal oils are applied
by spraying it on the surface of water by means of a knap-sack
sprayer or hand pumps or by a mop stick.
Resistance of Vectors to Insecticides
History : Ever since the introduction of the potent synthetic
insecticides into public health programmes at the close of the
Second World War, the main problem has been the development
of resistance to them by the arthropods they formerly controlled.
In 1947, DDT resistance was discovered in the housefly and
Culex molestus in Italy. In 1951, DDT resistance was noticed in
body louse in Korea and in Anopheles sacharovi in Greece. In
1955, Dieldrin resistance was discovered in Anopheles gambiae
in Northern Nigeria. In 1959, in Western India the oriental rat
flea was found to have developed resistance to DDT. The number
of arthropods showing resistance is on the increase.
Definition : Resistance is defined as “the development of an
ability in a strain of insects to tolerate doses of toxicants which
would prove lethal to the majority of individuals in a normal
population of the same species”. A more pertinent definition
of resistance promoted by the Insecticide Resistance Action
Committee (IRAC) is “the selection of a heritable characteristic
in an insect population that results in the repeated failure of
an insecticide product to provide the intended level of control
when used as recommended”. It is important to remember that
resistance does not mean that it is impossible to control the
population.
The word tolerance is normally used when the increase in
LC
50
is less than the indicated minimum for the tests, but is
nevertheless statistically significant. It is generally due to sub-
lethal exposure to insecticide and is not passed on to offspring.
Vigour tolerance is a term, which has been applied to enhanced
insecticidal tolerance resulting from extra vigour of the strain
rather than from any specific defence mechanism.
Types : Resistance is of two types i.e. physiological and
behaviouristic. Physiological resistance is the one described
above. Behaviouristic resistance means the development of
ability to avoid a lethal dose. This term is applied most often to
mosquitoes in relation to DDT.
Resistance Mechanisms
Metabolic Resistance : This is the most common resistance
mechanism encountered amongst insects, based on
detoxification enzymes. In resistant strains, these enzymes
are generally more enhanced thus enabling the insects to
metabolize or degrade the insecticide before their lethal effect
is exerted. The enzyme systems involved in these processes
are Esterases, Monooxygenases and Glutathione-S-tranferases
(Table - 4).
(i) Esterases - Dominant mechanism of resistance conferring
resistance to OP compounds. It also affects Pyrethroids
and Carbamtes.
(ii) Monooxygenases are importantly involved in the resistance
mechanism in insects to Pyrethroid class of insecticides. It
also affects DDT and OPs.
(iii) Glutathion S transferase - It has been implicated in DDT
resistance.
Target - Site Resistance : This is the second most common
resistance mechanism encountered in insects. The site of action
of the insecticide is modified so that the insecticide no longer
binds effectively at that site. Resistance to DDT and pyrethroids
is due to the modified gene Kdr, which leads to reduction in the
sensitivity of the site to the binding of DDT and Pyrethroids. On
the other hand, an altered site of action as a cause of resistance
to Organophosphates (OPs) and Carbamates has been definitely
established with cholinesterase inhibitors. In these cases,
mutated forms of AChE (MACE - modified acetylcholinesterase)
are produced that is inhibited more slowly than the normal
enzyme in susceptible strains. This produces resistance against
a large number of compounds of the OPs and Carbamates and
the resultant extensive cross-resistance which makes it a
serious type of resistance.
Reduced Penetration : Not a major resistance mechanism,
may contribute to the overall development of resistance to
an insecticide. Modification in cuticle (applicable for contact
poison) or digestive tract linings (in case of stomach poisons)
Table - 4 : Biochemical resistance mechanisms conferring resistance to important classes of insecticides in mosquitoes
(dot size gives the relative impact of the mechanism on resistance)
Biochemical Mechanism of Resistance
Metabolic Target-site
Esterases Monooxygenases
GSH-S
transferases
kdr MACE
Pyrethroids
DDT
Carbamates
Organo-phosphates
Source : Prevention & management of insecticide resistance in vectors and pests of public health importance. Manual by Insecticide Resistance Action Committee
(IRAC).

21. • 921 •
leads to prevention or reduced penetration of insecticides in the
body of the insects has been reported in a number of resistant
insects e.g. houseflies.
Behavioural Resistance : It is a modification in the behaviour
of insects to avoid toxic doses of the insecticide. It is also a
contributory factor in development of resistance and not an
important resistance mechanism. It may be stimulus dependent
(when insects avoid insecticide treated surface) or independent
(selective and sustained occupation of an untreated area).
Cross Resistance : Cross resistance occurs when a resistance
mechanism that allows insects to resist one insecticide also
confers resistance to insecticides of the same chemical class or
different chemical class which act on the same target site in the
insect. e.g. kdr resistance to DDT and to Pyrethroids, OPs and
Carbamates due to altered AChE.
Multiple Resistance : It is a common type of resistance when
different resistance mechanisms are present simultaneously
in resistant insects. It may combine to produce resistance to
multiple classes of insecticides.
Genetic Basis of Resistance : Resistance develops in arthropods
after a long period of insecticidal pressure. It is brought about by
the accumulation of the contributing genes through successive
selection with a number of insecticides, each of which confers
some cross-resistance. This is called polygenic or multiplicate
resistance. In contrast, the resistance may be due to a single
gene and bear no similarity to the complexities involved in the
multiplicate resistance. Monogenic resistant strains are more
vulnerable to counter measures such as addition of synergists;
hence the importance of distinction between the two types.
Resistance genes may be dominant, semi-dominant or
recessive; most of the resistance reported so far are either
semi-dominant or recessive and therefore may be managed
effectively as compared to situations when resistance is
genetically dominant.
Resistance Management : The factors that favour development
of resistance are frequency of application, dosage and
persistence of insecticide, rate of reproduction (short lifecycle
and high rate of reproduction) and population isolation.
Resistance management can be attained by rotation of
insecticides, using mixture of insecticides at their optimum
dosage (efficacy & persistence of the two insecticides being
mixed should be broadly similar) and fine scale mosaic (using
two insecticides in different houses in the same village).
Biological Control
Intentional manipulation of populations of living beneficial
organisms, called natural enemies, in order to reduce the
numbers of pests or amount of damage is called Biological
Control.
Natural control strategies that employ biological agents for
pest suppression are classified as biological control tactics. In
conventional usage, this term usually refers to the practice of
rearing and releasing natural enemies like parasites, predators,
or pathogens. Biological control is a particularly appealing pest
control alternative because, unlike most other tactics, it does
not always have to be reapplied each time a pest outbreak
occurs. However, Biological control is not a “quick fix” for
most pest problems. Natural enemies usually take longer to
suppress a pest population than other forms of pest control
and therefore often regarded as a disadvantage or limitation of
biological control. It also may be difficult to “integrate” natural
enemies when pesticides are still in use. Beneficial insects
are often highly sensitive to pesticides and their resurgence
(recovery to pre-spray densities) is usually much slower than
that of pest populations. Rapid pest resurgence often leads to a
vicious cycle of continued chemical usage that prevents natural
enemies from ever becoming re-established.
The biological agents are broadly classified as Predators,
Parasites and Pathogens.
Predators
Predators are insects or other insectivorous animals, each of
which consumes much insect prey during its lifetime. Predators
are often large, active, and/or conspicuous in their behaviour,
and are therefore more readily recognized than are parasites and
pathogens. Most commonly used predators are the larvivorous
fishes for the control of mosquitoes.
Larvivorous Fish : There are areas and habitats where
larvivorous fish, such as Gambusia affinis (Fig. - 22) and Poecilia
reticulata (Fig. - 23), can make considerable contribution to
vector control. The larvivorous efficiency of Gambusia is due
to the fact that a single full grown fish eats about 100 to 300
mosquito larvae per day, is a surface feeder, hence it is suitable
for feeding on both Anophelines and Culicines, is small and
inedible and can tolerate salinity. Poecilia’s larvivorous
efficiency is due to its capability to negotiate margins of ponds
more easily, tolerate handling and transportation very well,
survives and reproduces when introduced into new water
bodies, survives in new places (water bodies) and multiplies
easily and can survive in good numbers for years and does not
require constant care.
Fig. - 22 : Gambusia affinis
Fig. - 23 : Poecilia reticulata

22. • 922 •
Release of fishes is done at the rate of 5-10 fish per linear
meter. If the larval density is high up to 20 fishes can be
released. Fishes should be released in the morning hours or in
the evening.
Criteria for selecting a water body for a fish hatchery are :
It should be a permanent water body.●●
Depth of water should be at least 1.5 metre or more.●●
Water should be confined and without big natural outlet.●●
The minimum size of water body should be at least ●●
5m X 4m. The water body of 10 m X 5 m can support
50,000 fish.
It should be free from other carnivorous fish.●●
Water should not be contaminated by chemical or other ●●
harmful substances.
Easily accessible for daily or periodic inspection and for ●●
collection of fish.
De-weeding in ponds and shallow water bodies and ●●
cleaning of margins should be carried out periodically.
Parasites
Parasites are those organisms which depend on their host for
shelter or food. Many parasites are very specific to the type
of host insect they can attack, and they are not harmful to
humans. Although insect parasites are very common, they
are not well known because of their small size. Some of the
categories of Parasites are as follows :
Nematodes : Nematode Romanomermis culicivorax and
R iyengari have been evaluated and have been found to give
variable control of mosquitoes. The mode of action of the
nematodes is presented in Fig. - 24.
Fig. - 24 : Mode of action of Romanomermis against
mosquito larvae
7days
Eggs Pre-parasiticnematodesinvademosql arvae
Laidinmud
Exitsthrough
cuticle
Kills
larvae
Dropsto
bottom
Moults,mates&f emales
startlayingEggs
Fungi : Fungal agents Lagenidium and Culicinomyces have
shown immense potential as mosquito larval control agents
and can be exploited for use against mosquitoes.
Pathogens
The pathogens which have been found promising are the
Bacterial agents’ Bacillus thuringiensis var israelensis and
Bacillus sphaericus in mosquito larval control. However, these
have been classified as Biocides or Microbial insecticides
(discussed earlier). Viruses like Nuclear Polyhedrosis virus and
Irido virus have also shown promise against mosquito larvae.
Genetic Control
This is defined as “the use of any condition or treatment that
can reduce the reproductive potential of noxious forms by
altering or replacing the hereditary material”. The various
methods of genetic control fall into two general groups : those
leading to population control, reduction or elimination through
the release of partially or completely sterile insects in sufficient
numbers to overcome the reproductive capability of the
natural population; and those leading to population control or
population replacement through the release of partially sterile
or fully fertile genetically altered insects.
New genetic control methods, such as those involving sex
distortion mechanisms or the selection and release of strains
refractory to pathogens, sensitive to selected ecological factors,
or susceptible to insecticides, are being tested under field
conditions. However, unless some new and revolutionary ideas
emerge, the genetic control measures so far known are capable
of achieving only “management” or “manipulation” of insect
population rather than complete suppression or reduction in
densities.
Personal Protective Measures
The role of personal protective measures in arthropod-borne
disease control is to prevent the arthropod vector from biting
and feeding on its host, whether susceptible or already infected,
thereby blocking the chain of transmission of disease from one
host to another. Biting can be prevented either by protective
clothing or chemically by using appropriate repellents.
Protective Clothing
Individual personal protection against bites of arthropods can
be achieved by wearing long trousers, rolled down sleeves of
shirts, socks, shoes and anklets, particularly when going out on
patrols and exercises in areas heavily infested with arthropods.
These measures will vary according to the nature of problem
faced in a particular locality. Personal protective measures have
already been described as part of control measures against
different arthropods elsewhere in this chapter.
Repellents
Insect repellents are chemicals which repel insects when
applied to body surfaces or clothing. The suitability of
substances for use as repellents is dependent primarily on
their inherent repellency and duration of effectiveness. The
important factors are the ease of application on the skin; odour,
appearance or feel on the skin e.g. oily or greasy; the likelihood
of being rubbed off, or absorbed by the skin; irritant effect or
toxicity if absorbed; and its stability under high humidity, high
temperature, rain and perspiration. The efficacy may also be
influenced by the amount of sweating, rubbing and the avidity
of the insect itself. Moreover as is the case with insecticides,
repellents exhibit specificity of action so that some species of
insects are more sensitive to one and some to other repellents.
Common Repellents : The following compounds are among
the most effective when used alone as repellents against
one or more groups of arthropods : benzyl benzoate, DEET
(N, N-diethyl-m-tolumide), dibutyl phthalate, DEPA (di ethyl
phenyl acetamide), and Neem oil. Repellents are formulated as
liquids, gels, creams and in pressurized containers. Some of
the common compounds are discussed below :
(a) N, N-diethyl m-toluamide (DEET) : DEET has been reported
to be an outstanding all-purpose repellent. It provides 6-8 h of
protection against mosquitoes, 2-3 hour against Chrysops, 9
hour against Culicoids. It feels less oily on the skin than the

23. • 923 •
other repellents. DEET can also be used very effectively for
impregnation of clothing. In experiments conducted at AFMC
it has been shown to provide repellence up to two launderings
of the clothing. As a skin application DEET may be used for
protection against mosquitoes, sand flies, fleas and other biting
Diptera. It is a good repellent against all haematophagous
arthropods and also against leeches.
(b) Dibutyl Phthalate (DBP) : It is more persistent but
somewhat less rapid repellent. When smeared on clothing,
its effect lasts up to 2-4 washes, ironing destroys it. It is
specifically useful against ticks and mites as it is an acaricidal,
as well as a repellent. DBP is a good repellent against leeches
and Dimdam flies.
(c) Benzyl Benzoate : The oily liquid has a faintly pleasant
aromatic odour and sharp bitter taste. It has been applied in
5% emulsion to skin as repellent for many arthropods. Clothing
impregnated with benzyl benzoate show repellence to fleas,
chiggers and other arthropods. A mixture of equal parts of
diethyltoluamide and benzyl benzoate with the addition of
an emulisifier acts as a good impregnant for clothing against
trombiculid mites.
(d) Diethyl phenyl acetamide (DEPA) : It is available as cream
and sprays. It is a broad spectrum repellent and can be used for
topical application against mosquitoes, ticks & mites or any
other haematophagous arthropod and leeches. It can also be
applied on clothing/uniform as repellent. It can withstand 2-3
launderings and ironing. It matches DEET in its spectrum and
efficacy.
Application Procedures
Skin Application : Repellents like DEET and DEPA are applied
to the skin as cream formulation. These are generally effective
against such pests as midges, mosquitoes, sandflies and so
on. A good repellent applied in this way gives protection from
insect bites for about five to seven hours.
Impregnation of Clothing : Application to clothing is carried
out when longer protection against insects is required.
Application of DBP, DEET or DEPA to clothing to protect one
self from mites and ticks is much more persistent than skin
treatment and remains effective for a period up to a month.
(i) Hand Application (Repellent Drill) : Hand Application of
repellent is the simplest way to treat the clothing. The fingers
of one hand are dipped into the chemical in an open container
or a few drops of the chemical are poured into one hand, both
the hands are rubbed together and then they are wiped lightly
on the inside and also on the outside of all the openings of all
garments to produce a thin layer of the chemical on them. The
chemical should be applied more particularly to the opening
such as inside the neckband of shirts, turn ups of trousers and
tops of socks turned inside out. 60 ml per man per fortnight of
DBP is enough to impregnate two shirts, two pairs of trousers,
2 pairs of socks, anklets and two sets of underclothing. The
application should be started a fortnight before the mite borne
disease (like scrub typhus) season in any area begins, and
repeated every fortnight thereafter until the season lasts. This
should be done on a parade as a drill supervised by a person who
has had training and experience of the procedure. The repellents
DEET, DEPA require lesser quantities for impregnation.
(ii) Spraying / Impregnation : The chemical can be applied
to the entire clothing by spraying or the clothing can be
impregnated with a solution or emulsion of the repellent when
large quantities of clothing are required to be treated. Clothing
should be soaked in the solution, then wrung out lightly and
dried. DEPA is available as spray formulation for treatment of
clothing.
Barrier Application : Considerable protection may also be
obtained by treating only the openings of the clothes inside
the neckband and cuffs of shirts, inside the waistband, fly and
turn ups of trousers and on the socks above and inside the
shoes and below its tongue. These methods, called the barrier
application, are particularly useful when people go for amateur
camping or trekking or when sufficient supplies of repellents
are not available.
Household Products
Use of mosquito coils, vapourising mats, liquid vapourisers
consisting of pyrethroids is an important measure in reducing
man-vector contact. These products basically aim at deterring
the insects from entering the rooms and with continued contact
bring about death of the exposed insects. The allergic risks
which these products pose on continued exposure especially to
children necessitates adoption of safer preventive measures for
personal protection.
Mosquito Nets
Mosquito nets are very effective means of protection against the
bites of haematophagous arthropods. Untreated or insecticide
treated nets may be used as per the availability. The bednets
may be treated manually or may be purchased as pre-treated
nets.
Manually Treated Bednets : Insecticide treated bednets may be
manually treated with Synthetic pyrethroids like Deltamethrin
2.5% SC or Cyfluthrin 05 EW. These nets have to be treated
every six months as per the procedure described in Box - 1.
Long Lasting Insecticide Nets (LLINs) : The advancement in
the insecticide treated net technology has seen the development
of pretreated or Long Lasting Insecticide Nets (LLIN’s). These
nets may also be treated manually or may be pretreated with
insecticide Permethrin or Deltamethrin (Box 2). The shelf life of
these nets is 5 years.
Mechanical Control & Physical Control
Use of flyswatters, fly traps for housefly control, use of lice
combs, glue traps for various pests like cockroaches and
rodents, other mechanical trapping devices used for other
vectors are few examples of the use of this control option in
vector control.
Physical control measures exploit devices which aim to control
vectors by affecting them physically or their environment.
Sometimes they may be indistinguishable from environmental
or cultural methods. Fly electrocutors and use of temperature
extremes (hot water for bedbug control, sun exposure and
boiling in hot water of infested material for lice control) are
some physical control measures used in public health for
vector/ pest control.

24. • 924 •
Legislative Control
Use of laws and bye-laws to prevent / reduce propagation and
spread of vectors is an effective means of vector management,
although stringent in approach.
Integrated Vector Management
Development of resistance, effects on non-target organisms
and damage to the environment can all be minimized with
selective and judicious use of multi-faceted control tactics
(Fig. - 25). This approach, commonly known as integrated
control, requires an understanding of ecological principles as
well as a thorough knowledge of the pest’s life history and
population dynamics. Today, integrated pest control forms the
foundation of Integrated Vector Management programs (IVM)
that take a comprehensive and multi-disciplinary approach to
solving pest problems. These programs emphasize management
rather than eradication. They take a broad ecological approach
to pest problems, focusing on all members of a pest complex in
an effort to identify the optimum combination of control tactics
that will reduce vector populations below economic thresholds
and maintain these levels with the least possible impact on the
rest of the environment.
IVM is a dynamic approach which requires a broad knowledge
of vector biology, ecology and behaviour on the one hand
and that of system analysis approach on the other so that a
variety of control measures, such as environmental, chemical,
biological, genetic and personal protective measures, can
be integrated with a view to achieve the ultimate aim of
combating human disease. Chemical and biological methods
may provide temporary control of vectors but implementation
Box - 1 : Steps for treatment of bednets
(a) Measure the total area of the net in m
2
.
(2 x length x height + 2 breadth x height + length x breadth) in metres.
Average area of a single net is 10 sq m.
(b) Measure the absorption capacity
Measure 1 litre of water and take it in a tub.●●
Immerse the dry net, when completely wet, take it out by gently wringing the net to prevent dripping of water. ●●
Measure the remaining water in the tub. 1 litre - the remaining water gives us the absorption capacity of the net.●●
(c) Wash the net to be treated and dry it.
(d) Calculate the dosage of the insecticide required
Deltamethrin 2.5% SC- dosage required 25 mg a.i. per m●●
2
. 1 ml of the 2.5% insecticide contains 25 mg; therefore the dosage
will be 1ml per m
2
to give the dosage of 25 mg a.i./m
2
. So if the net is of 10 m
2
and the absorption capacity is say 500 ml
then we need to add 10 ml of insecticide in 500 ml of water to give the reqd. dosage.
Cyfluthrin 05% EW- dosages required 50 mg a.i. / m●●
2
. 1 ml of the 05% insecticide contains 50 mg; therefore the dosage will
be 1ml per m
2
to give the dosage of 50 mg a.i. /m
2
. So if the net is of 10 m
2
and the absorption capacity is say 500 ml then
we need to add 10 ml of insecticide in 500 ml of water to give the reqd. dosage.
(e) Put the net in the insecticide solution prepared as per the procedure given above and knead it well to ensure the net is
completely soaked in soln.
(f) Take out the net and spread it in shade, once semi dry it can be hung for drying.
Box - 2 : Long Lasting Insecticide Nets (LLIN’s)
A Long lasting insecticide net is a treated net which can withstand insecticidal efficacy up to at least 20 washes with more than
80% mortality after 24 hrs (95% Knockdown after 60 min).
The treatment is done by either incorporating Insecticide
In the yarn itself which is made into textile and then used for making nets e.g. ●● Olyset Net.
It is Permethrin impregnated (500 mg a.i./m2 net) Polyethylene net.--
Washing cleans the surface and hence it is recommended to be heated up (i.e. sun dried) after washing for the active --
ingredient to “migrate ” to the surface.
OR
By coating around fibres, e.g. ●● Perma Net.
It is a regular polyester net treated with 55 mg a.i./ m--
2
Deltamethrin.
Resists multiple washes.--
Biological activity lasts as long as the net itself (3 to 4 years for polyester nets, 4 to 5 years for polyethylene ones).--
Manually treated LLIN
Any untreated net can be treated with long lasting formulation of Deltamethrin using K-O Tab. ●●
Low cost as compared to pretreated nets.●●

25. • 925 •
of environmental control measures leads to permanent control.
In this approach, initial costs may be high and programmes
may require years for implementation, but authorities at all
levels should be advised to include environmental changes
and improvements relating to vector control in all long
term planning. However, these methods require elaborate
organization, longer time and liberal finances. Species control
and vector control are the two modifications circumscribing the
wider concept of vector control.
Fig. - 25 : Integrated Vector Management
Future Policy
The aim of future vector control by use of insecticides should
be to reduce the intensity of chemical selection by reducing the
frequency and coverage of insecticide sprays in public health
programmes, minimizing the agricultural use of persistent
chemicals as far as possible and by supplementing the chemical
control methods by other methods whenever feasible. There
is a need to strengthen existing surveillance methods and
incorporating the benefits of the newer methods like Remote
sensing, Geographical Information System, Global Positioning
System etc. whenever and wherever, feasible. There is a
continued effort to evolve safer alternatives for vector control
coupled with intensive research using molecular biology tools
for production of Genetically Modified Vectors (GMV) to address
the problems of vector control.
Summary
Arthropod control is one of the key strategies in the management
of vector borne diseases. This requires a sound knowledge of
bionomics, insecticide susceptibility & role in arthropod borne
disease transmission. The various control options available
are environmental, biological, chemical, personal protective
measures, mechanical, physical, genetic & legislative control.
Environmental control consists of a naturalistic approach with
the aim of minimizing vector propagation & reducing man-
vector-pathogen contact. Environmental modification consists
of transformation that is permanent like drainage, filling
and velocity alteration. Environmental manipulation aims at
producing temporary conditions unfavourable to the breeding
of vectors in their habitats like water salinity changes, stream
flushing etc.
Chemical control began with the discovery of the insecticidal
value of Dichloro Diphenyl Trichloro ethane (DDT). Insecticides
form a part of extensive countrywide programmes for control
& eradication of diseases and are broadly divided into natural
& synthetic. Natural insecticides can be plant & mineral based
whereas Synthetic insecticides can be organic or inorganic.
The organic Insecticides fall into four major groups’ viz.
Organochlorines, Organophosphates, Carbamates and Synthetic
pyrethroids. Organochlorine Compounds are contact poisons
and act on the nervous system, the only member of this group
used in Public Health is DDT.
Currently DDT is being used only in North Eastern states of
India for Indoor Residual Spray. A deposit of 1 g a.i of DDT/m
2

of surface area of walls and ceilings up to a height of 3.5 m in
all dwellings applied at 8 weeks’ interval effectively controls
majority of the mosquitoes and also other arthropods resting
on the treated wall. Organophosphorus Compounds - insects
which have become resistant to Organochlorines are still
susceptible to the members of this group. Some of the common
compounds are Malathion, Temephos, Fenthion, Dichlorovos
(DDVP) and Fenitrothion. Malathion is one of the least toxic
Organophosphorus compounds. Malathion is a broad spectrum
insecticide with efficacy against a large number of pests
ranging from mosquitoes, houseflies, cockroaches, bedbugs,
lice etc. Malathion (25% WP), under the National Vector Borne
Diseases Control Programme is being used as Indoor Residual
Spray against mosquitoes in areas where the vectors have
become resistant to DDT. As ULV spray, Malathion has been
very widely used during outbreaks of Dengue and JE as an
anti adult mosquito measure. Temephos (Abate) is the only
insecticide approved for use in potable water (20ml a.i/sqm).
Fenthion (Baytex) 82.5% EC is a good mosquito larvicide but
cannot be used in potable water bodies. It is highly effective as
a larvicide against Culex quinquefasciatus. Dichloro-dimethyl-
dichlorvinyl-phosphate (DDVP or Dichlorvos) produces fatal
insecticidal vapour and can be combined with solid substances
like wax and used as tablets or bricks. It is one of the common
insecticides used for disinfecting aircraft and is also an
effective housefly larvicide. Carbamates are derivatives of
carbamic acid; some of the compounds in common use are
Propoxur, Carbaryl and Bendiocarb. Synthetic Pyrethroids
are broad spectrum, highly potent, have quick knock down
action and long residual life. Synthetic pyrethroids are many
times more effective & safer than the previously available
insecticides and include insecticides like Permethrin, Allethrin,
Phenothrin, Cypermethrin, Cyfluthrin, Deltamethrin, Bifenthrin
etc. Deltamethrin, Cyfluthrin are used for bednet treatment &
routine household pest control activity.
Equipment for vector control can be broadly classified as
ground equipment and equipment used for aerial applications.
Commonly used for spraying various insecticidal formulations
are the hand operated sprayers, power operated sprayers,
aerosol dispensers, fog generators and dusters. Residual
Spraying is the application of insecticides to surfaces so that the
insecticide particles remain on the surface in the form, size and
quantity suitable for insects to pick up on contact and sufficient
to exert a lethal effect over a long period. Organochlorine,
Organophosphorus, Synthetic pyrethroids and Carbamate
compounds can thus be applied on the inside walls of houses
and also on thick bushes. Before starting a spray operation, the

26. • 926 •
equipment must be checked; the average discharge of an 8002
nozzle is about 750 ml per minute. Spraying in a room should
commence from the backside of a door clockwise completing the
plain surfaces of walls to the crevices on the walls and inside
portion of windows etc. The pillars, under surfaces of furniture
and lastly the ceilings should be taken for spray. Spray is done
from roof to floor, using downward motion, to complete one
swath; then stepping sideways and spraying upwards from
floor to roof. Spray is applied in vertical swaths 52-56 cm wide.
Swaths should overlap by 5 cm.
Space Spraying is an ideal method for bringing about rapid
control of vectors in emergency or epidemic situations and
may also be used for seasonal control of flying insects, pests
or vectors. Space sprays are applied mainly as thermal fogs or
cold fogs. Thermal fog is produced by special devices known as
thermal foggers that use heat to break up the chemical into very
small droplets (usually in 5-30 micron diameter range) which
then disperse in the air. The insecticide used in thermal fogs is
diluted in a carrier liquid, which is usually oil-based. Cold fog
is produced by a special device which breaks up the chemical
into microscopic droplets by mechanical means basically with
a high-pressure pump and an extremely fine nozzle. The spray
droplets are generated without any external heat. With cold
fogs, the volume of spray is kept to a minimum. Ultra-low-
volume insecticide formulations are commonly used for such
applications. The cold fogger may dispense formulations in a
very concentrated form and generate the droplets (usually in
the 5-30 micron diameter range) in a precise manner. However,
its ability to penetrate dense foliage or obstacles is not as good
as that of thermal fogging. Cold fogging is sometimes called
Ultra Low Volume (ULV) treatment as it allows the utilization
of only a very small amount of chemical for coverage of a
large area. Space-spraying formulations are generally oil-
based. Diesel is used as a carrier for thermal fogging, but
creates a thick smoke and oily deposits, which may lead to
public rejection. Insecticides used for fogging are Pyrethrum
2% Extract, EC, Malathion Tech, Deltamethrin 1.25 ULV. The
lower the concentration of active ingredient, the larger the
number of droplets of a given size required to achieve a lethal
dose. Suppression of vector populations over large areas can be
carried out using space sprays released from aircraft, especially
over areas where access with ground equipment is difficult and
extensive areas need to be treated very rapidly. Area Spraying
is carried out for treatment of land against mites and ticks and
also as an anti-larval measure over vast water surfaces.
Biological Control is the manipulation of populations of living
beneficial organisms in order to reduce the numbers of pests
or amount of damage. The biological agents are broadly
classified as Predators, Parasites and Pathogens. Predators are
often large, active, and/or conspicuous in their behaviour e.g.
larvivorous fishes for the control of mosquitoes like Gambusia
affinis. Parasites like Nematodes - Romanomermis culicivorax
and R iyengari have been evaluated and have been found to
give variable control of mosquitoes. Fungi Lagenidium and
Culicinomyces have shown immense potential as mosquito
larval control agents and can be exploited for use against
mosquitoes. Pathogens - Bacillus sphaericus and Bacillus
thuringiensis var israelensis which acts via delta endotoxin are
promising control options. Bti 12 AS is used @ 20ml/m
2
.
Genetic Control is the” use of any condition or treatment that
can reduce the reproductive potential of noxious forms by
altering or replacing the hereditary material”.
Personal protective measures to prevent the arthropod vectors
from biting and feeding on their host can be achieved by
use of mosquito nets, wearing of long trousers, rolled down
sleeves of shirts, socks, shoes and anklets (particularly when
going out and in areas heavily infested with arthropods) and
use of repellents by application on body surfaces or clothing
(are effective in reducing man-vector contact). The commonly
used repellents are benzyl benzoate, N, N-diethyl-m-tolumide
(DEET), Dibutyl phthalate (DBP), Diethyl phenyl acetamide
(DEPA) and Neem oil.
The development of resistance, effects on non-target organisms
and damage to the environment can all be minimized with
selective and judicious use of multi-faceted control tactics.
Integrated Vector Management is a dynamic approach which
requires a broad knowledge of vector biology, ecology and
behaviour on one hand and that of system analysis approach
on the other so that a variety of control measures such as
environmental, chemical, biological, genetic and personal
protective measures can be integrated with a view to achieve
the ultimate aim of combating human disease.
Study Exercises
Short Notes : (1) Environmental Management (2) Biological
control (3) Biorational insecticides (4) IRS
MCQs & Exercises
1. Organophosporus insecticides include all except
(a) Temephos (b) Malathion (c) Propoxur (d) Dichlorvos
2. Deltamethrin belongs to which group of insecticides
(a) Carbamates (b) Synthetic Pyrethroids
(c) Organophosphates (d) Organochlorides.
3. Which of the following is a chitin synthesis inhibitor :
(a) Imidacloprid (b) Novaluron (c) Fenoxycarb
(d) Cyfluthrin.
4. Maximum amount of pesticide on surface is available for
killing pests in this formulation (a) Dusts (b) Granules
(c) Wettable powder (d) Liquid formulation.
5. Hand operated equipment for vector control does not
include : (a) ULV fogging machine (b) Hand sprayer
(c) Knapsack sprayer (d) Compression pneumatic sprayer
6. Commonest type of equipment used in National
Vector Borne Disease Control Programme (a) Power
operated sprayer (b) Hand sprayer (c) Knapsack sprayer
(d) Compression pneumatic sprayer
7. Insecticide not used for bed net treatment : ( a) Deltamethrin
(b) Permethrin (c) Cyfluthrin (d) Allethrin
8. Space spraying is used as method of bringing down
rapid control of vector in (a) Emergency (b) Epidemic
(c) Seasonal control of flying insects (d) All.
9. Which of these is not true for thermal fogging
(a) easily visible fog (b) Low conc.of active ingredient
(c) Pyrethrum2% & Malathion tech used (d) Mostly water
diluted formulations used.

27. • 927 •
10. Predators used in biological control are all except
(a) Poecelia reticulata (b) Gambusia affinis (c) Bacillus
sphaericus (d) Gaurami
11. Match the Following :
1. Pyrethrum (a) Malathion
2. DDT (b) Temephos
3. Broad spectrum
insecticide used in IRS
where resistance to
DDT has occurred
(c) Available as 2% Extract.
Require 20 times dilution
to get 0.1%solution. Has
rapid knockdown action.
4. Available as 50% EC.
Used in potable water
(d) Used as IRS in the
dose of 1g/m
2
of surface
area of walls & ceilings
up to a height of 3.5m.
5. Mosquito larvicide,
cannot be used in potable
water bodies, highly
effective against Culex
quinquefasciatus
(e) Fenthion
6. Effective housefly
larvicide, available as 76%EC
& in aerosol formulations.
(f) Propoxur
7. Flushing out effect makes
it useful for cockroach
&bed bug control
(g) Available as 2.5%SC,
2.5%WP & 1.25 ULV
in space sprays
8. Deltamethrin (h) DDVP
9. Cannot be used in potable
water , available as 12AS
(i) Available as 2.15% gel
for use against cockroaches
& bait against houseflies
10. Imidacloprid (j) Bti
Fill in the Blanks :
12) Residual insecticidal property of DDT was discovered
by __________________
13) Mechanism of action of Pyrethrum __________________
14) __________________ effect is unique action of Fipronil.
15) ______________ powder & ______________ tablet is used
for rodent control.
16) Average discharge of an 8002 nozzle is ______________
17) Mechanism of action of Bti is by the action of
______________
Answers : (1) c; (2) b; (3) b; (4) c; (5) a; (6) d; (7) d; (8) d;
(9) d; (10) c; (11) 1-c; 2-d; 3-a; 4-b; 5-e;6-h;7-f;8-g;9-j;10-i;
(12) Paul Muller; (13) Rapid knockdown; (14) Cascade
(15) Calcium cyanide & Aluminium phosphide (16) 750ml/ min
(17) Delta-endotoxin
Further Suggested reading
WHO. Manual on Environmental management for mosquito control with 1.
special emphasis on malaria vectors. WHO offset Publications. WHO,
Geneva, 1982; 66 : 1-238.
WHO. Environmental management for vector control. Third report of the 2.
WHO Expert Committee on Vector Biology and Control. Technical Report
Series. WHO, Geneva, 1980; 649 : 1-75.
Sharma VP. Environmental management in malaria control in India. In : 3.
Targett GAT, editors. Malaria - waiting for the vaccine. John Wiley publishers,
USA 1991 : 49- 66.
Rozendaal Jan A. Vector control : Methods for use by individuals and 4.
communities. Geneva : World Health Organization, 1997.
Curtis CF, editor. Control of Disease Vectors in the Community. 1st ed. London: 5.
Wolfe, 1991.
Prevention &management of insecticide resistance in vectors and pests 6.
of public health importance. Manual by Insecticide Resistance Action
Committee (IRAC).
WHO. Chemical Methods for the control of Arthropod Vectors and Pests of 7.
Public Health Importance. Geneva : World Health Organization, 1984.
World Health Organization. Safe use of Pesticides. Tech Rep Ser no 513. 8.
WHO, Geneva 1973.
WHO. Implementation of global malaria control strategy. Technical Report 9.
Series. WHO, Geneva, 1993; 839 : 1-57.
WHO. A global strategy for malaria control. Geneva : World Health 10.
Organization, 1993.
Matthews GA. Pesticide application methods. 3rd ed. Blackwell Science, 11.
2002.
Reiter P, Nathan MB. Guidelines for assessing the efficacy of insecticidal 12.
space spray for control of the dengue vector Aedes aegypti. Geneva, World
Health Organization (document WHO/CDS/CPE/PVC/2001.1), 2001.
Najera J, Zaim M. Malaria vector control : decision making criteria 13.
and procedures for judicious use of insecticides. Geneva, World Health
Organization (document WHO/CDS/WHOPES/2002.5), 2002.
WHO. Equipment for vector control, 3rd ed. Geneva, World Health 14.
Organization, 1990.
156Housefly
Rina Tilak
Houseflies live in close association with man. Despite the best
and most extensive efforts taken to control it, housefly control
has remained a challenge. The important genera include
Musca, Fannia and the biting flies, Stomoxys, Sarcophaga and
the various blowflies viz. Chrysomya, Calliphora and Lucilia .
However, the most abundant and widely distributed is the
Housefly.
Classification and Distribution
The houseflies have a world wide distribution with about 1700
genera under family Muscidae, order Diptera to which the
medically important flies - houseflies and stable flies belong.
There are 70 species of flies in genus Musca, of which Musca
domestica (common housefly) and Musca sorbens (bazaar
fly) are abundantly encountered in tropical and subtropical
countries.

28. • 928 •
Morphology
Adult houseflies are 6-9 mm in length, greyish in colour with
4 distinct longitudinal black stripes on the thorax. The head
bears a pair of compound eyes which are close together in
males but are widely separated in females. The mouth parts,
collectively known as the proboscis are capable of considerable
extension and retraction and are adapted for lapping-sponging.
The antennae generally remain hidden in the antennal groove
on the head. The thorax bears four narrow black stripes, a
pair of clear transparent wings and a pair of halteres (knob
like structure behind the wing), three pairs of legs - each
terminating in five segments of tarsus; the last segment bears
a pair of claws and pair of pad-like ‘pulvilli’ provided with a
large number of glandular hairs. These glandular hairs secrete
a substance which keeps the pads wet and sticky and enables
the flies to cling to vertical and smooth surfaces. The abdomen
is short, broadly oval with five visible segments (Fig. - 1).
Fig. - 1 : Musca domestica
Life History
The life cycle of housefly undergoes complete metamorphosis
with egg, larva (maggot), pupa and adult stages (Fig 2).
Fig 2 : House fly - Life cycle
8-24hrs
2-7days
3-6days
2-5days
Larva
Pupa
Adult
Egg
Houseflies breed in decaying organic matter of animal or plant
origin. The eggs are pearly white, oval in shape and measure
about 1 mm in length. They are laid in cracks & crevices in
moist manure heaps or any decaying animal or vegetable
matter. A female fly may lay 300 to 900 or more eggs in 5 to
6 batches during her lifetime. In summer, the eggs hatch after
8-12 h, whereas in winter it may take 2 to 3 days. There are
three larval stages or instars in the life of a fly. The larvae
are photophobic and thus are found in the deeper layers of the
manure. After 3-5 days, the third stage larva moves from deep
moist burrows to the neighbouring dry soil and contracts to
form a dark brown barrel shaped pupa about 6 mm in length.
The pupa neither feeds nor grows. Within 2-5 days, the adult
fly emerges out of the pupal case. Under favourable conditions
of temperature and food supply, the whole life cycle from egg
to adult may be completed in about less than a week’s time.
During winter it may take as many as 20 to 22 days. The length
of life is usually 2-3 weeks but in cooler conditions it may be as
long as three months.
Bionomics
The housefly has a remarkable capacity to reproduce. It is
estimated that at an average of 7 day’s developmental cycle
for each generation, one female housefly, laying about 120
eggs, could produce a progeny of 5,598,72000,000 adult flies
by the end of 5 months in summer. However, nature does not
allow this as a large number of eggs, larvae and pupae are lost
due to desiccation, starvation, predators and adverse climatic
conditions. A high percentage of flies remain near the breeding
places. Depending on the prevailing wind and availability of
food, some of them may migrate up to 20 km from breeding
places (however most stay within 1-2 km of the larval habitat
if sufficient food is available). High temperature is lethal to
larvae and so the heat generated in tightly packed manure
heap quickly kills them. The adult houseflies are attracted to
light. The housefly is omnivorous and a voracious feeder; it is
particularly partial to faecal matter, sputum, discharges from
wounds and open sores. It is also easily attracted to sugars,
milk and other articles of food meant for human consumption.
The solid or semi-solid foods are softened by extrusion of a
vomit drop and then sucked up. Well fed fly defecates every 5
min, particularly while feeding, and vomits every 2-3 minutes.
Water is an essential part of a fly’s diet and flies do not
ordinarily live more than 48 hours without access to it.
Ecology of Adult Flies
An understanding of the ecology of flies is essential for
effective management of houseflies. Flies are mainly active
during daytime when they feed and mate. They are generally
found resting indoors on floors, on the ground and walls at
less than 5 feet from the ground; whereas outdoors they rest on
fences, walls, steps, pit latrines, clothes lines, grasses, weeds
and garbage cans. At night, flies rest above 5 feet height on
walls, on hanging wires and vertically suspended articles and
ceilings, usually close to their daytime food sources.
Vector Potential
Immediately after visiting a dirty place, the fly may rest on any
foodstuff or drink meant for human consumption or an exposed
part of body e.g. mouth, eyes or a wound, and deposit the

29. • 929 •
disease producing organisms. The housefly is thus a mechanical
carrier of the causative organisms of diarrhoeas, dysenteries,
gastroenteritis, cholera, enteric group of fevers, intestinal
worms, poliomyelitis, viral hepatitis A, other entero- viruses,
trachoma, conjunctivitis, anthrax, yaws and tuberculosis. At
times, the housefly may cause conditions known as internal
and external myiasis, in which the flies breed in sloughing
wound, intestinal contents and suppurating cavities.
Fly Control
(a) Environmental Control : The best method of control of
houseflies is to eliminate their breeding places and to maintain
a high standard of environmental sanitation, especially by
proper disposal of human and animal excreta, swill, garbage
and all other decaying organic rubbish, offal and carcasses.
Access of flies to faeces should be prevented by fly proofing
the latrines and latrine pans and prompt removal of faeces.
Their access to food is prevented by fly-proofing cook houses
and messing blocks and by use of fly-proof cupboards and
containers. The doors of all entrances and windows should
open outwards and preferably should have vacuum levers
especially in cookhouses. Constant vigilance is necessary to
destroy all flies that gain entrance, otherwise the fly-proofed
rooms become large fly-traps. In pantries and mess rooms,
fly-proof cupboards for food storage and wire gauze, weighted
with beads afford protection to food in jugs or bowls but their
repair and cleanliness requires constant supervision. When
the table is being laid, cups should be inverted in saucers and
bowls should be kept either upside down or under cover when
not in use.
(b) Insecticidal Control (Box - 1)
(i) Space Spray : For immediate destruction of flies and
especially for prevention of fly borne diseases, pyrethrum
(0.1%) spray is useful, mainly in cook houses and dining rooms
before meal times and in canteens. Certain combinations of
space sprays containing Pyrethrum or Synthetic pyrethroids
and/or Organophosphorus/Carbamate compounds are available
commercially. ULV spray in large areas may also prove effective
in controlling houseflies.
(ii) Baits : Propoxur baits have been in use since long for fly
control. Recent introduction in this concept is Imidacloprid
baits containing Imidacloprid as the toxicant with Pheromone
- Muscalure, which helps in attracting the flies to the bait. This
bait has been found to be effective for use in areas with low to
moderate fly infestation. However, while using these baits in
cookhouses/ dining areas, care should be taken that they are
not placed close to cooking or serving place.
(iii) Cord and Ribbons : During the night, houseflies prefer
to rest on strings and hanging wires or any object; this fact
is utilized for killing them by use of insecticide treated cords
and strips which are hung from ceilings in kitchens, dining
halls, store rooms, dairy farms and poultry houses to provide
effective control during the fly season. Dark coloured material
is preferred for treatment @ 1m cord or strip for each square
metre of floor space. The period of effectiveness ranges from
1 to 6 months. For this any insecticide with high vapour
pressure and quick knock down effect should be used. These
treated materials should not be hung over food containers,
water containers/ troughs or within reach of animals or pets.
Curtains treated with Synthetic pyrethroids will be of additional
benefit.
(iv) Larvicides : Insecticides such as DDVP (2%), Fenthion (4%)
have been used as larvicides to control fly breeding but the
use of larvicides may favour the development of resistance,
the choice should therefore be made carefully. Insecticides
like Dimilin (IGR) may be used to retard development of
resistance. Larvicides should be applied at a rate sufficient to
wet the upper 10-15 cm of the breeding medium thoroughly
i.e. 0.5 - 5 l/m
2
.
Box - 1 : Newer Insecticides Against Houseflies
Insecticide
Formu-
lation
Dilution
Area to be
sprayed
Cyphenothrin 5%EC
10 ml +
990 ml
of water
500 ml/sq m
Diflubenzuron
2.5%
WP
8 g in 1
litre water
500 ml/sq m
Diflubenzuron 5% G As it is50 g/sq m
Imidacloprid
bait
0.5% As it is5 g / bait station
Propoxur bait 2% As it is
Moisten and keep
in bait stations
(v) Paints : The concept of using insecticidal paint for housefly
is catching up. Imidacloprid baits wetted with water may be
used as paint on housefly resting places.
(vi) Residual Spray : The housefly has developed resistance
to most of the Organochlorine as well as Organophosphorus
and Carbamate group of insecticides routinely used in public
health. Residual sprays are ideally not recommended for fly
control.
(c) Mechanical Control
(i) Fly Traps : Various types of fly traps such as the cage trap
and the kerosene tin trap were used in the past with fairly good
results. These are no longer in use because of the availability
of more potent and convenient methods. Newer mechanical fly
catching devices have been developed which have bags with
attractants inside, which attract the houseflies and on entry
inside the bag, they get trapped and eventually are killed.
(ii) Swatting : It is used in situations where infestation is so
low that routine fly control measures are either not indicated
or feasible. However, it is important to remember that fly
population of a cook house or dining room cannot be greatly
reduced by persistent swatting. A good swat is the one, which
is resistant enough to affect a rapid hit. The flaps should be
perforated and washable.
(iii) Fly Paper : Commercially available fly papers may be used
or alternately sticky fly papers (Fig. - 3) can be prepared by
mixing 8 parts of powdered resin and 5 parts by weight of crude
castor oil and heating the same in a water bath while stirring
constantly. The paste mixture is spread on glazed paper. The
latter can be prepared by coating an ordinary paper with a hot

30. • 930 •
solution of 1 g of glue in 3ml of water and allowing it to dry.
The fly papers do not give lasting results and hence are not
much in use for control purpose. They are however considered
an effective method for monitoring fly density.
Fig. - 3 : Fly paper
(d) Physical Control : Use of light traps (electrocutors) is very
useful in the dining areas & other public eating places. The
light traps should be placed away from dining tables & food.
Summary
Houseflies live in close association with man. The most
abundant and widely distributed is the Housefly - Musca
domestica. Adult houseflies are 6-9 mm in length, greyish in
colour with 4 distinct longitudinal black stripes on the thorax.
The head bears a pair of compound eyes. The thorax bears a
pair of clear transparent wings, three pairs of legs - the last
segment of the tarsus bears a pair of claws and a pair of
pad-like ‘pulvilli’ provided with a large number of glandular
hairs. These secrete a substance which keeps the pads wet
and sticky for clinging to vertical and smooth surfaces. The
life cycle of housefly undergoes complete metamorphosis
with egg, larva, pupa and adult stages. Houseflies breed in
decaying organic matter of animal or plant origin, eggs are laid
in cracks & crevices in moist manure heaps or any decaying
animal or vegetable matter. The larvae are photophobic and
thus, are found in the deeper layers of the manure. After 3-5
days, the third stage larva moves from deep moist burrows to
the neighbouring dry soil and contracts to form a dark brown
barrel shaped pupa. Within 2-5 days, the adult fly emerges out
of the pupal case. Under favourable conditions of temperature
and food supply, the whole life cycle from egg to adult may be
completed in about less than a week’s time. During winter it
may take as many as 20 to 22 days.
The housefly has a remarkable capacity to reproduce. High
temperature is lethal to larvae and so the heat generated in
tightly packed manure heap quickly kills them. The adult
houseflies are attracted to light. The housefly is omnivorous
and a voracious feeder. It is particularly partial to faecal
matter, sputum, discharges from wounds and open sores. The
housefly is a mechanical carrier of the causative organisms of
diarrhoeas, dysenteries, gastroenteritis, cholera, enteric group
of fevers, intestinal worms, poliomyelitis, viral hepatitis A,
other entero- viruses, trachoma, conjunctivitis, anthrax, yaws
and tuberculosis.
The best control of houseflies is to eliminate their breeding
places and to maintain a high standard of environmental
sanitation, especially by proper disposal of human and animal
excreta. Access of flies to faeces should be prevented by fly
proofing the latrines and latrine pans and prompt removal of
faeces. Their access to food is prevented by fly-proofing cook
houses and messing blocks and by use of fly-proof cupboards
and containers. The doors of all entrances and windows
should open outwards and preferably should have vacuum
levers. Insecticidal Control : Space Spray of pyrethrum (0.1%)
useful for immediate destruction of flies & for prevention of
fly borne diseases. Propoxur baits have been in use since long
for fly control. Recent introduction is Imidacloprid baits. Use
of insecticide treated cords and strips provide effective control
during the fly season. Other useful insecticides are DDVP (2%),
Fenthion (4%), Dimilin etc. Mechanical fly catching devices
have been developed which have bags with attractants inside,
which attract the houseflies and on entry inside the bag, they
get trapped and eventually are killed. Swatting is used in
situations where infestation is so low that routine fly control
measures are either not indicated or feasible. Commercially
available fly papers may be used or alternately sticky fly papers
can be prepared. Use of light traps (electrocutors) is very useful
in the dining areas & other public eating places, however, they
should be placed away from dining tables & food.
Study Exercises
MCQs
1) Housefly transmits all except (a) Poliomyelitis
(b) Hepatitis B (c) Tuberculosis (d) Intestinal worms.
2) Environmental control measures for housefly control do
not include (a) Fly proofing (b) Pyrethrum spray (c) Proper
garbage disposal (d) Vacuum levers in doors.
3) Anti adult control measure is (a) Fenthion (b) DDVP
(c) Dimilin (d) Imidacloprid bait.
Fill in the Blanks
4) Size of a housefly is ______________
5) ______________ secrete a substance which keeps pads wet
& sticky for clinging to vertical &smooth surfaces.
6) Larvae of housefly are ______________
Answers : (1) b; (2) b; (3) d; (4) 6-9mm; (5) Pulvilli;
(6) Photophobic.
Further Suggested Reading
Keiding J. The housefly biology and control. Training and information guide. 1.
Geneva : World Health Organization, 1986.
Zurek L, Denning SS, Schal C, Watson DW. Vector competence of Musca 2.
domestica (Diptera : Muscidae) for Yersinia pseudotuberculosis. J Med
Entomol 2001; 38 : 333-5.
Fotedar R. Vector potential of houseflies (Musca domestica) in the 3.
transmission of Vibrio cholerae in India. Acta Trop 2001; 78 : 31-4.
Nayduch D, Noblet GP, Stutzenberger FJ. Vector potential of houseflies for the 4.
bacterium Aeromonas caviae. Med Vet Entomol 2002; 16 : 193-8.

31. • 931 •
157Mosquitoes
Rina Tilak
Mosquitoes qualify to be rated as one of the most important
vectors, the world over, from amongst the numerous species
of blood sucking arthropods, due to the sheer magnitude of
morbidity and mortality caused by them. The mosquitoes belong
to phylum Arthropoda, class Insecta, order Diptera and family
Culicidae. The family Culicidae is divided into sub-families
Culicinae, the Chaoborinae and the Dixinae. Of these, only the
sub-family Culicinae, which comprises all the true mosquitoes,
is of medical importance. Amongst the mosquito genera, only
Anopheles, Culex, Aedes and Mansonia are of importance in
India. The mosquitoes are further classified as ‘Anophelines’,
which comprises only one important genera - Anopheles and
‘Culicines’ comprising three important genera viz. Culex, Aedes
and Mansonia.
Distribution
Mosquitoes have a worldwide distribution, being found in the
tropics, temperate zones and also in the arctic circles. They
have even been found breeding in underground tunnels, deep
mines and at altitudes as high as 4000 m above sea level.
Morphology
Mosquitoes are about a centimetre long and greyish black
in colour. The division of the body into the head, thorax
and abdomen is sharply defined. The head bears two large
compound eyes, a pair of antennae and the mouthparts which
are collectively called ‘proboscis’. The mandibles and maxillae
of only the female are developed for cutting the human skin
and therefore only the female mosquitoes can suck blood and
transmit diseases.
Sexual dimorphism is clearly seen in mosquitoes; males can be
identified by their antennae, which are densely haired and look
like moustache, whereas, in females the antennae is sparsely
haired.
The palps are also helpful in identification of males and females
of Anophelines and Culicines; In Anopheles male, the palpi are
long and club shaped at the termination; in the females, they
are as long as proboscis and are straight. In Culicines, the
male palpi are long and tapering and deflected out, whereas
in females, the palpi are much shorter than proboscis and
budlike.
The thorax bears a pair of wings and three pairs of legs. The
thorax of Culicines is humped in all the three genera, giving them
a hunchback appearance while resting; whereas, Anophelines
rest with their head, thorax and abdomen in the same line and
forming an angle of 45° with the surface, with the exception of
Anopheles culicifacies which rests like the Culicines i.e. with
the body parallel to the surface. The abdomen of mosquitoes
consists of 10 segments of which 7 or 8 are clearly marked
out and the terminal ones form the male and female external
genitalia.
Life History
Mosquitoes undergo complete metamorphosis through the
stages of egg, larva, pupa and adult. Water is required for egg
laying with variations existing among genera and species for
the type of water desired by them. The number of eggs laid
at each oviposition varies between 50 and 150. Anophelines
prefer to lay their eggs singly in clean water collections and
have lateral floats; Culex sp prefers dirty/ polluted water and
the eggs are deposited as raft; Aedes sp prefers to lay eggs
singly on some substratum/ debris in containers (natural or
artificial), whereas Mansonia sp breeds in water bodies with
aquatic vegetation namely Pistia and lay their eggs on the
under surface of the leaves in star shaped clusters. The eggs
hatch into larvae in one to two days, but in cold weather, the
hatching may be delayed. Mosquito larvae feed voraciously
on micro-organisms, water algae or other organic matter and
breathe through spiracles. Larvae pass through four stages or
“instars’ in five days depending on the species, the temperature
of the water and availability of food supply. In Anophelines, the
larvae rests parallel to the surface and have palmate hair on
the dorsal surface of the abdomen and do not possess siphon
tube, whereas in case of Culicines, the larvae rest at an angle
with the head downwards and possess a single siphon tube. At
the end of the fourth instar, the fully grown larva casts its skin
and becomes a comma shaped ‘pupa’, which is a motile but
non-feeding stage and has two respiratory trumpets originating
from the cephalothoracic area. During this stage, it undergoes
transformation to the adult usually within 1-2 days. The adult
mosquito wriggles out of the pupal skin through a ‘T’ shaped
slit and balances itself on the water surface or some near by
floating object until its wings are dry and then flies off. The
total duration of the life cycle varies between seven days to one
month. The life span of adult mosquitoes is up to a maximum
of 6 months in the temperate zones, but in the tropics they
seldom survive for more than a month.
Bionomics
The females of all the medically important mosquitoes are
normally bloodsuckers, as they require a blood meal for
maturation of eggs. Females are fertilized during swarming
(nuptial dance) at dusk. The source of the blood meal varies
with the species. Those feeding on human blood are called
anthropophilic and those feeding on animals are called
zoophilic. They are attracted by the body odours, carbon
dioxide and heat emitted from animals or humans. Majority
of species are nocturnal in their feeding habits, while others
feed indiscriminately by day or night. Some are outdoor biters
(exophagous) and some are indoor biters (endophagous). After
blood meal, female goes in search of a quiet place indoors
(endophilic) or outdoors (exophilic) to rest for a variable period,
usually 2 days and matures her eggs. When the eggs are fully
matured, she goes in search of water collection for oviposition
(act of laying eggs). Male mosquitoes feed on flower-nectar and
plant-juices and do not survive long after fertilizing the female
mosquito.
Genus Anopheles
Members of this genus have 58 species in India. Females of
only 9 species of Anopheline mosquitoes are the vectors of
human Plasmodia in India (Fig. - 1). In certain parts of the
world, some species of Anopheline mosquitoes transmit
Wuchereria bancrofti and Brugia malayi infections as well.

32. • 932 •
All important vector species are preferentially anthropophilic.
The time of feeding is also variable; some species prefer dusk
for their blood meals, others late night while still others select
early morning. Knowledge of the habits is useful in designing
control measures. An identification key for common Anophelines
of India is presented at the end of the section. Detailed
entomological techniques are given in the WHO manual and
a pictorial identification key has also been published by Das
et al.
Fig. - 1 : Anopheles Mosquito
Egg
Larva
Pupa
RestingposeHeadoffemaleHeadofmale
Lateralfloats
Genus Culex
Members of this genus are found in temperate and tropical
zones throughout the world. There are 240 Indian species in
this Genus. Adult mosquitoes of this genus are generally dull
in colour and inconspicuous due to unspotted wings (Fig. - 2).
Their breeding sites vary from clear water, such as wells and
springs, to collections of muddy, brackish or polluted water;
but unlike Anopheline mosquitoes, these mosquitoes generally
prefer stagnant and muddy pools. C quinquefasciatus is the
common house mosquito and is prevalent universally; it bites
at night and is the most important vector of Filariasis caused
by Wuchereria bancrofti.
Genus Aedes
Some members of the genus Aedes have almost worldwide
distribution while others have restricted habitats. The chief
species in India are Aedes aegypti, Ae albopictus and Ae vittatus.
They are black or dense brown and medium sized mosquitoes
with silvery white scales forming patterns on the thorax, bands
on the legs and rings around each abdominal segment.
These mosquitoes are mostly anthropophilic and are adapted
to domestic or semi-domestic environments. These are
container breeders. The cigar shaped eggs are laid singly on
damp surfaces on stagnant water. During the pre-monsoon
period, the breeding is restricted to water collections meant
for domestic use. Communities or sections of the cities with
water scarcity, which leads to water storage practices, are
mostly harassed by Ae aegypti. In the shore areas, barges and
country crafts provide ample place for Aedes breeding and
constitute a permanent source of Aedes infiltration in the shore
establishments and cities. They are well adapted for breeding
in small collections of water in a wide variety of natural and
artificial containers such as masonry tanks, earthenware pots,
small and large tins, barrel drums, coconut shells, stored or
discarded motor car tyres, junk and hardware, flower pots, fire
buckets, depressions in tree trunks, leaf axils etc. They may
breed in tree holes if these are situated within about 20 metres
of human habitat.
The eggs after maturing may remain viable for considerable
periods even after drying-up of the breeding sites, and hatch
out during rains. Such surviving eggs rapidly build up the
adult mosquito population, once it starts raining. Their
capacity to complete life cycle indoors enables them to breed in
urban areas throughout the year, irrespective of the prevailing
external climate.
Aedes mosquitoes are day biters with dawn and dusk as peak
biting time. They may feed indoors or outdoors and rest near
the breeding places in dark, shady corners outdoors, whereas
indoors they rest behind cupboards, hanging clothes, inside
shoes, umbrellas, below the furniture and in containers
providing breeding sites. Aedes aegypti prefers to breed in
artificial containers, whereas Aedes albopictus prefers natural
containers.
Aedes mosquitoes are the vectors of urban and rural yellow
fever (not in India), dengue, dengue haemorrhagic fever
Fig. - 2 : Culex Mosquito
Egg
Larva
Pupa
Resting positionHead of femaleHead of male

33. • 933 •
and chikungunya. Ae niveus has been reported as vector of
W bancrofti (diurnally sub periodic) infection in Nicobar
Islands. Ae aegypti and Ae albopictus, the two important vector
species can be easily distinguished by their thoracic pattern.
Ae aegypti has sickle or lyre shaped pattern on the thorax,
whereas Ae albopictus has a single central mark present on the
thorax (Fig 3).
Fig. - 3 : Aedes species identification
Aedesalbopictus Aedesaegypti
Genus Mansonia (Fig 4)
This has wide distribution in tropical countries. In India,
Mansonia annulifera, M uniformis and M indiana are the
prevalent species in Kerala. The adult mosquitoes are robust and
yellowish brown. The wings are covered with flat, broad scales,
which give the wings a speckled appearance as if sprinkled
with mixed salt and pepper. The female lays eggs in cluster
anchored to the under surface of the leaves of aquatic plants
such as Pistia, Lemna, Eichhornia and Salvinia. On hatching
out, the larvae obtain oxygen from the plant cells through their
modified siphon tubes by attaching themselves to the rootlets
of these plants. The pupae are similarly attached to the plant
stems by the modified breathing trumpets. When matured
they detach themselves and come to the water surface. They
are persistent biters, particularly during darkness. Mansonia
mosquitoes are the vectors of B malayi infection of filariasis in
several pockets in rural areas of Kerala, Tamil Nadu, Andhra
Pradesh, Madhya Pradesh, Assam and West Bengal.
Fig. - 4 : Mansonia adult with wing depicting typical salt
& pepper appearance
MansoniawingMansoniaadult
Some of the important differences between Anopheline and
Culicine mosquitoes are shown in Table - 1.
Table - 1 : Important differences in Anopheline and Culicine mosquitoes
Stage Anopheline Culicine
Egg
Boat shaped●●
Laid singly ●●
Possess lateral floats●●
Elongated●●
Aggregation occurs into rafts of hundreds of eggs in ●● Culex
Aedes●● eggs are laid singly
Mansonia●● eggs are laid on under surface of leaves of aquatic
plants in star shaped clusters
Larva
No siphon tube but only apertures on 8●●
th

abdominal segment
Larvae rest parallel to the surface of ●●
water
Swim with swift wriggling movements ●●
Palmate hairs for floatation arranged in ●●
pairs on all abdominal segments
Single siphon tube on 8●●
th
abdominal segment
- In Culex, siphon tube is long and narrow
- In Aedes, it is short and broad
- In Mansonia, larvae are attached through siphon tube to
roots of aquatic plants
Larvae rest at an angle to surface ●●
Swim with slow snail or worm like movements●●
No palmate hairs for floatation●●
Pupa
Pupa is comma shaped ●●
In Anophelines, respiratory trumpets are ●●
short stumpy and funnel shaped
Pupa is comma shaped●●
In Culicines, respiratory trumpets are longer, slender and ●●
trumpet shaped
Adult
Wings usually spotted●●
Rests at an angle to surface, with the ●●
exception of A culicifacies
In the males, palpi are long and club shaped ●●
at the termination; in females, they are as
long as proboscis and are straight
Wings usually not spotted●●
Rests parallel to the surface●●
Thorax is humped●●
In the males, palpi long and tapering and deflected out; in ●●
females, palpi are much shorter than proboscis and budlike

34. • 934 •
Vectors of Malaria
There are 58 species of Anopheline mosquitoes in India but
only 9 are incriminated as vectors and another 5 species
have been found to be of local importance in transmission of
malaria. In Northern and Peninsular India, the main vectors
are A culicifacies, A stephensi and A fluviatilis, while the
local vectors are A sundaicus, A annularis and A varuna. In
Eastern India, the main vectors are A dirus, A sundaicus, A
philippinensis, A minimus, and A maculatus. In Andaman and
Nicobar Islands, the main vector is A sundaicus while the new
vectors are A dirus, A maculatus and A tesselatus. A detailed
list of these vectors is presented in Table - 2.
The following characteristics of vector mosquitoes play an
important role in the epidemiology of malaria.
(a) Breeding Habits : The breeding habits of mosquitoes
show a lot of variation and hence vector mosquitoes tend to be
confined to certain geographical areas only. A few examples are
as shown in Box - 1.
Box - 1 : Breeding Habits of Vectors
Slow moving water, seepages, terraced
rice fields
A fluviatilis
Brackish waters A sundaicus
Wells, cisterns and over head tanksA stephensi
Tanks, pools, burrow pits and ditchesA philippinensis,
A annularis
Forest pools, streams and slit trenchesA dirus
(b) Vectorial Capacity : Only certain species act as vectors;
moreover, within the known vector species, some are more
efficient while some are less. The exact reasons for this are
not known. Certain new species are emerging as secondary
vectors in different parts of the country. The Vectorial capacity
(C) can be expressed as a mathematical expression to measure
vector efficiency and assess risk and impact of interventions
as follows :
map
(-logp)
2n
e
C=where :
m = density of vectors in relation to man
a = number of blood meals taken on man per vector per day
p = proportion of vectors surviving per day
n = incubation period in the vector (days) - 8 days
when they survive (1/-log
e
p) days.
Theoretically, incidence of infection rises when C>1, incidence
falls when C<1.
(c) Density : For effective transmission of malaria in a locality,
the mosquito vector must attain and maintain a certain density.
This is called critical density and it varies from one mosquito
to another and also under different environmental conditions.
A culicifacies needs a very high density for transmission of
malaria, while philippinensis, dirus and fluviatilis need much
lower critical density.
(d) Longevity : A mosquito, after an infective blood meal, must
live for at least 10 days to complete the development of malaria
parasites. Therefore aim in malaria control programme is to
reduce the life span of mosquitoes to less than 10 days.
(e) Tropism : Some mosquitoes like A fluviatilis prefer human
blood and are called anthropophilic. Others like A culicifacies
preferably feed on animal blood and are called zoophilic. This
preferential feeding habit is called tropism.Table - 2 : Malaria Vectors of India and Vectors of local importance
S No. Name of Species Role in transmission
1 A stephensi Primary urban vector in most of India except North-East and Rajasthan
2 A culicifacies Primary rural vector all over India except North-East
3 A fluviatilis Primary rural vector in foothill areas of the country
4 A minimus Primary vector in North Eastern regions, West Bengal
5 A phillippinensis Primary vector in rice field ecosystem of North Eastern regions
6 A dirus Primary vector in regions of evergreen forests in the North East
7 A sundaicus Primary vector in Andaman & Nicobar Islands
8 A varuna Eastern ghats (Andhra Pradesh), Singbhum hills (Bihar) & Kerala
9 A annularis Secondary vector in rice field ecosystem in Uttar Pradesh and coastal areas of Orissa
Vectors of local importance
1 A aconitus Orissa, Assam
2 A jeyporiensis (var
candidiensis)
In certain localities in Kerala, Karnataka and Assam
3 A maculatus In certain localities in Assam & Meghalaya
4 A tesselatus Lakshadweep Islands (on epidemiological grounds)
5 A subpictus In Madhya Pradesh

35. • 935 •
(f) Biting Behaviour : Some vector mosquitoes bite at or soon
after dusk, others either during late night or early hours of the
morning. However, some species may be active at two different
periods during the same night.
(g) Resting Habits : A female mosquito after a blood meal
rests either indoors (endophilic) or outdoors (exophilic) for
maturation of its eggs. The common resting places are either
human dwellings, cattle sheds or mixed dwellings.
(h) Flight Range : The range of flight and dispersion varies
from one vector to another. Some have a short flight range e.g.
flight range for A dirus, A annularis and A fluviatilis is upto 1
km distance; whereas A culicifacies and A stephensi fly up to 2
km and A sundaicus may fly even up to 8 or 10 km.
(j) Resistance to Insecticides : When a vector mosquito in a
locality becomes resistant to a particular insecticide, use of an
alternative insecticide is recommended.
Vectors of Filariasis
(a) Culex quinquefasciatus : This species is the main vector of
bancroftian filariasis in India. It preferentially breeds in dirty
water collections such as in drains, cesspools, soakwells and
septic tanks. When denied such opportunities, it can also breed
in clean water.
(b) Mansonoides : Mansonoides species are the vectors of
B malayi infection in India. In Kerala, M. annulifera and M.
uniformis are the major vector species. These mosquitoes are
associated with aquatic plants like Pistia stratiotes, Eichhornia
speciosa and Salvinia auriculata .
(c) Aedes niveus : This species has been incriminated as a
vector of diurnally subperiodic form of Bancroftian filariasis in
Nicobar group of islands.
(d) Other Species : In India, Anopheline mosquitoes have not
been found to play any significant role in the transmission of
any type of filarial infection unlike in some other parts of the
world.
Mosquito Surveillance
In places which are endemic for mosquito borne diseases
or outbreak prone, it is deemed mandatory that mosquito
surveillance system be established. The aim of the surveillance
should be to inform about changes in density and major
characteristics of vectors, to forecast an impending outbreak
and to recommend appropriate strategies for mosquito control
which would prevent outbreak.
Steps for Establishing Mosquito Surveillance System
Step 1 : Acquaint yourself with basics of mosquito identification
both larval and adult mosquitoes up to genera level (Culex,
Anopheles and Aedes) and mosquito bionomics.
Adult
Identification of mosquito : Mosquitoes are identified from
other such flies by the presence of forward projecting mouthparts
or proboscis, wing veins (veins 2, 4 and 5 are branched) and a
fringe of scales along the posterior margin of wings (Fig. - 5).
Fig. - 5 : Identification of mosquito
ABDOMEN
Cercus
HIND-LEG
Tarsus
Claw
MID-LEG
Tibia
Femur
Fringe
Veins
WING
Cells
HALTER
FORELEG
Proboscis
Antenna
Palp
Eye
Scutum
Scutellum
HEAD
THORAX
1
2
3
4
5
6
7
8
1
2
3
4
5
Identification of Male & Female Mosquito : The mosquito
sexes can be identified by their antennae. It’s bushy in males
and not so bushy in females (Fig. - 6).
Fig. - 6 : Identification of Male & Female Mosquito
Identification of Anopheles, Culex and Aedes : If the wings
are spotted, it is anopheline vector species; whereas, if the legs
are having silvery stripes against dark black legs it is Aedes
and if there are no spots on the wings or stripes or legs, it is
Culex adult. The length and shape of terminal area of palpi
can also be used for discriminating, as explained in Table - 1
earlier.

36. • 936 •
Larvae : If the larvae have siphon tube at its tail end, it belongs
to Culicine group and could be Culex or Aedes. For this reason,
the larvae of Culicines are suspended upwards down, at an
angle with the water surface. If the siphon tube is long and
narrow, it is Culex species, whereas short and broad siphon
tube indicates it is Aedes. It is also important to remember that
Aedes is essentially a container breeder and will be found only
in artificial or natural containers. The Culicine larvae float at
an angle to the water surface as shown in Fig. - 7.
Fig. - 7 : Culicine Larva
Anopheles in contrast does not have siphon tube and floats
parallel to the water surface of water as shown in Fig. - 8.
Fig. - 8 : Anopheline Larva
Step 2 - Sector Allocation and Mapping : Once armed with
the basic information, start by dividing the entire area of your
responsibility into four or five sectors and make a map of the
sector. Each sector should be covered every fortnight or at least
once a month for surveillance.
Step 3 - Conduct a Preliminary Survey : First a baseline survey
(larval and adult) is conducted to determine the prevalence,
abundance and distribution of the vector species.
Step 4 - Data Compilation and Preparation of Spot Map : The
data collected in preliminary survey is compiled and analyzed
to determine the above mentioned variables and a spot map is
prepared with clearly marked out areas where density is high,
medium or low or where potential of breeding exists.
Step 5 - Establish Monitoring / Catching Stations : In each of
the four or five sectors, establish four fixed stations (minimum
two if manpower is less), with preferably two from the high
density and two from the medium density spots and an equal
number of random stations (2 each from the high density and
the medium density areas). Poor or worst ventilated houses
located close to the breeding places, discarded bathrooms,
abandoned buildings are considered ideal; for room selection,
the room which has fewer openings is selected.
Step 6 - Conduct Fortnightly Surveys : Conduct surveys in the
fixed and random stations and record your data in the format
given in Appendix ‘A’ and ‘B’ at the end of this chapter.
Larval Sampling Procedure : The mosquito larval sampling
should be done by standard larval ladles. The method is as
follows :
Dip the ladle sideways : A minimum of five dips may be taken
for calculation of larval density (Fig. - 9).
Fig. - 9 : Larval Sampling
Transfer the larvae in enamel bowl : count the total number
of larvae in the bowl after five dips.
Calculate larval density : For example :
Total no. of dips taken-5●●
Total number of larvae counted -50●●
Larval density (Total no. of larvae/ no. of dips)-50/5 = 10●●
Other larval sampling procedures like larval nets (when the
water body has vegetation) or well nets (when mosquito
breeding is noticed in wells) may be used in specific situations.
The density is calculated in the same way and given as larvae/
larval net or well net.
Adult Sampling Procedure
Aspirators / Suction tube : This is the most common method
of sampling adult mosquitoes (Fig. - 10). It is normally
undertaken in the mornings. Before using suction tube ensure
that muslin cloth or gauge piece is placed between the glass
tube and the rubber tubing to prevent mosquitoes being sucked
inside your mouth.
Fig. - 10 : Aspirator / Suction Tube
With the aid of a torch look for resting mosquitoes on the ●●
walls, ceilings (when it’s low), behind and under furniture/
wall hangings etc.

37. • 937 •
For practical purposes, while undertaking mosquito ●●
surveillance, one insect collector should spend at least 15
minutes in each of the 4 fixed and each of the 4 random
stations. Thus, 2 insect catchers should be deputed; one
for the 4 random stations and one for the fixed stations (15
minutes at each station) on the day surveillance is being
conducted in that sector.
While using suction tube, keep the end of rubber tubing ●●
in your mouth and place the opening of the glass tube 1-2
cm from the resting mosquito. Move the end closer to the
mosquito by applying gentle suction to draw the mosquito
inside the tube, now place your finger over the tube to keep
the mosquito from flying away.
Do not collect more than five mosquitoes in one tube. After ●●
collection, transfer the mosquitoes in transport cages by
gentle blowing.
Density of mosquitoes is calculated by the following ●●
formula :
Density(PerManHour)=
TotalNo.ofmosquitoescollected
ManHourspentincollection
If 2 persons have collected 18 mosquitoes and each man
has spent 1 hr each (15 minutes per station), the density is
calculated by the following method :
2 Persons X 1 hr= 2 Man Hour
Total mosquitoes collected in 2 Man Hour - 18
\ Mosquitoes collected Per Man Hour (PMH) - 18/2 = 9
Density = 9 PMH
Total Catch or spray sheet collection : Involves the use of
Pyrethrum for collection of mosquitoes resting indoors (Box -
2). This is a more efficient method of sampling as it can also
collect those mosquitoes which are hiding under furniture or
resting on high ceilings or where the density of mosquitoes
resting is low.
Other types of mosquito sampling devices : The other
sampling tools are the window traps, magoon traps, direct bait
collection and light traps.
Surveillance of Aedes mosquitoes
(i) Larval survey : Three indices are commonly used to record
Ae aegypti and Ae albopictus density levels :
House Index (HI) : Percentage of houses or premises positive
for Aedes larvae.
HI=
No.ofhousespositiveforAedeslarvae
No.ofhousesinspected
x100
Container Index (CI) : Percentage of water-holding containers
positive for Aedes larvae.
=CI
No.ofpositivecontainers
No.ofcontainersinspected
x100
Breteau Index (BI) : Number of positive containers per 100
houses in a specific location.
=BI
No.ofpositivecontainers
No.ofhousesinspected
x100
(An HI >5% &/or a BI >20 for any locality is an indication
that the locality is dengue sensitive and therefore adequate
preventive measures should be taken).
(ii) Oviposition Traps : “Ovitraps” provide a sensitive and
economical method for detecting the presence of Ae aegypti
and Ae albopictus in situations where Aedes density is low
and general larval surveys produce unsatisfactory results
(e.g. when the Breteau Index is < 5). The standard ovitrap is
a wide-mouthed glass jar of approximately 250 ml which is
painted black on the outside to attract the Aedes females to
oviposit. A piece of hardboard/ wooden paddle or filter paper is
placed diagonally inside the glass as an oviposition substrate.
In addition, the jar is partially filled with clean water and
placed in a shaded and protected place (rain, people/ animal)
for oviposition.
(iii) Adult Survey : Human bare-leg catches (landing / biting
catches) of Aedes adults (both male & female) or indoor resting
collections of adults are normally used to assess adult Aedes
populations. The data collected are calculated to reflect the
number of female Aedes mosquitoes landing/biting on single
human bait per hour (e.g. number per man hour). The collectors
should move from house to house and not collect in one place
for more than 15 or 20 minutes. In a similar manner, indoor
resting collections can be made and the data expressed as
numbers collected per man-hour or per house. Adult indices
can also be elicited just like the larval indices i.e. ‘Adult House
Index’, ‘Adult Room Index’ and ‘Adult Breteau Index’.
Box - 2 : Total Catch or Spray sheet collection
Remove all animals, small items of furniture, food items. Close all windows and doors and close / cover all openings with ●●
cloth.
Spread white cotton bed sheets to cover the entire surface area (no. of sheets will depend on the size of the room). Ensure ●●
that sheets have been placed under furniture also. Sheets placed on furniture should not touch the floor as it will prevent
the insecticide from reaching underneath.
Prepare the Pyrethrum solution from 2% Pyrethrum extract available to a workable strength of 0.2% by mixing 100 ml of ●●
Pyrethrum with 900 ml of Kerosene to make a total volume of 1 litre of prepared soln. of 0.2%.
Use a hand sprayer (Flit gun) for spraying. After entering the room first close the door of the room and then start spraying ●●
in open spaces and holes in the wall and thereafter proceed to apply spray towards the ceiling until the room is filled with
fine mist; always taking care to move in a clockwise direction. After spraying, close the door and keep the room closed for
at least 10 min.
After 10 min, open the door, move gradually from the doorway picking up the mosquitoes by forceps in a container. A torch ●●
may be needed for collection of mosquitoes in rooms.

38. • 938 •
Mosquito Control
Anti-adult measures and anti-larval measures are the two most
important mosquito control measures. Personal protection
against their bites aids these measures in control of disease.
Methods related to environmental management by way of
minor manipulations or major engineering steps should always
be an important consideration in overall anti-malaria plan. The
details of these various mosquito control methods have already
been presented in detail in an earlier chapter on principles of
vector control and the readers are suggested to go through the
details.
Anti-adult Measures
(a) Residual Insecticides : Indoor residual spray is considered
to be the most important tool for controlling mosquito borne
diseases. This is a more practicable and simpler method
of interruption of transmission of disease. However, there
are certain conditions under which the absolute efficacy of
this procedure may be doubted, for instance, where vector is
exophilic though biting indoors or where the surfaces sprayed
are subject to frequent mud plastering or white washing. Even
when local conditions do not appear to be absolutely favourable,
the application of residual insecticide gives relative success in
disease control. DDT, Malathion and the members of synthetic
pyrethroids like Cyfluthrin, Deltamethrin etc. are the residual
insecticides of choice depending upon the susceptibility of the
vectors. The dosages and formulations of common adulticides
used in mosquito control are given in Appendix ‘C’.
(b) Space Sprays : Space treatments are usually designed
to provide rapid knock-down and mortality with little or
no residual effect. Such treatments must be considered in
conjunction with other control methods as part of an integrated
vector management programme. The details of space spraying
and other adult measures are given in the Chapter on “Principles
of Vector Control”.
Anti-larval Measures
Larval control is the only effective method of radical mosquito
control. In urban areas, this method complements the adult
mosquito control. Anti-larval work is carried out by preventing
breeding and destruction of larvae and pupae. For long term
and permanent mosquito control, greater emphasis should be
placed on the prevention of breeding during non-transmission
season than on larvicidal measures during breeding season.
(a) Vector Engineering : Avoidance of man-made mosquitogenic
conditions is of primary importance. The details are presented
in the Chapter on “Principles of Vector Control”.
(b) Dry Day : Intermittent drying once a week is an effective
method of prevention of breeding especially for container
breeders like Aedes by observance of a weekly ‘dry day’. All fire
fighting tanks, ornamental ponds or water storage tanks, fire
buckets, and all domestic water containers should be emptied,
scrubbed and allowed to remain dry for a few hours on the
weekly ‘dry day’.
(c) Larvicidal Measures : Destruction of larvae is achieved by
application of larvicidal oils, Organophosphorus insecticides,
use of IGR’s, biocides and use of larvivorous fish. The details
are presented in the Chapter on “Principles of Vector Control”.
The list of anti-larval chemicals along with their dosages is
presented in Appendix ‘D’.
Personal Protection
Individual personal protection against mosquito bites is
achieved by use of mosquito nets, repellents and protective
clothing. The details are presented in the Chapter on “Principles
of Vector Control”.
Summary
The mosquitoes are rated as the most important vectors
amongst the blood sucking arthropods. They belong to phylum
Arthropoda, class Insecta, order Diptera & family Culicidae.
Amongst the mosquito genera, only Anopheles, Culex, Aedes
and Mansonia are of importance in India. The mosquitoes are
further classified as Anophelines which comprises only genus
Anopheles & Culicine comprising three important genera viz.
Culex, Aedes & Mansonia.
The length of the mosquito is about 1 cm and the body is
divided into head, thorax & abdomen. Head has two compound
eyes, antennae & mouth parts collectively called proboscis.
Males can be identified by their densely haired antennae which
look like moustache. Thorax in all the three genera of Culicines
is humped. Mosquito undergoes complete metamorphosis
through stages of egg larva, pupa & adult. Numbers of eggs
laid vary between 50 & 150. Larvae pass through 4 instars in
5 days and become a pupa. Duration of lifecycle varies between
7 days to 1 month. Female mosquitoes are blood suckers
(anthropophilic - prefer to feed on humans & zoophilic- prefer
to feed on animals). Mosquitoes as per their biting habits may
either be exophagus or endophagus and depending on their
resting preference may be endophilic or exophilic. Males feed
on plant juices & flower nectar.
The genus Anopheles comprises 58 species in India, of which 9
are vector species with most of them being anthropophilic. Culex
adults are dull in colour & have unspotted wings and prefer
stagnant water. Culex quinquefasciatus is the main vector of
bancroftian filariasis in India. Chief species of Aedes in India
are aegypti, albopictus & vittatus. They are well adapted for
breeding in small containers of water such as masonary tanks,
earthenware pots, small and large tins, barrel drums, coconut
shells, stored or discarded motor car tyres, junk and hardware,
flower pots, fire buckets, depressions in tree trunks, axils of
leaves & tree holes. Aedes mosquitoes are the vectors of urban
and rural yellow fever (not found in India), dengue, dengue
haemorrhagic fever and chikungunya. Mansonia species lay
eggs in cluster anchored to the under surface of the leaves of
aquatic plants.
Mosquito surveillance is done to elicit information on vector
density and major characteristics of vectors, to forecast an
impending outbreak and to recommend appropriate strategies
for mosquito control which would prevent outbreak. It is thus
important to identify mosquitoes from other such flies; they
are identified by the presence of forward projecting mouthparts
or proboscis and wing veination (veins 2, 4 and 5 are branched)
besides other characteristics. If the wings are spotted, it is
generally anopheline mosquitoes; whereas, if the legs are
having silvery stripes against dark black legs, it is Aedes

39. • 939 •
and if there are no spots on the wings or stripes on legs, it is
Culex adult. If larvae have siphon tube at its abdominal end, it
belongs to Culicine group. Larvae are sampled using standard
larval laddles & larval density is calculated by dividing the total
no. of larvae by the no. of dips. Adult sampling is generally
done using aspirators & suction tube. The mosquito density
is calculated by Total no. of mosquitoes collected divided by
the man hours spent in collection. Other methods are spray
sheet collection & use of window trap, magoon trap or direct
bait collection. Aedes larval surveys include computing indices
viz. Container, House & Breteau index. The adult sampling
procedures are the landing catch and resting catch, whereas
breeding can be detected by oviposition traps.
Study Exercises
MCQs & Exercises
1) Wings in mosquito are attached to (a) Thorax (b) Abdomen
(c) Both (d) None
2) Branched veins in a mosquito wing are (a) 1,3,6 (b) 2,4,5
(c) 2,4 (d) 1,6.
3) Eggs are laid in rafts in (a) Aedes (b) Culex (c) Anopheles
(d) Mansonia.
4) Boat shaped eggs with lateral floats and larva without
siphon tubes are features of (a) Anopheles (b) Aedes
(c) Culex (d) Mansonia.
Fill in the Blanks :
5) ________________ is the main rural vector of Malaria.
6) Respiratory apparatus in larvae of Culicine is __________
7) Most important vector of Wuchereria bancrofti is _______
8) Vector species can be distinguished by their thoracic
pattern in _______________
True or false
9) Mosquitoes can be found in altitudes as high as 4000mts.
10) The palpi & proboscis in Culex female are equal.
Answers : (1) a; (2) b; (3) b; (4) a; (5) Anopheles culicifacies;
(6) Siphon tube; (7) Culex quinquefasciatus; (8) Aedes;
(9) True; (10) False.
Further Suggested Reading
Service Mike. Medical Entomology for students. 3rd ed. Cambridge : 1.
Cambridge University Press, 2004.
Foster WA, Walker ED. Mosquitoes (Culicidae). In : Medical and Veterinary 2.
Entomology, ed. Mullen G, Durden L. Amsterdam : Academic Press, 2002;
203-62.
Kettle DS. Medical and Veterinary Entomology. 2nd ed. CAB International, 3.
1995.
Hati AK. Medical Entomology. Allied Book Agency, Kolkata, 2001.4.
Rozendaal Jan A. Vector control : Methods for use by individuals and 5.
communities. Geneva : World Health Organization, 1997.
Nelson MJ. Aedes aegypti : Biology and Ecology. Pan American Health 6.
Organisation. Washington, 1986.
Rao TR. The Anophelines of India. Revised Ed 1983. Malaria Research Centre 7.
(Indian Council of Medical Research), Govt of India, Delhi.
WHO. Entomological Field techniques for Malaria control. Part 1 : Learners 8.
Guide. WHO, Geneva, 1992.
WHO. Entomological Field techniques for Malaria control. Part 1 : Tutor’s 9.
Guide. WHO, Geneva, 1992.
Das Bina Pani, Rajagopal R, Akiyama J. Pictorial key to the species of Indian 10.
Anopheline Mosquitoes. Journal of Pure & Appl Zoology 1990; 2(3) : 131-
62.
Appendix ‘A’ : Mosquito Larval Surveillance Register
Sector
No.
Date
Anopheles Culex Aedes
No. of
dips
Total
larvae &
pupae
Larvae/
dip and
Pupae/
dip
No. of
dips
Total
larvae &
pupae
Larvae/
dip and
Pupae/
dip
Type of
container
No. of
container
Positive
for
breeding
Remarks
(**)
(**) Remarks to include details of House index, Container and Breteau index for Aedes if situation warrants.
Appendix ‘B’ : Adult Mosquito Surveillance Register
Date
Sector
No.
Species
Fixed catching
station
Random catching
station
Total Density
Per Man
Hour
Time
spent
No.
collected
Time
spent
No.
collected
Time
spent
No. collected
M F M F M F Total
Anopheles
Culex
Aedes
Anopheles
Culex
Aedes

40. • 940 •
Appendix ‘C’: Insecticides Used for Indoor Residual Spray with Dosage and Residual Efficacy
Insecticide
Preparation
of
suspension
in water

Dosage
of a.i.
/ sq m
Residual
effect in
weeks
No. of spray
rounds/
annum
Area to be
sprayed by
10 lit of
suspension
Remarks
DDT 50% WP 1 kg/ 10 lit1 g 10 - 12 2 500 sq m In North East only
Malathion 25% WP 2 kg/ 10 lit2 g 6 - 8 3 250 sq m In DDT resistant areas
Deltamethrin 2.5% WP400 g/ 10 lit20 mg 10 - 12 2 500 sq m In Malathion resistant areas
Cyfluthrin 10% WP 125 g/ 10 lit25 mg 10 - 12 2 500 sq m In Malathion resistant areas
Appendix ‘D’ : Insecticides Used for Mosquito Larval Control
Insecticide Dilution rate Dosage /sq m
Area in linear metre
to be sprayed by
10 lit of solution/
suspension
Frequency of
application
Remarks
MLO As it is 20 ml 500 Weekly Applied along shore
of water body
Temephos
50% EC
2.5 ml in 10 lit for water
depth up to 50 cm. 5 ml in
10 lit for >50 cm depth
20 ml 500 Weekly Applied in all water
bodies
Fenthion
82.5% EC
5 ml in 10 lit for water depth
up to 50 cm. 25 ml in 10 lit
for >50 cm depth
20 ml 500 Weekly Not used in potable
water
Fenthion 2G - Not used in potable
water
Bacillus
thuringiensis
H -14 (12 AS)
250 g in 10 lit 20 ml 500 FortnightlyNot used in potable
water
Bacillus
sphaericus
500 g in 10 lit 20 ml 500 Once in three
weeks
Not used in potable
water
158Fleas
Rina Tilak
Fleas are one of the few important vectors which have been
historically linked with mankind the world over, since time
immemorial. Plague, transmitted by rat flea, was one of the
vector borne diseases which played an important role in
redefining geographical boundaries for many centuries.
Distribution
Fleas are distributed all over the world and belong to the order
Siphonaptera comprising about twenty five hundred species
and sub-species. Fleas can be classified into two main groups
viz. the 'combless' fleas and the 'combed' fleas, depending on
whether they have chitinsed teeth like structure called ‘genal
comb’ around the mouth, or not.
The Combless fleas contain the important genus Xenopsylla
which has about sixty species and sub-species including the
well known vectors of plague viz, X cheopis, X astia and X
braziliensis. The oriental rat flea (Fig. - 1), X cheopis, is widely
distributed in the tropics and is the principal vector of plague
in India. X astia is also found in India, Burma, Sri Lanka,
Hongkong and Iran. X braziliensis is found in Africa, especially
in Nigeria, Congo, Kenya and in South America but in India its
distribution is very restricted. The other important combless
flea is Pulex irritans (Human flea), which occurs only in the

41. • 941 •
hills of the tropical countries of the Eastern Hemisphere. It
breeds in and around dwellings and principally attacks man
besides animals and rats.
Fig. - 1 : Adult Rat Flea
The Combed fleas are the cat fleas - Ctenocephalides felis (Fig.
- 2), the dog flea - C canis and the rat fleas of temperate zones,
Nosopsyllus fasciatus. In addition to the genal comb, they also
possess ‘pronotal comb’ on the thorax (Fig. - 2). These fleas
serve as intermediate host of certain veterinary cestodes (dog-
tapeworm) but are more of a biting nuisance to man. Tunga
penetrans, a sandflea is found in tropical and sub-tropical
regions of North and South Americas, Africa and occasionally
in Western India.
Fig. - 2 : Adult Cat Flea
Morphology
Adult fleas are small, bilaterally compressed, highly chitinised,
wingless, 6 legged, blood sucking ectoparasites of many warm
blooded vertebrates. The size varies from 1.5 to 6 mm in length
and the colour from light amber to dark brown.
They have a compact appearance without a sharp division
between the head, thorax and abdomen. The head is roughly
triangular and bears a pair of three segmented antennae, the
mouth parts and in certain flea species, a row of powerful teeth
like spines collectively known as the ‘genal comb’, arranged
on the lower border of the head, and a set of ‘pronotal combs’
on the thorax. However, the rat fleas are devoid of both these
combs. The mouth parts are adapted for biting, piercing and
sucking blood, which forms the only food for both sexes. The
thorax of the flea is compact without any wings. The legs are
long and powerful and are adapted for the purpose of hopping
and jumping. The abdomen consists of 10 segments, the 9
th
and 10
th
being modified for sexual functions. In the female,
the abdomen has a rounded terminal outline whereas in the
male it has a rather cocked up appearance. The body and
the legs are provided with stiff setae, which give the insect a
bristly appearance. The tapering pharynx continues into the
oesophagus leading into the conical proventriculus. This is an
important structure involved in the transmission of bubonic
plague.
Life History
The flea undergoes a complete metamorphosis through the
successive stages of egg, larva, pupa and adult (Fig. - 3).
When the female is ready to lay eggs, it leaves the body of the
vertebrate host and lays eggs in dark place in the host's nest,
debris, accumulation of dust, in cracks or crevices in the floor
of granaries etc. or under carpets in houses. During her lifetime
of 6 months or a year, the female lays 300 to 500 eggs in small
batches of about a dozen at a time. A temperature between
18°C and 27°C and humidity about 70% favour egg laying.
However, most fleas complete their life cycle in one to two
months. The eggs are just visible to the naked eye and hatch in
2-10 days depending on temperature and humidity. The larvae
are very active, slender, 13 segmented and yellowish white
with a number of bristles. They feed on the excreta of rodents
and on partially digested blood discharged from the faeces of
adult fleas. Larvae complete their development in a week or
two and enter quiescent stage, spin cocoons which are whitish,
translucent and so loosely spun that the pupae can be seen
within them. Hence the pupa closely resembles the adult which
usually emerges within a week. It is important to note that
presence of ground vibration (caused by movement of hosts i.e.
rodents, animals, humans or earthquakes) is essential for the
emergence of adult from the pupa. The whole metamorphosis
takes two to four weeks, but may need several months under
less favourable conditions.
Fig. - 3 : Life Cycle of Flea

42. • 942 •
Bionomics
The adult fleas are temporarily parasitic on their host while
their immature forms are free living. Both, males and females
are haematophagous and frequently leave the host between
blood feeds. After the death of the host, its body becomes cold
and the flea seeks a new host. Fleas feed frequently and much
more than their actual requirements, and as a result much of
the ingested blood is passed out in a semi-digested state. Fleas
are not strictly host specific and may attack unusual hosts
when hungry or with rise of ambient temperature, when they
feed more frequently. They are very sensitive to light and air
currents. They always hide under dark objects and when blown
up, they at once get agitated. Adult fleas can survive several
months without food. They are able to jump up to 16 cm and
hop 30 cm.
Vector Potential
Flea transmits mainly the zoonoses to man, chiefly from
rodents and also from dogs and cat. The most important
microorganism that is conveyed to man from rat is Yersinia
pestis causing bubonic plague. The most important vector
species is X cheopis, however, X astia and X braziliensis are
also effective vectors. Rickettsia typhi, causative organism of
endemic (murine) typhus, is also transmitted from its rodent
reservoir to man by the same rat fleas. Cat and dog tapeworms
use fleas as their intermediate hosts for the development of
cysticercoid stages. Cats and dogs become infested by the
ingestion of infested fleas. Children also get infected similarly
due to accidental ingestion of the infected fleas. The South
American and African flea Tunga penetrans burrows under the
soft skin in between toes and under the nail bed and causes a
disease called 'chigger', 'jigger' or 'chigoe' in endemic areas.
Modes of Transmission of Diseases
Bubonic plague is transmitted by infected fleas called ‘Blocked
flea’. When a flea (Xenopsylla species) feeds on a host suffering
from plague, it ingests plague bacilli (Y pestis) along with the
blood meal. The blood is digested in the stomach, however, the
bacilli rapidly multiply and block the proventriculus thereby
rendering the proventriculus partially blocked and incapable
of preventing regurgitation of stomach contents (the normal
role or functioning of the proventriculus) while feeding. Since
the stomach is filled with plague bacilli, the amount of blood
digested is minimal and hence this flea feels hungrier and bites
repeatedly and in the process, regurgitates plague bacilli in the
wound thus causing plague in the host. After a few days, the
multiplying plague bacilli completely block the proventriculus
thus rendering it totally ineffective in preventing regurgitation.
Since no amount of blood gets digested, this flea though bites
repeatedly and inoculates the bite wound with bacilli, lives
for a very short duration. A partially blocked flea is thus more
efficient in transmitting plague as compared to a completely
blocked flea.
Endemic typhus is transmitted through the faeces of the rat
flea which contains the semi digested blood along with the
causative organism R typhi. The flea has a habit of defaecating
while feeding and hence while scratching the bitten area; the
organism finds its way in to the host’s body. The organism
can also find an entry into the body through conjunctiva,
inhalation and skin abrasions and also by accidental ingestion
of infected flea.
Flea Control
Chemical control of fleas is one of the best methods of flea
control. These are effective against both adult as well as
larval fleas. The areas or places generally frequented by fleas
like rodent burrows and rat runs are treated. The insecticidal
treatment is either done in the form of residual sprays, dusting
or treatment of rodent burrows. Dusting is done by applying a
patch of insecticide dust of about 20-25cm wide and 0.5cm thick
in all infested areas. For rodent burrows, 30 g of insecticidal
dust is used.
In plague susceptible areas, treatment is undertaken when flea
index i.e. X cheopis index exceeds 1 (the other flea indices are
presented in Box - 1). However, during an outbreak, no rodent
control activity is undertaken. In the event of a plague case
occurring, immediate treatment of the patients dwelling and of
other dwellings within 200 m is undertaken.
(a) Vector Control : DDT has become non-effective against
fleas in many parts of the world. Malathion resistance has also
appeared in certain parts of India. Prior susceptibility tests
should be carried out to find out the most effective insecticide.
Indoor residual spraying at the lower one metre of the wall
surface and adjacent floor area is effective. Patch dusting also
brings about marked reduction in flea density. For this, dusts of
Propoxur (1%), Malathion (5%) or Carbaryl (5%) may be applied
at a dosage of 2 to 3 g per m
2
of surface area under grain bins,
on rat runs, furniture, upholstery, rugs and bedding. The dust
of Deltamethrin (0.05%) may also be used for dusting in rodent
infested area.
(b) Disinfestation : Disinfestations of pet animals like dogs
and cats along with good environmental sanitation of the
household and public places (by keeping houses well swept
and floors washed) help in flea control. Pet animals may be
treated with dusts, sprays or dips of Malathion, Propoxur,
Permethrin or Pyriproxyfen. Animal premises may be sprayed
with insecticides (Malathion, Deltamethrin, Pyrethrum etc.)
@ 4 - 8 l/100 m
2
. Insecticidal treatment of animals and their
premises should be carried out simultaneously. Lufenuron
Box - 1 : Flea Indices
General flea index●● - Average number of fleas (all species included) per rodent. e.g. if, in all, 20 fleas were recovered from
a total of 4 rodents examined, the index would be 20/4 = 5.
Specific flea index●● - (important is X cheopis index). This is same as general flea index but calculated exclusively for X
cheopis. e.g. in above example, if a total of 2 of the 20 fleas were X cheopis, this index would be 2 / 4 = 0.5
Percentage incidence of flea species●● - This is the percentage of each species of fleas, out of the total fleas sampled per
rodent.
Rodent infestation rate●● - If 10 rodents were caught and 6 were infested with fleas, this index would be 6/10=60%

43. • 943 •
tablets are recent introduction in the armoury of flea control
measures against cats and dogs; it is taken up by the female
flea during feeding and acts by inhibiting egg development.
A dose of 30 mg Lufenuron per Kg of body weight for cats
& 10 mg/Kg of body weight for dogs is ideal for flea control.
Details of insecticides used in flea control are shown in
Table - 1.
Table - 1 : Flea control measures
Insecticide
Concen-
tration
Dilution
a) For Fleas :
Propoxur 20% 1% Mix 50 ml + 950 ml water
Malathion 50% 5% Mix 100 ml + 900 ml water
Deltamethrin 2.5%0.05%Mix 20 ml + 980 ml water
b) For Animal treatment:
Propoxur 1% 1% spray/
dust
As it is
Malathion 50% 0.25% DipMix 5ml in 1 l of water
Deltamethrin
0.0025%
0.0025%
shampoo
As it is
Permethrin 1% 1%
shampoo
As it is
c) For Premises :
Pyrethrum
2% Extract
0.2% Mix 100 ml + 900 ml water
Malathion 50% 2% Mix 40 ml + 960 ml water
Chlorpyrifos 20% 0.5% Mix 25 ml + 975 ml water
(c) Rodent Control : It is an indirect method of flea control.
Though a radically effective method during non-epidemic
period, it is dangerous during epidemics, because the fleas leave
dead rats quickly and starts attacking human beings. However,
a constantly sustained campaign keeps the rodent population
down and aids significantly in keeping the flea index constantly
low. In general, during control of urban plague, insecticides to
kill rat fleas should be applied a few days earlier or at least
at same time (and not after) when rat poison baits are being
applied. The detail on rat poisons is presented in the chapter
on rodents.
(d) Personal Protection : This is achieved by the use of
protective clothing such as wearing long trousers, socks and
shoes. Use of a high charpoy with the net, and application of
repellents viz. DEET are necessary precautions while in endemic
or epidemic areas of flea borne disease. Using flea collars for
pets are effective means of keeping them free from infestation
(for 3-4 months).
Summary
Fleas are important vectors, linked with mankind since long.
Fleas can be classified into two main groups, 'combless' fleas
and the 'combed' fleas. The combs can be genal comb arranged
on the lower border of the head or pronotal combs on the thoax.
Xenopsylla species are combless fleas and include the well
known vectors of plague viz, X cheopis (Rat flea), X astia and X
braziliensis. The combed fleas are the cat fleas - Ctenocephalides
felis, the dog flea, C canis and the rat fleas of temperate
zones, Nosopsyllus fasciatus. Adult fleas are small, bilaterally
compressed, highly chitinised, wingless, 6 legged, blood
sucking ectoparasites of many warm blooded vertebrates. The
mouth parts are adapted for biting, piercing and sucking blood,
which forms the only food for both sexes. The flea undergoes a
complete metamorphosis through the successive stages of egg,
larva, pupa and adult. Eggs are laid in dark place in the host's
nest, debris, accumulation of dust, in cracks or crevices in the
floor of granaries etc. or under carpets in dwelling houses. A
temperature of 18° and 27°C and humidity about 70% favour
egg laying. The adult fleas are temporarily parasitic on their
host while their immature forms are free living. Both males
and females are haematophagous.
Flea transmits mainly the zoonoses to man, chiefly from
rodents and also from dogs and cat. The most important
microorganism that is conveyed to man from rat is Yersinia
pestis causing bubonic plague. Rickettsia typhi, causative
organism of endemic typhus, is also transmitted from its
rodent reservoir to man by the same rat fleas. Bubonic plague
is transmitted by infected fleas called ‘Blocked flea’; Endemic
typhus is transmitted through the faeces of the rat flea.
Chemical control of fleas is one of the best methods of flea
control effective against both adults & larvae. The insecticidal
treatment is either done in the form of residual sprays, dusting
or treatment of rodent burrows. In Plague susceptible areas,
control is undertaken when flea index i.e. X cheopis index
(the average number of fleas per rodent) exceeds 1. However,
during an outbreak, no rodent control activity is undertaken.
Flea control can be achieved by Indoor Residual Spraying using
Propoxur (1%), Malathion (5%) or Carbaryl (5%). Disinfestation
of pet animals like dogs and cats and use of flea collar, along
with good environmental sanitation of the household and
public places helps in flea control. Use of a high charpoy,
insecticide treated nets and application of repellents viz. DEET
are necessary precautions while in endemic or epidemic areas
of flea borne disease.
Study Exercises
Short Notes : (1) Blocked Flea (2) Flea Index.
MCQs
1) Combed flea is (a) Xenopsylla cheopis (b) Xenopsylla astia
(c) Pulex irritans (d) Ctenocephalides felis.
2) Flea transmits all except (a) Bubonic Plague (b) Endemic
typhus (c) Epidemic typhus (d) Chiggerosis.
3) The insecticides that can be used for flea control are
(a) Propoxur (b) Malathion (c) Deltamethrin (d) All.
Fill in the Blanks
4) The Rat flea lives for a duration of ______________
5) Average no. of fleas per rodent gives the ______________
True or false
6) Presence of ground vibration (caused by movement of hosts
i.e. rodents, animals, humans or earthquakes) is essential
for the emergence of adult from the pupa.
7) A partially blocked flea is more efficient in transmitting
plague as compared to a completely blocked flea.

44. • 944 •
Answers : (1) d; (2) c; (3) d (4) 06 months to 01 year; (5) Flea
Index; (6) True; (7) True.
Further Suggested Reading
Gratz NG, Brown AWA. Fleas biology and control. Geneva : World Health 1.
Organisation, 1983. WHO/VBC/83.874.
Rothschild M. Recent advances in our knowledge of the order Siphonaptera. 2.
Annual review of Entomology 1975; 20 : 241-59.
WHO. Fleas Training and Information guide. WHO, Geneva, 1985. WHO/VBC/3.
TS/85.1.
Hinkle NC, Rust MK, Reierson DA. Biorational approaches to flea 4.
(Siphonaptera : Pulicidae) suppression : present and future. Journal of
Agricultural Entomology 1997; 14: 309-21.
159Human Lice
Rina Tilak
Human lice are true ecto-parasites of man. There are three
species of human lice viz. Pediculus capitis (head louse),
Pediculus humanus (body louse) and Phthirus pubis (crab or
pubic louse). Adults (both female & male) as well as nymphs
are haematophagous, however, only body louse has been
incriminated as vector.
The head louse infests the hair on the head and may be found
in the neck region and behind the ears. The body louse infests
the hairs of chest and axilla, seams of clothing in contact with
the body and sometimes linen. The crab louse infests the hair
of the pubic region and occasionally invades eyelashes and
eyebrows.
Morphology
Lice are small (4.5mm), dorso-ventrally flattened, wingless
insects with simple metamorphosis. They are permanent
obligatory ecto-parasites living entirely on mammals. The
mouthparts are of a sucking and piercing type. They have
no eyes. The legs are short, stout, and thick with claws for
grasping hairs and fibres. The abdomen is oval or somewhat
circular in shape. In the females, the last abdominal segment
is bilobed while in the male, it is pointed from which the
aedeagus (penis) projects. Phthirus resembles Pediculus in its
general morphology, but its body is almost circular, all the three
pairs of legs of Pediculus are equal whereas in Phthirus, the
first pair is less developed. P capitis has a smaller and deeply
pigmented body, while that of P humanus is larger and non-
pigmented. Abdominal segments of P humanus are rounded
with shallower inter segmental indentations while those of
P capitis are clearly marked and deeper. Antennae and legs
of P humanus are longer and thinner than those of P capitis
(Fig. - 1 & 2).
Life History
The life histories of all the three varieties are similar
(Fig. - 3). After fertilization, the female lays eggs either on the
hairs or under clothing chiefly along the seams of the vests, pants
and shirts etc. Freshly laid eggs are white and proportionally
large for the size of the insect. They are firmly cemented to
the hair or seams of the clothing, singly or in groups. As the
embryos develop they become yellowish. The number of eggs
laid depends upon the food supply and the temperature. Under
optimum favourable conditions, the louse lays 4 to 9 eggs in
each batch. Total number of eggs laid during the life span of
4 to 5 weeks may be 300 in a body louse, 150 in a head louse
and 50 in a crab louse. Within a weeks time, the immature
stages called nymph emerge and begin sucking blood at once
and throughout their development feed frequently during the
Fig. - 1 : Head Louse Fig. - 2 : Body Louse

45. • 945 •
day and night, mostly when the host is quiet. There are three
nymphal stages and the young ones resemble the adults except
in size. It takes about 21 days between hatching of the eggs
and appearance of the adults.
Fig. - 3 : Life history of Lice
Bionomics
Lice prefer warm and moist environments; 38°C is the optimum
temperature. Higher temperature and death of the host are
detrimental and make lice leave the body of the host. The
average life of a louse is 30 to 50 days. Females live longer
than the males.
Once lice are acquired by a human host, their multiplication
depends on the neglect of personal hygiene. Following factors
are responsible for the dissemination of lice.
(a) Close contact with lousy persons; sharing the same bed
and clothing etc. In fact any prolonged crowding of human
beings in unsanitary surroundings will spread lousiness.
Hence lice and louse borne diseases are closely associated
with wars and disasters among prisoners and refugees.
(b) Indirect contact - for example exchange of beddings,
clothing, blankets, towels, hats, combs and brushes.
(c) Hair bearing eggs from lousy persons scattered in public
conveyance are picked up from the seats and cushions of
railway carriages and buses etc.
(d) Head lice easily pass from one child to another in school by
close contact while playing and also by sharing of combs.
(e) The pubic or crab lice spread through sexual contact and
sometimes from toilet seats, beds and by close personal
contact. Small children may become infested with crab lice
on their eyebrows and eyelashes from their mothers or
other close contact.
Vector Potential
Body lice are responsible for the transmission of Rickettsia
prowazeki, causing Epidemic typhus, Bartonella quintana
causing Trench fever, and Borrelia recurrentis causing
Relapsing fever. The presence of lice on any part of the body
is termed ‘pediculosis’ which causes irritation with loss of
sleep and scratching which may lead to secondary infections.
Toxic reactions to the saliva injected into the skin may lead to
weariness and a general feeling of illness. The skin of a heavily
louse infested person becomes hardened and deeply pigmented
and results in a condition known as ‘Vagabonds’ disease or
melanoderma.
Prevention and Control
Regular washing of hair with soap and warm water and combing
with lice comb may prevent/reduce head lice infestation.
Similarly, regular washing of clothes with hot water (more
than 60°C) and ironing and changing of clothes prevents body
louse infestations.
In the past, use of anti louse powder (10% DDT) was used for
reduction of infestation in a controlled community by dusting
the lousy individuals and garments. Currently, the insecticides
of choice are Permethrin dust (0.5%), Propoxur dust (1.0%);
for mass treatments against body louse, dusts should be
applied through neck openings, up sleeves and from all sides
of the loosened waist of trousers. Socks, head coverings, the
inner surfaces of extra garments and bedding should also
be treated. Treatment of clothing with synthetic pyrethroid
or use of pretreated uniforms may prove an effective means
of prevention of body lice infestation amongst Armed Forces
personnel during war.
Shampoo formulations like Phenothrin (0.2-0.4%), Permethrin
(1%) and Malathion (5%) lotion are ideally used for head lice
infestation. For application on hair, the hair of the infested
persons should be wetted thoroughly before application. The
insecticidal shampoo is thoroughly massaged on the head and
left for minimum 10 min. Thereafter, the shampoo is rinsed off
from the hair, hair is towel dried and combed with lice comb to
remove dead/ stunned lice.
Summary
Human lice are true ecto-parasites of man. There are three
species of human lice viz. Pediculus capitis (head lice),
Pediculus humanus (body louse) and Phthirus pubis (crab or
pubic). They are obligatory ecto-parasites living entirely on
mammals. After fertilization, the female lays eggs either on
the hairs or under clothing chiefly along the seams of the vests,
pants and shirts etc. The number of eggs laid depends upon the
food supply and the temperature (38°C). It takes about 18 days
between hatching of the eggs and appearance of the adults.
Dissemination of lice occurs with close contact, in unsanitary
surroundings, exchange of beddings, hats, comb & brushes.
Transmission occurs through public conveyance, in school &
through sexual contact.
Body lice are responsible for the transmission of Rickettsia
prowazeki, causing Epidemic typhus, Bartonella quintana
causing Trench fever, and Borrelia recurrentis causing Relapsing
fever. Insecticides of choice are Permethrin dust (0.5%), Propoxur

46. • 946 •
dust (1.0%) and insecticide treated uniforms for body louse and
shampoo formulations like Phenothrin (0.2-0.4%), Permethrin
(1%) and Malathion (5%) for head lice infestation.
Study Exercises
MCQs & Exercises
1) Lice cause all of the following except (a) Epidemic Typhus
(b) Trench fever (c) Relapsing fever (d) Endemic Typhus
2) Body louse infests (a) Hair on chest & axilla (b) Seams of
clothing (c) Linen (d) All
Fill in the Blanks :
3) Presence of lice on any part of the body is called as _____
4) Dust application through neck openings, up sleeves &
trousers is used for _________ against __________ louse.
Answers : (1) d; (2) d; (3) Pediculosis; (4) Mass treatment,
body.
Further Suggested Reading
Vector control : Methods for use by individuals and communities, Jan 1.
A.Rozendaal World Health Organization Geneva 1997.
Birendra Nath Ghosh; A treatise on hygiene and public health, 14th edition, 2.
1965, Calcutta scientific publishing co.
Gratz, N. Human lice, their prevalence and resistance to insecticides. 1998. 3.
Geneva : WHO.
Chunge RN, Scott FE, Underwood JE, et al. A review of the epidemiology, 4.
public health importance, treatment and control of head lice. Can J Public
Health 1991; 82 : 196-200.
Reeves JR. Head lice and scabies in children. Pediatr Infect Dis J 1987; 6 : 5.
598-602.
Bachok N, Nordin RB, Awang CW, Ibrahim NA, Naing L Prevalence and 6.
associated factors of head lice infestation among primary schoolchildren
in Kelantan, Malaysia. Southeast Asian J Trop Med Public Health. 2006;
37(3):536-43.
160Sand Flies
Rina Tilak
Sand-flies bite humans and transmit diseases to them. There
are about 700 species of sand-flies of which only 70 species
have been incriminated as vectors so far. The sand-flies belong
to the subfamily Phlebotominae of the family Psychodidae.
The haematophagous species belong to the three genera of
Phlebotomus, Lutzomyia and Sergentomyiay, the former two
being more important as vectors of diseases. The medically
important Phlebotomus vector species in India include
P argentipes, P papatasii, P sergenti and P braziliensis.
Morphology
The adult sand-fly is a small, greyish yellow to brown insect
and about 1.5 to 4.0 mm in size. The insect is typically
characterized by large conspicuous eyes and stilt like legs. The
entire body is densely covered with hair. The antennae are long
filamentous and give a beaded appearance. The mouth parts
are very short and are adapted for biting and piercing in the
females. The thorax is markedly humped and bears a pair of
lanceolate wings which are held erect over the body when the
fly is at rest. The wings are densely hairy and the second vein
branches twice, the first branching in the centre of the wing
and second at the margin. Legs are long, slender and used for
hopping as sand-flies are poor fliers. The abdomen consists of
10 segments; the last two are modified for sexual functions.
The abdomen of a female is rounded posteriorly; in the males,
it is modified and bears claspers (Fig. - 1).
Life History
Fig. - 1 : Sand Fly - Morphology and Life Cycle
Egg Larva Pupa
Wing
Venation
Female
Male
The sand-flies prefer to breed in dark places rich in organic
matter and moisture. Sand-flies lay torpedo shaped eggs in small
batches which hatch out in one or two weeks under optimum

47. • 947 •
favourable conditions. The larvae are legless and whitish with
a dark head capsule and pass through four instars. The larvae
feed on organic excrement of lizards and mammals and other
decaying material. Its life span is from 2 to 6 weeks, depending
on the temperature and humidity. The larva bears two anal
spines. The pupa is golden brown in colour and naked and
requires about 10 days for development after which the adult
emerges. Male sand flies emerge about 24 h before females,
for their external genitalia to rotate 180° for achieving the
correct position for mating before the females have emerged.
The total period required from egg to the adult stage is about 4
weeks under favourable conditions. In the tropics, the breeding
goes on throughout the year. In north India, they appear about
the middle of March and persist until November, with their
maximum density in March and April.
Bionomics
The sandflies live entirely on plant juices or similar fluids from
other available sources but the females need a blood meal
in order to develop eggs. Phlebotomines are crepuscular or
nocturnal biters and most of the biting occurs outdoors with
only a few species feeding indoors. The adults are weak fliers
and generally confine themselves up to 50 yd from their breeding
place and are not found resting beyond 3 ft on the wall. After
fertilization and a blood meal, the female lays eggs in shady,
damp and warm places with sufficient supply of organic matter
such as insect remnants and faeces and excrements of tiny
animals which form the future larval food. Such conditions
are found under stones, in stables and poultry houses, around
soakage pits, grease traps and water sinks, in hollowed trees
and rodent burrows, bases of walls and embankments. Large
population of sandflies can build up in dwellings where cattle
are kept at night; the cattle provide an abundant source of
blood, while the stables and houses provide suitable resting
place.
Vector Potential
Sand-flies are responsible for the transmission of various
species of Leishmania causing Kala-azar or Visceral
leishmaniasis, Oriental sore or Cutaneous leishmaniasis,
and Espundia or Mucocutaneous leishmniasis (naso-oral).
Sand-flies also transmit the virus of sand-fly fever, also
known as papatasi or Phlebotomus fever or 3 day fever and
the re-emerging viral disease - Chandipura disease. It also
transmits Bartonella bacilliformis or Oraya fever also known
as Bartonellosis or Carrion’s disease. In addition, the sand-
flies have biting nuisance causing skin reactions (Herara) in
sensitized persons.
Phlebotomus Control
(a) Prevention of Breeding : This is primarily achieved by
good environmental tidiness. Places providing humidity,
darkness and organic matter should be dealt with by removing
all collections of rubble and heaps of rubbish; obliterating all
cracks and fissures in the floors of the buildings and indoor
constructions, sides of culverts, gutters, nullahs, cattle sheds
and poultry houses which are common breeding places for
sand-flies. Cracks and holes in the walls up to a metre from the
ground should be sealed by plastering and the earthen floor of
cattle sheds should be rammed down and made hard to make
it difficult for the larvae to burrow. Empty buildings should be
kept in good repairs; soak pits and grease traps should be well
maintained.
(b) Anti-larval Measures : Anti larval measures are generally
difficult to undertake as identification of larval breeding sites
is difficult. Even if insecticidal control is planned, it has been
found to be of little importance in the control of sand-flies.
(c) Anti-adult Measures : Anti adult measures are based on the
principle that sand-flies make short flights with relatively long
pauses on entering or leaving any place or shelter. Therefore,
any surface treated with residual insecticide on which the flies
rest will have a lethal impact. The anti adult measures are the
same as followed under the National Programme for Indoor
residual spraying against mosquitoes; this strategy has proved
to have a dramatic impact on the density of sand-flies in the
area. If outdoor resting sites have been identified, they can also
be sprayed with residual insecticides. Outdoor fogging may
provide additional benefit in reduction of Sand-fly density.
(d) Personal Protection : Use of repellents viz. DEET is one of
the most efficient methods of preventing bites from sand-flies;
the repellents may be applied topically or sprayed on clothes.
A sand-fly net is useful, but it reduces air movements and
causes great discomfort. Use of insecticide treated mosquito
nets has been found very effective in protecting against bites of
sand-flies. Personal protection may also be achieved by barrier
clothing.
(e) Other Measures: These include encouragement of gardening
(cultivation of ground) and planning of embankments with
native aromatic plants. Free cross ventilation and ingress of
sunlight keeps the sand-fly out of habitations or animal sheds.
Electric fans are useful as the air current drives them away.
Electric light shades smeared with Vaseline, traps a large
number of sand-flies. Siting of human habitation beyond 50
yards of the breeding place is an effective method of preventing
transmission of sand-fly borne diseases as also sleeping on
cots.
(f) Treatment of Animals: Earlier practice of culling of dogs
or killing of rodents is no more undertaken. Dogs are treated
by dipping in insecticide solution (Deltamethrin 50 ppm) or
applying insecticide solution (1-2 ml of 65% Permethrin or
Imidacloprid 10%). Even insecticide treated dog collars and
treatment of non-reservoir animals reduces transmission of
Leishmaniasis.
Summary
Sand-flies belong to the subfamily Phlebotominae, bite humans
and transmit diseases to them. The medically important vector
species in India include P argentipes, P papatasii, P sergenti
and P braziliensis. The adult sand-fly is a small, greyish yellow
to brown insect about 1.5 to 4.0 mm in size; body is densely
covered with hair. Sandflies possess a pair of lanceolate wings.
The wings are densely hairy and the second vein branches
twice. Sand-flies prefer to breed in dark places rich in organic
matter and moisture. Females have piercing mouth parts and
are blood suckers. The males live entirely on plant juices or
similar fluids from other available sources. After fertilization

48. • 948 •
and a blood meal, the female lays eggs in shady, damp and
warm places with sufficient supply of organic matter such as
insect remnants and faeces.
Prevention of breeding is achieved by good environmental
tidiness. All collections of rubbish should be removed from
places providing humidity, darkness and organic matter.
Cracks and holes should be sealed. Earthen floor of cattle sheds
should be rammed down and made hard to make it difficult
for the larvae to burrow. The anti adult measures are the same
as followed under the National Programme for Indoor residual
spraying against mosquitoes. Use of repellents like DEET is one
of the most efficient methods of preventing bites from sand-
flies. Other methods like treated nets or barrier clothing may be
used. Free cross ventilation and ingress of sunlight keeps the
sandflies out of habitations or animal sheds. Study Exercises
MCQs & Exercises
1) The vein which branches twice in a sandfly wing is (a) 4 (b)
5 (c) 6 (d) 2
2) Sandfly eggs can be laid in (a) Stables (b) Hollow trees
(c) Rodent burrows (d) All of these
3) Sandfly does not transmit (a) Kala-azar (b) Oriental sore
(c) Chikungunya (d) Sandfly fever
4) Which of these preventive measures is difficult to undertake
(a) Prevention of breeding (b) Antilarval measures (c) Anti
adult measures (d) Personal protection
Fill in the Blanks
5) The sand-flies belong to the subfamily ______________
6) The males of sandfly live entirely on _______________
Answers : (1) d; (2) d; (3) c; (4) b; (5) Phlebotominae (6) Plant
juices.
161Some Annoying Pests
Rina Tilak
Simulium Flies
Simulium flies are commonly known as Black flies. They have
a world wide distribution. They belong to the family Simulidae,
which contains over 1800 species; however, the important
genera are only four which bite humans and out of these four
genera, Simulium is the most important. The members of
this family are found from sea level to a height of 2000 m. In
India, these have been reported from Kumaon Hills, Himachal
Pradesh, Kashmir, Assam, Arunachal Pradesh, Manipur,
Nagaland, Bengal, Bihar, Maharashtra, Tamil Nadu and Nilgiri
Hills. In Arunachal Pradesh, they are known as Dimdam flies.
Life History
The adults are small; stout bodied with a humped thorax and
blood sucking flies varying in length from 1 to 5 mm. The
colour varies from dark amber to bright yellow to orange. The
name “black fly” is therefore, a misnomer.
Simulium flies breed in fast flowing turbulent mountain
streams and torrents because they require well aerated water.
A female lives for 2 to 3 months and lays several batches of
eggs under the rocks, stones, vegetation or debris submerged
just below the water surface (Fig. - 1). Eggs hatch in one to two
days; in temperate zones the hatching period may be one week.
The larvae are attached to submerged objects with their heads
downstream. They feed on microscopic animals and plants and
do not swim. The larva undergoes six moults in about 10 to
14 days in the tropics and 3 to 4 weeks in temperate regions.
The silken pupa is also firmly anchored to the substratum.
The pupal stage may extend from 4 to 5 days to 2 to 3 weeks.
The imago emerges from the submerged pupal case and comes
on to the water surface where it rests for a while and after
sometime starts flying. In the tropics and subtropics breeding
is continuous throughout the year. The life span of the adult is
about three months.
Fig. - 1 : Black Fly
Bionomics
Simulids are strong fliers. Their normal flight range is about
4 to 5 km. Flights up to 20 to 40 km with favourable wind
are not unusual. Both male and female Simulids feed on plant
juices, nectar and pollens of flowers; the females however,

49. • 949 •
require blood meal for development of eggs and are voracious
and persistent biters. They may enter through any opening in
the clothing such as sleeves or through the lower opening of
trousers for biting. They bite only by day in the open and are
especially active on bright sunny days and retire at night to the
neighbouring vegetation where the females mature their eggs.
Vector Potential
Several species of simulids are known vectors of Onchocerciasis,
a filarial disease due to Onchocerca volvulus occurring in tropical
Africa, Central America and Venezuela, where S damnosum,
S mettallicum and S neavei are the vectors. In India, Simulids
however, are not vectors of any known human disease. But the
very annoying and persistent attacks in large numbers make
working in the open virtually impossible. The immediate trauma
caused by its bite produces a red haemorrhagic spot leading to
papule formation. In certain cases, it may lead to secondary
infections and ulcers like ‘ulcus tropicum.’ In sensitized
persons, allergic reactions like lymphangitis, lymphadenitis,
rhinitis and fever may occur. Their bites are responsible for loss
of livestock.
Control Measures
Control of the biting flies has been achieved by use of
larvicides and aerosol treatment. In the Onchocerciasis control
programme in Africa, Temephos 200 g/l emulsion has been
used as a larvicide with good results. BTI has also been used
@ 0.54 - 0.72 l/m
2
with great success. Aerosols and fogs
produced by fogging machines are useful in killing adult
flies. Clearing of vegetation around the perimeter also reduces
Dimdam fly nuisance. Other compounds like Permethrin and
Etofenprox have also been evaluated and found effective. Use
of protective clothing will prevent the flies from ascending
up the sleeves and trousers or entering into the shirt front.
Socks should be pulled over the bottom of trousers. Additional
protection may be obtained by treating the clothing and the
exposed parts of the body with any of the repellents such as
Dibutyl phthalate (DBP), Diethyl toluamide (DEET) or Diethyl
phenyl acetamide (DEPA).
Bugs
Bugs (Order Hemiptera) have been associated with man since
antiquity. They have a world wide distribution and consist
of two important families’ viz. Cimicidae and Reduviidae.
Family Cimicidae includes the ‘bed bugs’, Cimex lectularius
of temperate regions and C rotundatus or C hemipterus of the
tropics. Family Reduviidae includes the cone nose Triatomine
bugs, also known as ‘kissing’ or Assasin bugs. In India, bedbugs
have a great nuisance value.
Morphology
Bed bugs are small, 5 to 6 mm long, dorso-ventrally flattened,
wingless, dark brown insects with a mahogany tint (Fig. - 2).
They have a very short and broad head attached to the thorax.
The head bears antennae and a pair of well developed eyes. On
either side of the thorax, the stink-glands are situated which
give off the nasty, pungent or offensive odour associated with
this group.
Fig. - 2 : Bed Bug - Adult
Life History
A bed bug passes through egg stage and 4 nymphal stages.
The fertilized females lay flask shaped, operculated eggs singly
in hidden sites, such as cracks and crevices in the walls and
floorings, spaces in the wood work of furniture, behind pictures,
mattresses, pillows etc. A female lays 2 to 10 eggs a day with
a total up to 200 to 300 in her life time of 6 to 8 months.
The eggs usually take 5 to 10 days to hatch. The nymph starts
feeding within an hour or two after emergence and continues
to feed intermittently in all the further stages of development.
There are four nymphal stages, each lasting 6 to 7 days; at
the end of each, a skin is cast off. It takes 4 to 6 weeks for the
development from egg stage to adult.
Bionomics
Adults can subsist without food for months under favourable
conditions. Bugs are disseminated through travelling bags,
laundry, furniture, bedding, old charpoys, soiled clothing,
infested household goods, public conveyance and public places.
Bed bugs like lice have been companions of man for centuries.
Hiding in cracks and crevices during the day, they become
active during the night and come out of their hiding places to
feed on hosts and engorge completely in 3-6 min. They may
travel long distances for sucking blood. They are gregarious,
occurring in great assemblages. All stages are parasitic and
thrive on human blood.
Medical Importance
Bedbugs have all along been suspected for the transmission of
various diseases but so far have not been incriminated for any
human disease. They are of public health importance primarily
for their biting nuisance and demoralizing effect as their
infestation may cause insomnia and pruritis / dermatitis.

50. • 950 •
Prevention and Control
The first and foremost principle for the prevention of bedbug
infestation is to maintain a very high standard of hygiene.
All furnitures and belongings of new occupants should be
thoroughly checked for the presence of bed bugs and immediate
measures taken to prevent their multiplication by one of the
appropriate insecticides. Residual insecticides applied directly
into the hiding places control the bedbugs. Solution of Malathion
@2% or Chlorpyrifos @0.5% may be used. Disinfestations
of blankets, beddings, mattresses and mosquito nets may
be carried out by subjecting them to heat at or above 70°C.
Synthetic pyrethroids like Bifenthrin (0.096%), Permethrin
(0.125%), Cyfluthrin (0.04%) and Deltamethrin (0.03%) can
also be used to achieve optimum results. Residual insecticidal
spraying for malaria control undertaken systematically and
methodically will also help in reducing the density of bed bugs
as a collateral benefit. Insecticide treated bed-nets will also
help in reducing the menace of bed bugs.
Debugging : The bed (Bed stand) need not be inclined against
the wall nor the coir netting loosened. The cots should be
thoroughly treated on all sides with insecticidal spray. All
cracks and crevices should be fully flooded. The chairs, tables
and other items of furniture may be similarly treated. The
insecticide formulation may also be directly applied to the
hiding places such as joints, cracks and crevices in the cots/
chairs/tables and folds or creases in the mattresses and other
items of beddings.
Slow drip technique involving the use of the common two
inches thick paint brush for treatment of the infested cots and
other items of furniture is reportedly superior as compared to
the routine method of spraying with compression sprayer. In
this technique, the ready to use solution of insecticide in water
is taken in a plastic mug of one litre capacity, a paint brush
is dipped in the solution, and the solution so lifted is slowly
drained into the cracks and crevices as well as the joint spaces
from different directions. The process is repeated by turning the
cot upside down so that all such hiding places are thoroughly
flooded with insecticide.
Cockroaches
One of the most annoying pests encountered in an urban
area are the Cockroaches. The common domestic species
which infest buildings are Blatella germanica, the German
roach; Periplaneta americana, the American roach and Blatta
orientalis, the Oriental roach (Fig. - 3). The German cockroach,
although a native of Europe, is the most widely distributed
species.
Morphology
Cockroaches are dorso-ventrally flattened creatures with colour
varying from dark brown to black. The head is flexed backward
and the antennae are filiform. Most of the species have two
pairs of wings. In some of them the wings are vestigial. In the
oriental cockroach, the wings are short in the females but much
developed in the males which possess the power of flight.
Life History
They have simple metamorphosis and lay 16 to 48 capsulated
eggs depending on the cockroach species (Fig. - 4). The eggs
hatch out in 2-6 months in most of the species, depending on
temperature and humidity. The young ones are almost white
and wingless. They moult a number of times and the total
developmental period may be 6 months to 1 year. They may
produce three generations in a year and usually have a long
life span.
Fig. - 3 : Cockroaches - Common Domestic Species
Periplanata
americana
(American
Cockroach)
Blatella
germanica
(German
Cockroach)
Blatta
orientalis
(Oriental
Cockroach)
Fig. - 4 : Egg cases of the domestic cockroaches
Bionomics
They breed in warm moist places in the humid microclimate
of the kitchen and pantry, laying eggs in cracks, crevices and
sinks. They can run swiftly by means of long well developed
legs. They are highly gregarious and primarily nocturnal in
habit, but may be seen during the day as well. The mouth parts
are adapted for biting and chewing and they are omnivorous,
feeding on any material meant for human consumption like
meat, milk, grains and sugar.
Disease Potential
They are filthy, annoying pests imparting a nauseating
‘cockroach’ odour to the food articles and utensils they come
in contact with and the places they infest. They destroy food,
damage fabrics, books and other household articles. They
may enter houses and other buildings from outdoors through
infested containers or from adjoining rooms and apartments
or through drains. On account of their indiscriminate roaming
and feeding habits, they mechanically spread diseases like
cholera, typhoid, dysentery, protozoal cysts, intestinal worms
etc. by polluting food with infective material carried on their
legs and bodies.

51. • 951 •
Control Measures
(a) Prevention : This includes :
(i) Good house-keeping is the key to cockroach control,
whether in the home, restaurant, hotel or grocery stores.
(ii) All cracks and crevices should be properly filled up.
(iii) All areas should be kept thoroughly clean so that no food
particles, debris, dust and rubbish remain to support and
nourish cockroaches.
(iv) Keeping surveillance on the occurrence or increase in the
density of cockroaches in a house by use of sticky traps or
else by use of visual assessment method, whereby light is
switched on late in night and the cockroaches counted for a
stipulated time period, say five minutes. This method also
indicates the hiding places in a room of the cockroaches
besides indicating the level of infestation.
(b) Control : Cockroach infestation can be controlled with
insecticidal sprays, dusts or baits. The insecticide should be
applied thoroughly to runways, cracks, crevices, undersides of
tables and even under the table spreads, rear of sinks, meat
safes and other harbourage areas. Use of 2-5% dust or 1-3%
solution or emulsion of Organophosphorous compounds like
Malathion or Carbamate insecticide such as Propoxur gives
excellent results. To obtain a quick effect in heavy infestations
or to drive them out from the hiding places, a direct spray
containing 0.3% Pyrethrum or 0.5 to 1.0% DDVP or Fenitrothion
may be used. Small pills of flour containing boric powder left on
dining table, food safes and pantry boards or under table cloth
also kill cockroaches. Abermectin and Synthetic pyrethroids
(Table - 1) are currently being used for control. Newer
insecticides, Fipronil and Imidacloprid Gel have been found to
be very effective in controlling cockroaches. Fipronil has been
demonstrated to have “cascade effect” (secondary killing of
cockroaches due to necrophagy amongst them, whereby they
consume Fipronil killed cockroaches and get killed in turn).
It is important to remember that chemical control gives only
temporary results and maximum efforts should be made to
improve the environmental sanitation and housing conditions.
Moreover, there are reports that the German cockroach has
become resistant to several Organo-chlorine, Organo-phosphate,
Carbamate & Pyrethroid insecticides. The insecticides used for
cockroach control are summarised in Table -1.
Table - 1 : Insecticides for Cockroach control
InsecticideConcentration Use
Boric acid 100%
Baits/ sprinkle
along corners
Imidacloprid Gel1.85 - 2.15%Cracks and crevices
Fipronil Gel 0.01 - 0.03%
Cracks and crevices
Has a cascade effect
Fenitrothion 1 - 2% Spray
Malathion 3% Spray
Cyphenothrin 0.5% Spray
Deltamethrin 0.03% Spray
Summary
Simulium flies have worldwide distribution & are commonly
known as Blackflies; in Arunachal Pradesh, they are known as
Dimdam flies. As they require well aerated water, they breed
in fast flowing turbulent mountain streams. The life span of
the adult is about three months. The colour varies from dark
amber to bright yellow to orange. Male and female Simulids
feed on plant juices; the females require blood meal for
development of eggs. They can enter through any opening in
the clothing and bite during the day. Simulids are vectors for
Onchocerciasis in some parts of the world; however, they do
not cause any disease in India. Control can be achieved by the
use of larvicides like temephos & BTI. Use of aerosols & fog
with clearing of vegetation is other means of achieving control,
besides protective clothing & use of repellants.
Bugs have a world wide distribution and consist of two
important families’ viz. Cimicidae (bed bugs) and Reduviidae
(kissing bugs). Bed bugs are small, 5-6 mm long, dorso-ventrally
flattened, wingless, dark brown insects. The nasty pungent
odour which this group emanates is because of the presence
of stink glands. Metamorphosis is incomplete with presence
of egg stage & 4 nymphal stages. Females lay flask shaped,
operculated eggs singly in hidden sites. It takes 4 to 6 weeks for
the development from egg stage to adult. Bugs can stay without
food for months & can travel long distances through traveling
bags, laundry, furniture, bedding, old charpoys, soiled clothing
etc. Bugs so far have not been incriminated for any human
disease; though have been suspected for transmitting diseases.
Infestation though can cause insomnia, pruritis, dermatitis
etc. Prevention includes maintenance of hygiene, thorough
check of furniture & belongings. Insecticides viz. Malathion
@2% or Chlorpyrifos @0.5% etc. may be used. Disinfestations
of blankets, beddings, mattresses and mosquito nets may be
carried out. Debugging can be used for treatment of the cots
(charpoy).
Cockroaches are one of the most annoying pests encountered
in an urban area. The common domestic species are Blatella
germanica (infest buildings), Periplaneta americana & Blatta
orientalis. Cockroaches are dorso-ventrally flattened with
colour varying from dark brown to black. They breed in warm
moist places (humid microclimate of the kitchen), laying eggs
in cracks, crevices and sinks. Feed on any material meant for
human consumption, their mouth parts are adapted for biting
& chewing. They destroy food, damage fabrics, books and
other household articles. Cockroaches spread diseases like
cholera, typhoid, dysentery, protozoal cysts, intestinal worms
etc. by polluting food with infective material carried on their
legs and bodies. Prevention comprises of good housekeeping,
filling up of all cracks & crevices. Infestation can be controlled
with insecticidal sprays (Malathion or Propoxur), dusts or
baits. Spray containing 0.3% Pyrethrum or 0.5 to 1.0% DDVP
brings about quick effect. Newer insecticides, Fipronil and
Imidacloprid have been found to be very effective in controlling
cockroaches.

52. • 952 •
Study Exercises
MCQs & Exercises
1) Cimex lectularius & C rotundatus consist of two important
families of (a) Simulium flies (b) Bugs (c) Cockroaches
(d) None.
2) Which of the following in relation to bugs is not correct
(a) Head bears antennae (b) Pair of well developed eyes
(c) 5 nymphal stages (d) Flask shaped operculated eggs.
3) Prevention & control of bed bugs can be achieved by
all except (a) Malathion @2% (b) Chlorpyrifos @ 0.5%
(c) Debugging of bed stands (d) Fipronil Gel.
4) Sticky traps and Visual assessment method are used for
surveillance of (a) Bed bugs (b) Cockroaches (c) Simulium
(d) All of these.
Fill in the Blanks
5) Simulium flies are known vectors of _______________
6) Nasty, pungent or offensive odour associated with bugs
due to the presence of _______________
7) _______________ gel is an effective insecticide against
cockroaches and has a cascade effect.
True or False
8) Black flies are black in colour.
9) A female bed bug lays 2 to 10 eggs a day with a total up to
200 to 300 in her life time of 6 to 8 months.
10) The mouth parts in cockroaches are adapted for biting and
chewing.
Answers : (1) b; (2) c; (3) d; (4) b; (5) Onchocerciasis; (6) Stink
glands; (7) Fipronil; (8) False; (9) True; (10) True.
162Envenomizing Pests
Rina Tilak
Scorpions
Scorpions are one of the commonly encountered venomous
arthropods of the class Arachnida (Fig. - 1). At least 1000
species of scorpions have been described, but only 20 species
are of medical importance. The last segment of their bodies is
modified to form a flexible tail, with a vesicle holding poison
gland and a sharp spine. They vary in size from about 2 to
20 cm and are cryptozoic and nocturnal, spending the day
concealed under stones or fallen tree branches or in burrows
and venturing out after sunset in search of food. The common
Indian species belongs to the genera Buthus (Mesobuthus) and
Palamnoeus; the former are more poisonous.
Fig. - 1 : Scorpion
Scorpion sting as a rule is not more dangerous than bee or
wasp sting as the chemical nature of the poison is similar to
formic acid. It is, however, much more painful, and if sufficient
poison has been injected, may cause distressing symptoms
which may take twenty four hours to pass off. Stings of red
scorpion (Mesobuthus tamulus) can be serious with massive
release of catecholamines, producing raised BP, arrhythmias,
cardiac failure and pulmonary oedema. Profuse sweating,
dilated pupils and priapism can occur.
Box - 1 : Prevention of Scorpion Sting
Do not encroach their hiding places especially if ill equipped
(i.e. barefoot or wearing loose / open sandals in areas with
loose stones, fallen debris etc.)
Scorpions are active at nights. Always carry a torch while
moving in infested areas at night.
Erect barrier up to 20 cm by means of tiles at the base of
walls and steps to prevent scorpion encroachment.
Fill cracks and crevices to deny hiding places.
In infested areas, people should sleep with mosquito nets
properly tucked. It is advisable to use treated bed nets.
Clear all junk and rubbish from around the house.
Shake the shoes / clothes well before putting them on.
Lastly use chemicals if problem still remains unresolved.
A Scorpion sting case needs immediate medical attention. A
local anaesthetic (1% Lignocaine) at the site of sting or a
strong oral pain killer is advised. For stings with less pain,
ice therapy works well.
There is no commercially available antivenin for treatment
of Mesobuthus tamulus stings in India. The effects are more
marked in children, it is, however, very rare that a fatal dose
of the venom is injected. If the sting is on the extremities, an
immediate ligature may be helpful. Application of a strong

53. • 953 •
solution of ammonia relieves pain in a majority of cases; a series
of injections of 1% Novocaine or Lignocaine and Adrenaline
at the spot and along the nerve may be necessary in others.
Barbiturates in large doses are useful in reducing restlessness.
Patients developing priapism, dilated pupils, sweating and
bradycardia may require early energetic treatment with
vasodilators. Preventive measures include alertness in avoiding
contact with scorpions in infested areas, putting on clothes
and shoes after shaking them well and proper housekeeping.
Propoxur 2% or Chlorpyrifos @ 0.2-0.5% may be used. Synthetic
pyrethroids are generally not used as they irritate the scorpions
and risk is increased further.
Ants
Ants are common annoying insects. They have also been
experimentally incriminated in the mechanical transmission
of excremental infections. They should therefore be kept away
from foodstuffs by placing the legs of food safes, tables etc.
in anti-formicas viz. bowls or tins containing water or waste
crude oil. Insecticidal sprays like Pyrethrum or Malathion are
effective. Ordinarily, the ant-bite causes only a sharp stinging;
the bites of some of the larger ants may be very painful involving
faintness and shivering. Dilute ammonia or any other alkaline
solution applied relieves the pain.
Bees, Wasps and Hornets
In bees and wasps, venom is produced in glands at the
posterior end of the abdomen and is expelled by contraction
of muscles of the venom sac, which has a capacity of up to
0.1 ml. Uncomplicated stings cause immediate pain, a wheal-
and-flare reaction and local edema and swelling that subside
in a few hours. In sensitized individuals there may be alarming
symptoms.
Honeybees often lose their stinging apparatus and the attached
venom sac in the act of stinging and subsequently die, it should
be removed gently by pulling it out, care being taken not to
squeeze the venom in the wound. The site should be cleansed
and disinfected and ice packs applied to slow the spread of
venom. Elevation of the affected site and administration of
analgesics, oral antihistamines and topical calamine lotion
relieve symptoms. Local application of an alkaline solution of
sodium bicarbonate or ammonia or soap and applying pressure
with a moistened piece of lint are useful in relieving pain.
Papain diluted roughly 1:5 with tap water is said to produce
immediate relief of pain. Disturbing the honey-comb may bring
the whole swarm on the person responsible or any one who
happens to be in the vicinity. Their stings may cause serious
allergic symptoms requiring adrenaline and/or morphine
administration. Wasp’s nests should not be destroyed in the
daytime; they are generally destroyed at night when all wasps
are inside. Insecticidal treatment includes use of 1% Dichlorvos,
0.015% Deltamethrin or 0.5% Chlorpyrifos.
Centipedes
Centipedes (Myriapoda) possess a pair of legs to each apparent
segment of the body; the first pair is modified to form poison
claws. The bites of small centipedes’ gives rise to mild local
inflammation but the larger centipede Scolopendra gigantia
may cause a severe painful bite with marked local and general
reaction (usually swelling, erythema and lymphangitis;
dizziness, nausea and anxiety are occasionally described and
rhabdomyolysis and renal failure have also been reported).
Solution of ammonia is useful for local application and in bites
of the larger centipedes morphia may be necessary to allay the
pain. 2-4% Malathion as spot application is very useful.
Leeches
Leeches (Hirudinea) are a class of annelid worms that attach to
their hosts with chitinous cutting jaws and draw blood through
muscular suckers. They are particularly troublesome near
streams and rivers, in leafy forests and marshy jungles.
The two important species are:
Haemadipsa zeylanica●● , which is a small land leech, about
2.5 cm long with great power of penetration into the
interstices of clothing, putties or laced boots. They often
drop from tree leaves onto man or animals passing by and
suck blood.
The other variety is ●● Limnatis nilotica - the large aquatic
leech which on being ingested, fastens itself to the mucus
lining of the mouth, pharynx, larynx or nasal cavities
of man or animal producing prolonged bleeding unless
removed. Leech bites are painless but the bleeding may be
prolonged (after the leech has detached) due to a powerful
anti-coagulant, hirudinin, present in its saliva.
Gum boots or jungle boots are very effective in protecting
from leech bite. A frequent search of the body for the presence
of leeches should be made. The leech should not be dragged
or pulled off the skin because of the risk of breaking and
leaving behind its suction apparatus which is liable to cause
inflammation and suppuration. Salt, vinegar or a tobacco
infusion application or a touch of the lighted end of a cigarette
induces the leech to relinquish its hold; tincture of iodine
should be applied to the bitten spot and a piece of adhesive
plaster may be applied on it. Internally attached leeches may
detach on exposure to gargled saline or may be removed by
forceps. Repellents DEET & DEPA can be used to provide
protection. The repellents can be applied on to the clothing as
well as topically over the skin. At night a properly adjusted
mosquito net, preferably insecticide treated bed net provides
good protection. Aquatic leeches can be removed from drinking
water by filtering through a sieve or a piece of muslin.
Spiders
There are more than 30,000 recognized species of spiders,
however, only about 100 can defend themselves aggressively
and have fangs sufficiently long to penetrate human skin. The
true spiders (Arachnida) have poison glands and inject venom
into their prey. The common species of spiders as a rule do not
bite man. If by chance it happens to bite, the bite amounts to
no more than a pin prick. Envenomations of the brown or fiddle
spiders (Loxosceles species) and widow spiders (Latrodectus
species) may be life-threatening. Some spiders, especially those
belonging to the genus Latrodectus produce severe effects in
man. Important species are L hasselti, the ‘red-backed’ spider
and L mactans the black widow and the allied species. The
acute symptoms generally subside after a few days, but pain
may persist for some time. In Latrodectus bites, the death rate

54. • 954 •
may amount to 6% or higher.
Identification of the offending spider should be attempted,
both because specific treatments exist for bites of spiders.
Initial management includes local cleansing, application of
sterile dressings and cold compresses, and elevation and loose
immobilization of the affected limb. Analgesics, antihistamines,
antibiotics, and tetanus prophylaxis should be administered
if indicated. Use of sedative is contraindicated. Intra venous
administration of widely available equine antivenom rapidly
relieves pain and can be life-saving. Because of the risk of
anaphylaxis and serum sickness, antivenom should be reserved
for severe cases involving respiratory arrest, uncontrollable
hypertension, seizures, or pregnancy. Intravenous Calcium
gluconate or Magnesium sulphate also gives dramatic relief
to cramps. Spot (Infested area) treatment with 3% Malathion,
0.03% Deltamethrin, Chlorpyrifos @ 0.2-0.5% has been found
to be effective against spiders.
Summary
Scorpions are one of the commonly encountered venomous
arthropods of the class Arachnida. The common Indian species
belongs to the genera Buthus (Mesobuthus) and Palamnoeus.
Scorpion sting (chemically similar to formic acid) is much more
painful than bee or wasp sting, though not more dangerous.
Hypertension, arrhythmia, cardiac failure and pulmonary
oedema may be encountered following stings of Mesobuthus
tamulus (Red scorpion). Prevention includes avoiding hiding
place of scorpions especially when ill equipped, erecting barriers
& filling up of cracks & crevices. There is no commercially
available antivenin for treatment of Mesobuthus tumulus.
Application of a strong solution of ammonia relieves pain in
a majority of cases; in others, injections of 1% Novocaine or
Lignocaine and Adrenaline bring relief. Preventive measures
include alertness in avoiding contact. Propoxur 2% or
Chlorpyrifos @ 0.2-0.5% may be used.
Ants are common annoying insects; need to be kept away from
food stuffs. Ants have been involved in mechanical transmission
of diseases & their bites may sometimes also be very painful.
Dilute ammonia relieves pain. Other means of prevention are
use of anti-formicas & Pyrethrum or Malathion spray.
Bee, Wasps & Hornet stings are often painful, the honey bee
often leaves sting along with poison gland in the puncture, this
should be promptly removed and pain relief may be obtained by
applying alkaline solution of sodium bicarbonate or ammonia.
Insecticides which can be used are Dichlorvos & Deltamethrin.
Centipedes possess a pair of legs on each segment of the body.
Bites can cause mild reaction to severe pain. Ammonia and
Morphine is used for treatment.
Leeches are a class of Annelid worms; the two important
species are the land leech & the aquatic leech. Leech bites are
painless but bleeding may be prolonged due to presence of anti
-coagulant. Ideally, leech should not be dragged or pulled off
the skin. Prevention is by wearing gum boots and searching
the body for leeches. DEET & DEPA can be used to provide
protection.
The true spiders (Arachnida) have poison glands. Important
species are the red backed spider & black widow spider.
Treatment of a bite is immediate washing and ligation. Aspirin
or morphine is used for pain relief. Intravenous Calcium
gluconate or Magnesium sulphate gives relief from cramps.
Study Exercises
MCQs & Exercises
1) Buthus (Mesobuthus) and Palamnoeus are common Indian
species of (a) spiders (b) scorpions (c) bugs (d) flies
2) Hypertension, arrhythmia, cardiac failure and pulmonary
oedema may be encountered following stings of
(a) Scorpion (b) Spider (c) Wasps (d) All of these
3) Scorpion sting can be treated by (a) Ammonia
(b) Lignocaine (c) Barbiturates (d) All
4) This should not be used to treat stings of bees, wasps
& hornets (a) Magnesium sulphate (b) Adrenaline
(c) Morphine (d) Papain
5) Scolopendra gigantia is a species of (a) Spider (b) Scorpion
(c) Centipede (d) None
Fill in the Blanks
6) Stings of red scorpion can be serious due to release of
__________
7) Class of annelid worms which suck blood, cause painful
bites __________
True or False
8) Erecting a barrier up to 20 cm by means of tiles at the base
of walls to prevent scorpion encroachment.
9) Stings of Bees, Wasps & Hornets should not be removed.
Answers : (1) b; (2) a; (3) d; (4) a; (5) c; (6) Catecholamines;
(7) Leeches; (8) True; (9) False.
Further Suggested Reading
Anthony S. Fauci, Eugene Braunwald, Harrison’s, Principles of Internal 1.
Medicine, Seventeenth Edition, 2008, Volume 2 ISBN 978-0-07-147693-5,
the McGraw-Hill Companies, United States of America.

55. • 955 •
163Ticks and Mites
Rina Tilak
Ticks and mites belong to class Arachnida, which are
characterized by the presence of two distinct body parts -
cephalothorax and abdomen and four pairs of legs. Antennae
are absent and eyes may or may not be present. Metamorphosis
is incomplete and the adults resemble the nymphs except for
the fact that nymphs lack genital aperture while in the adult
the sexes are distinct. The larvae are morphologically distinct
with three pairs of legs.
Ticks
Taxonomy & Morphology
Ticks belong to the super-family Ixodoidea. They are
distinguished from other acarines by their relatively large size
and absence of prominent hairs on the body. They are oval in
shape and of varying colours and dorsoventrally compressed.
Females are larger than males and are capable of great
distention. Both sexes as well as the other stages i.e. nymphs
and larvae thrive on blood alone and lead an intermittent
parasitic life during a major part of their life cycle. They are
free living on the ground in between various moults during
development. There are two families, Family Ixodidae which is
the hard tick and Family Argasidae which is the soft tick. The
hard tick is more a jungle tick while the soft tick is a domestic
or household tick like a bedbug.
Ixodidae or Hard Ticks
The dorsum of the adult male is covered by a dark shield, like
that of the tortoise, called the scutum. This may be ornate with
grey or white ‘patterns’. In females and immature males, it
covers only the anterior part behind ‘the capitulum’ which is
the false head, actually formed by the mouthparts anteriorly
and therefore visible from above (Fig. - 1).
Fig. - 1 : Hard Tick - Dorsal and Ventral Aspect
Argasidae or Soft Ticks
These are oval with leathery cuticle and devoid of scutum.
Their mouth parts are placed ventrally and hence not visible
from above and they possess no festoons (Fig. - 2).
Life History
All species of ticks pass through four stages during their
development viz. egg, larva, nymph and adult. The total period
required for full development of a tick is from six weeks to
2 years. Fully engorged fertilized female drops off to the
ground and lays eggs in cracks and crevices in the soil under
stones or among roots of shrubs and grass and such other
sheltered spots. Hard ticks deposit all their eggs in a single
act of oviposition after which they die. Eggs take a few weeks
to several months to hatch. Larvae are six legged and do not
feed for about a week after emergence. Thereafter, they become
hungry and active and climb on vegetation for attachment to
passing hosts (this is called questing). They feed for about
three days and drop off when engorged and remain quiescent
for digestion of blood. After the first moulting, the nymphs
emerge with their fourth pair of legs and seek a new host in
the same manner as the larvae, feed and again drop off. They
again moult and become sexually mature. The adults are also
parasitic and exhibit questing. Copulation takes place after the
last moult and the male dies after fertilizing the female. The
female engorges and then deposits eggs.
Fig. - 2 : Soft Tick Dorsal and Ventral Aspect
Bionomics
Hard ticks are open jungle dwellers and thrive on animal
hosts; hence they do not attach themselves to human
beings voluntarily, except when a person comes across them
accidentally. Argasidae (soft ticks), on the other hand, although
preferentially parasitic on animals and birds attack man
voluntarily. These are found in human dwellings and cattle
sheds and attack man and animals during their sleep. They
however, live away from their hosts, like bedbugs, in cracks
and crevices and only emerge at night to feed on the host. Ticks
can survive starvation for a long time.
Vector Potential
Ticks produce diseases in man by transmitting the viruses,
rickettsiae, spirochaetes and bacilli of infectious diseases
and through toxin present in their saliva. Some of the factors
which account for high vector potential are that all the stages
are essentially haematophagous and are persistent blood
suckers and while feeding they attach firmly and cannot be
easily removed. They are resistant to varying environmental
conditions and relatively protected from natural enemies.
The trans-stadial and transovarian transmission of infection
helps in maintaining infection for several years. Ticks have the
power to regenerate lost parts such as amputated legs and also
the ability to repair mutilated mouth parts, which conserves
them for long.

56. • 956 •
Soft Ticks
Soft ticks of the genus Ornithodorus transmit various types of
spirochaetae causing relapsing fevers in certain parts of the
world. O moubata is the vector of Borrelia duttoni in Africa;
O lahorensis has a very wide distribution in Central Asia and
North West India. In India, they are found in Kashmir and are
known as O crossi.
Hard Ticks
These are much more ubiquitous and produce larger varieties
of human diseases. The most important of all are the various
rickettsial infections transmitted by the hard ticks of the
genera Ixodes, Dermacentor, Amblyomma, Haemaphysalis,
Rhipicephalus, Hylomma and Boophilus. Viruses causing
Kyasanur Forest Disease, Colorado tick fever and other
Haemorrhagic fevers and Encephalitides are transmitted.
These also transmit P tularensis, the causative organism
of Tularaemia. Tick paralysis is an acute ascending flaccid
paralysis due to an unknown toxin the tick’s saliva introduces
through the bite of certain species of ticks of the genera
Dermacentor, Ixodes and Amblyomma. It affects mostly children
and young domestic animals in Australia, South Africa, North
America, Southern U.S.A. and N.W. Pacific. Even a single tick
bite may cause fatal paralysis. In certain cases, reaction from
improper or partial removal of ticks or due to the bite itself may
cause itching, swelling and ulceration at the site of the bite.
Otoacarisis is an invasion of the auditory canal by ticks.
Mites
The vector mites belong to the order Acarina and family
Trombiculidae which comprises many hundreds of species of
world wide distribution. They are found in great abundance in
areas with hot, humid climate, thick vegetation and presence of
small vertebrates like rodents. The foothills in subtropical and
temperate regions offer them ideal conditions. In the tropics,
they are found even at heights in mountain valleys. These
have also been found in the Alpine-subarctic terrain in the
Himalayas as well as at the level of coniferous forest-glacial
valleys in Pakistan. They are known by various names such
as chiggers, harvest mites, kedany or scrub mite. Important
species of the genus Leptotrombidium are akamushi which is
distributed widely in Japan, South East China, Korea, Malaysia
and Philippines and L deliense which is vastly distributed in the
tropical regions of South East Asia, Indian sub-continent, Sri
Lanka and Maldive Islands. In India, it is present in the whole
of the Shivalik range from Kashmir to Assam, the Eastern half
of the plains adjoining the foothill ranges, the Eastern and
Western ghats and the Vindhyachal range in Central India.
Morphology
The adult is about 3-4mm in size, is densely covered with hair
and has a figure of eight (Fig. - 3). The mouth parts consist
of a pair of chelicerae and a pair of palps, together giving the
mite an appearance of having a false head. The adult resembles
the nymph except that it is larger and more densely covered
with hairs; ‘Kedany’ meaning hair in Japanese language. The
larva is pale yellow to orange-red in colour with a circular body
bearing three pairs of legs and branched hairs on the body
and on the legs. They are found in nature in the interior of ear
cusps or on rumps of rats, mice, shrews, bandicoots and other
small mammals, reptiles and birds in orange coloured clusters
of as many as 50 to 200 larvae. A cluster of 5 to 6 larvae is as
big as a pin-head. When the cluster detaches, a scab is formed
showing an evidence of recent infestation.
Fig. - 3 : Trombiculid Mite (Adult)
Life History
The stages in the life history of a mite are egg, larva, nymph
and adult. The eggs are laid singly on the surface of the soil.
The larva leaves the egg-shell and becomes very active. Moving
quickly over the surface of the soil and low lying vegetation,
it seeks a suitable host such as rat, mouse, bandicoot and
shrew and so on. While feeding, it buries the whole length of
its chelicerae in the host’s skin and injects an irritant secretion
which causes tissue lysis. The larva feeds on the lymph and the
tissue fluid but not on blood of the host. The orange red colour
of the larva is therefore not due to ingested blood. The larvae
feed for 2 to 3 days and then drop off to the ground concealing
themselves in loose soil, and here they develop into nymphs
and finally into adults. Generally, it takes 6 to 12 weeks for its
development from egg to adult stage. Only the larval stages are
parasitic; the nymphs and adults are never parasitic and feed
on small insects in the soil or their eggs.
Bionomics
The mites are distributed in areas ideal for their survival
called as “Mite Islands”. Mite Islands are patches of ground
characterized by thick vegetation cover, mainly the scrub jungles
or other tall grasses offering protection from direct sunrays
and desiccation, nearly 100% relative humidity at ground level
and ideal ambient temperature of 27± 5°C. Such conditions
also provide sanctuaries for small vertebrate life such as rats,
mice, bandicoots, and shrews which are hosts for larval mites
(shrews are considered as the index animal for scrub typhus).
These animals are also the reservoirs of rickettsiae for which
the trombiculid mites are vectors. Hence, these mite islands
may also become typhus endemic foci. Mites are most active
during the whole rainy season and their prevalence in such mite
islands is related to the intensity and length of the monsoons.
In dry season, the adults migrate deeper into the soil, the egg
laying ceases and the mite islands shrink; during monsoon,

57. • 957 •
there is prolific activity and the mite islands expand. Patchy
distribution of mite islands and their selective choice of locality
explain the patchy nature of typhus endemic foci. The typical
terrains favourable for the mites to thrive and propagate are
as under :
Man-made rural and urban wastelands like overgrown ●●
clearings produced by shifting cultivations.
Domestic sub-urban waste lands produced around neglected ●●
patches in and around villages and even big towns, such as
neglected gardens and plantations or overgrown clearings
therein; deserted villages are heavily infested.
Around the edges of moist depressions, water meadows, ●●
grassy but not swampy river banks and moist sites such
as seepages along over ground canal areas.
The hedgerow types of features ranging from a simple ●●
bushy hedgerow to belts of forests following water courses
and ravines which are commonly left in deforested areas in
and below the foothills.
The scrub at the outskirts of the forests and low lying ●●
patches overgrown with elephant grass in sunny clearings
inside thick forests.
Vector Potential
Larval mites belonging to several genera attack man but
only the Genus Leptotrombidium contains species of medical
importance. In India, Leptotrombidium deliense is the vector
of Orientia tsutsugamushi causing Scrub typhus; in Japan,
the closely related variety L akamushi (kedani mite) transmits
Scrub typhus. Rickettsiae taken up by larvae while feeding on
rodents are carried through its nymph, adult stages and then
its eggs (trans-stadial transmission). The larvae hatching
out of these infected eggs are capable of transmitting the
rickettsiae to the next host. The infection is thus trans-ovarially
transmitted for some generations and hence the mite also acts
as a reservoir of infection. Larvae feed only once during their
life time. Therefore, transmission of infection occurs in second
or subsequent generations. When a larva lying on the ground
comes in contact with a human being (instead of a rodent as
would happen in the normal course), it attaches on to him and
feeds on his lymph; in the process, the rickettsiae contained in
the mouth parts are injected into the lymph of the human being
thereby causing scrub typhus.
Control of Ticks and Mites
A. Insecticidal Control
a. Area Treatment : This is the only reliable Acarine control
method in camp sited areas or areas amenable for such
treatments. Before the application of an insecticide to the
areas infested with hard ticks and mites, clearing of bushes by
cutting them is advantageous. If possible a bulldozer should be
employed. When the top soil is bare and dry, an area becomes
considerably safe and more suitable for insecticidal action.
Initial coverage of the area should be thorough. It may require
repetition after 8 weeks and occasionally a third time during
the hot-humid season. If people have to go to some nearby
stream for bathing or washing their clothes, the selected area
should be similarly treated as the stream edges covered with
vegetation are favourite sites for acarines. “Malathion 50% EC”
(as 5% solution) should be sprayed in a dosage of 4 kilograms
of active ingredient (a.i.) per hectare of ground surface area, or
else “Cyfluthrin EC” applied in dosage of 0.1 Kg (100 grams) of
active ingredient per hectare. Malathion 5% may also be used
(4 Kg of active ingredient of malathion will be present in 8 litres
of 50% EC commercial supply; this 8 litres will be mixed with
72 litres of water to get 80 litres of 5% solution which should
then be sprayed over one hectare of ground surface area).
b. On Vegetation : In areas where vegetation cannot be
removed for various reasons, control of ticks on vegetation can
be achieved by insecticide dusting or spraying from the ground
or air at the dosage varying from 0.5 to 2 Kg/hectare. In woody
and bushy areas, the dosage is increased proportionately.
Malathion, Fenthion, Propoxur and Permethrin are suitable.
c. Premises : Against soft ticks, application of insecticide to
floors and walls of infested premises on alternate days after
initial scraping and scorching are necessary. Treatment for 2
weeks before occupation gives good control. During this period
sweeping of floors should be discontinued. Such a series of
applications repeated 6 weeks afterwards gives adequate
protection to people staying in such habitations as camps
during disasters or migratory labour camps, etc. The insecticides
should also be applied to beds, mattresses, rugs and furniture. In
known tick infested areas, particularly where there is a history
of relapsing fever, infested houses/ areas should be avoided as
far as possible. Organophosphorus compounds like Malathion
and Fenitrothion or Carbamate compounds like Propoxur can
be used either as 0.5 to 1.0% spray or as 5 to 10% dust.
d. Domestic Animals : Dogs and other domestic animals can
be freed of ticks by a wash or spray containing 2% Malathion,
1% Propoxur, Deltamethrin (0.025%) etc. Only half these
concentrations should be used if the animal is to be dipped and
the entire animal should be immersed except the head. Dusts
containing 5% Malathion, Propoxur (1%), Cyfluthrin (0.1%),
Deltamethrin (0.05%), Temephos (2%), Fenthion (2%) etc. may
also be used. The premises which animals visit or is tied in,
should also be treated.
B. Personal Protection
a. The repellant materials used for personal protection
against ticks and mites are Dibutyl-phthlate (DBP), Diethyl
phenyl acetamide (DEPA) and Diethyltoluamide (DEET).
These are more effective when applied to the clothing than
to the skin. The effect may last for nearly six washings or
weeks which ever is earlier. However, if it is ironed, the
concentration falls below effective limits. DEET/ DEPA may
be used for application on the exposed parts of the body to
reinforce the use of protective clothing treated with DEET/
DBP/DEPA when working in an uncontrolled area or under
acute emergency when application of repellent on the
clothing prior to entry in an unknown or uncontrolled area
is absolutely impossible. The persistence of DEPA on clothes
post ironing is superior to that of DEET, whereas both are
equally effective when applied topically. Permethrin may
also be used for treatment of clothing.
b. Wearing shirts with rolled down sleeves tightly buttoned
at the cuffs, the lower ends of trousers tucked in socks
and wearing of proper boots considerably reduces the risk
against ticks and mites.

58. • 958 •
c. Clothes for drying should be hung on ropes especially fixed
for the purpose and not on the vegetation. Bush and grass
on the periphery of a camp becomes infested by larval mites
and ticks brought in by the rats migrating into the camp.
Therefore, purposeless wandering in such areas should be
discouraged.
d. More mites and ticks are picked up by standing or sitting
than by walking over the infested ground. Therefore, while
in such areas it is unsafe to lie down on a grassy ground.
The immediate vicinity of a tree base should be avoided for
resting, so also the green edges of a stream or an irrigation
channel. Open grassy grounds should be avoided in tick
infested areas.
e. Before retiring at night or after leaving a tick infested area
one should take a bath and carefully search one’s body and
clothing for presence of ticks. If a tick is found attached to
the body it should be removed immediately, because every
added moment of its attachment increases the danger of
transmission of infection. Pulling of a tick has the danger
of breaking off its parts, therefore, it should be removed
by making the surrounding skin taut, slipping the point
of a flat needle or a scalpel under the mouth parts and
then removing the mouth parts by raising the point of the
needle with a minimum of tissue damage. Iodine or any
other antiseptic should then be applied to the site.
f. Use of Insecticide treated mosquito net gives some
protection against soft ticks.
C. Habitat Management : Vegetation management is another
control option for ticks. Removal of shrubs, trees, or tall
grass can be useful in recreational areas. Wherever this is not
possible, area treatment with insecticide may be the only viable
option.
Anti-Rodent Measures
Persistent anti-rat hygiene is of great value in reducing the
risk of diseases conveyed to man through ticks and mites.
The main objective should be to reduce ingress of rodents by
proper disposal of camp and kitchen refuse and removal of
overgrown vegetation and rubble which afford them shelter.
Rat destruction requires forethought; because if the feeding
of larval ticks and mites is interrupted by the death of rodent
hosts, a number of released acarines may reattach themselves
to another host, which may be man. Active rat destruction
may be adopted when the first infestation is at its peak i.e. a
month or so after the rain starts. It is better to trap and then
destroy them so that their parasites do not escape. When dead
rats are collected from any endemic foci, the soil under and
immediately around them should be treated with insecticide.
Soft tick control is further achieved by rat-and-tick-proofing
of dwellings. All cracks and crevices, fissures and other points
of ingress should be closed and all doors should be made tight
fitting to keep away rodents.
Camp Siting
In civil life, camps may need to be sited for temporary
settlements of labour population when large industrial /
urbanization projects are being launched in possibly infested
areas or during disaster like situations. Before any area in
the known endemic tract is selected for camping or before the
insecticide treatment is undertaken, the degree of risk should
be assessed by determining the prevalence of adult and larval
Acari.
(a) Mite Survey : Superficial layers of earth are scraped from
moist areas around the roots of scrub and mixed with water
in a bowl. Adult mites resembling a figure of 8 float in a few
minutes. For non parasitic larvae, pieces of dark cardboard are
placed edge wise forming tent fashion structures on the ground
at intervals, larvae crawl up the cardboard and congregate
at its top edge within a few minutes. Rats caught from the
area should be examined for the clusters of larvae or scabs
in ear-cusps and shrews for clusters on their rumps. If the
ears or rump is infested, they should be carefully cut with fine
scissors and placed in 70% alcohol vials. Rodent trapping is
done in field (Camps, fringe areas) by specialized traps called
Sherman traps, whereas in peri-domestic areas, Wonder traps
may also be used. The trapping procedure is described in a
subsequent chapter on rodents. Once the traps are brought to
the laboratory, the rats are transferred in to large polythene
bags, anaesthetized and thereafter ectoparasite screening is
undertaken. While carrying out the survey, one must protect
oneself adequately with protective clothing and repellents.
As a rough guide, it can be said that the Scrub typhus risk in
any area during the monsoon is considered low if only up to
10% of the rats have been found infested on consecutive two
surveys unless cases have occurred already; if 20-40% of rats
have been found infested the contraction of infection is very
probable; and if 50% or more rats have been found infested the
risk is high and the site should be considered as dangerous.
Similarly, even if a single rat is found infested with more than
100 larval mites, the area should be avoided, being a very high
risk area.
(b) Tick-Survey : Ticks are collected by sweeping flags made
of white flannel across the vegetation (Flagging/dragging
method). The larvae, nymphs and adults get attached to them
and are easily detected against the white background of the
flag. They should be picked up by forceps and placed in 70%
alcohol vials. Parasitic stages of ticks on various animals can
be collected by catching rodents, shrews and other animals.
Physical examination of a volunteer for ticks attached on his
body after he has travelled for a stipulated / fixed period of
time in an area delineated for survey also provides information
on tick presence/ abundance. Dry ice traps are one of the most
effective means of sampling ticks.
Summary
Ticks and mites belong to class Arachnida; body is divided into
cephalothorax & abdomen with 4 pairs of legs. Ticks belong to
two families : Ixodoidea (hard tick) and Argasidae (soft tick).
All stages thrive on blood & lead an intermittent parasitic life.
Dorsum of hard tick is covered by scutum which is replaced
by the leathery cuticle in soft ticks. All species of ticks pass
through 4 stages - egg, larva, nymph & adult. Eggs are laid in
cracks & crevices. Hard ticks deposit all their eggs in a single act
of oviposition, after which they die. Larvae are six legged and
do not feed for about a week, thereafter they attach to passing
hosts. They feed on blood & drop off. The nymphs and adults
(both sexes) are parasitic and exhibit questing. Male dies after

59. • 959 •
fertilizing the female. Hard ticks thrive on animal hosts & soft
ticks attack man voluntarily. Ticks produce diseases in man
by transmitting viruses, rickettsiae, spirochaetes and bacilli of
infectious diseases. Rickettsial infections are transmitted by
the hard ticks of the genera Ixodes, Dermacentor, Amblyomma,
Haemaphysalis, Rhipicephalus, Hylomma and Boophilus. Hard
ticks also transmit virus causing KFD and P tularensis, the
causative organism of Tularaemia. Soft ticks transmit various
types of spirochaetes causing relapsing fever.
The vector mites belong to the order Acarina, family
Trombiculidae. Found in great abundance in areas with
hot, humid climate, thick vegetation and presence of small
vertebrates like rodents (Mite Island). They are known by various
names such as chiggers, harvest mites, Kedany or scrub mite.
Important species of the genus Leptotrombidium are akamushi
and deliense. They possess a figure of eight shaped body with
eight legs. They are found in nature in the interior of ear cusps
or on rumps of rats, mice, shrews, bandicoots and other small
mammals. The stages in the life history of a mite are egg, larva,
nymph and adult. The eggs are laid singly. The larva seeks a
suitable host such as rat, mouse, bandicoot or shrew and feeds
on the lymph and tissue fluid. Mites are vectors for various
rickettsial diseases. Leptotrombidium deliense transmits
Orientia tsutsugamushi causing Scrub typhus.
Control of ticks & mites can be achieved by clearing of bushes
in areas infested by ticks & mites followed by application
of insecticides. Malathion 50% EC (@ 5% solution) should
be sprayed in a dosage of 4 kg of active ingredient (a.i.) per
hectare. In areas where vegetation cannot be removed, dusting
can be done by Malathion or Fenthion. Insecticides should
also be applied to beds, mattresses, rugs and furniture. Dogs
and other domestic animals can be freed of ticks by a wash or
spray containing 2% Malathion or 1% Propoxur. The repellants
used for personal protection against ticks and mites are
Dibutyl-phthlate (DBP), Diethyl phenyl acetamide (DEPA) and
Diethyltoluamide (DEET). Clothes for drying should be hung on
ropes. Before retiring at night or after leaving a tick infested
area, one should take a bath and carefully search one’s body
and clothing for presence of ticks. Anti rodent measures like
proper disposal of refuse & removal of overgrown vegetation
helps in reducing the risk of diseases conveyed to man through
ticks and mites. It is better to trap and then destroy rats so that
their parasites do not escape.
Mite survey for nymphs and adults is done by soil sampling
method, whereas larvae are sampled by rodent trapping. Ticks
are surveyed by flagging/ dragging method as well as by direct
host collection method through rodent trapping or by screening
the host body for attached tick stages. Scrub typhus risk is
considered low if only up to 10% of the rats have been found
infested on consecutive two surveys unless cases have occurred
already. Study Exercises
MCQs & Exercises
1) Which of these regarding ticks is not true (a) Females are
larger than males (b) Both sexes thrive on blood alone
(c) They are free living on the ground in between various
moults (d)All are true.
2) Soft ticks are involved in transmission of which of these
(a) Relapsing fever (b) Kyasanur forest disease (c) Tick
paralysis (d) Tularaemia.
3) Mite island is an area having (a) 100% relative humidity
(b) Temperature of 27± 5°C (c) Scrub jungles or other tall
grasses (d) Rats, mice, bandicoots, and shrews (e )All of
these
4) Which is the parasitic stage in mites (a) Egg (b) Larva
(c) Nymph (d) Adults
Fill in the Blanks
5) Ticks & mites belong to the class _____________
6) Hard ticks deposit all their eggs in a single act of
____________ after which they die.
7) The most important vector species of trombiculid mite
transmitting scrub typhus is __________
8) The most effective means of sampling ticks is __________
True or false
9) Hard ticks belong to Family Ixodidae and soft tick to Family
Argasidae.
10) The total period required for full development of a tick is
from six weeks to 2 years.
11) Copulation in case of ticks takes place after the last moult
& the male dies after fertilizing the female.
12) The mite larva feeds on lymph and the tissue fluid but not
on blood.
13) Only the adult stages of ticks are parasitic, whereas the
larvae and nymph feed on plant juice.
Answers : (1) d; (2) a; (3) e; (4) b; (5)Arachnida; (6) Oviposition;
(7) Leptotrombidium deliense; (8) Dry ice collection; (9) True;
(10) True; (11) True; (12) True; (13) False.
Suggested reading
Schuster R, Murphy PW, editors. The Acari : Reproduction, Development and 1.
Life History Strategies. London : Chapman and Hall, 1991.
Evans GO. Principles of Acaralogy. Wallingford : CAB International, 1992.2.
Varma MGR. Ticks and Mites. In : Cook G, editors : Manson’s Tropical 3.
Diseases. 20th ed London : English Language Book Society and WB
Saunders, 1996; 1650 -9.
Sehgal S, Bhatia R. Manual on Zoonoses. National Institute of Communicable 4.
Diseases (Govt of India, Min of Health and Family welfare). New Delhi
1981.
Sonenshine DE, Lane RS, Nicholson WL. Ticks (Ixodida). In : Mullen G, 5.
Durden L, editors. Medical and Veterinary Entomology. Amsterdam :
Academic Press, 2002; 517-58.
World Health Organisation. Arthropod borne and Rodent borne viral diseases. 6.
Tech Rep Ser No 719. WHO, Geneva, 1985.
Fernandez Stan, Kulkarni SM. Studies on Trombiculid Mite Fauna of India. 7.
Rec zool Surv India Occ Paper No. 212 : 1-539. Kolkata : Zool Surv India.
2003.

60. • 960 •
164Rodents
Rina Tilak
Rodents are the largest order of mammals with over 2000
living species placed in about 30 families. The word ‘Rodent’
has been derived from “rodere”, which means to gnaw; the
incisors of the rodents keep growing and to keep them in check,
the rodents gnaw continuously. Rodents range in size from 5
gm (pygmy mice) to over 70 kg (capybaras). They have a world
wide distribution with the exception of Antarctica. The habitat
of the rodents is equally diverse with some preferring terrestrial
life, while some live underground, others are adapted to desert
life while some have inhabited the aquatic environment.
Public Health Importance
Rodents are capable of transmitting a large number of diseases
to man through transfer of ectoparasites, which are vectors
of diseases like murine typhus, plague, scrub typhus etc.
(Table - 1 & 2) besides consuming and contaminating stored
food with their urine and droppings, destruction to immovable
property, gnawing on wiring and electrical insulations etc.,
which at times may lead to fire hazard.
Bionomics
Rodents use their sense of vision, hearing, touch, smell and
taste for exploring new areas or food with certain senses more
developed amongst rodents based on their needs for e.g. diurnal
rodents have better vision as compared to their counterparts
who are active at night. The rodents also possess the ability
to perceive ultrasounds and use chemical, tactile, visual,
auditory cues and intraspecific chemicals viz. pheromones for
communication. Rats tend to be cautious and mice are more
curious. Most rodents are omnivorous with few exceptions.
They reproduce rapidly and exhibit crepuscular to nocturnal
habit.
Types of Rodents
Rodents may be classified broadly into two types : viz.
commensal (domestic rodents) and sylvatic or wild rodents.
Three species of urban rodents, Mus musculus (house mouse),
Rattus norvegicus (Norway or brown rat) and Rattus rattus
(roof or black rat) are the principal rodents encountered in
peridomestic and domestic environments.
Table - 2 : Diseases transmitted through ectoparasites
being harboured by the rodents.
Disease Agent Vector
Murine typhus RickettsiaFlea
Plague Bacteria Flea
Scrub typhus RickettsiaTrombiculid mite
Indian tick typhus RickettsiaHard Tick
Rickettsial Pox RickettsiaMouse mite
Kyasanur Forest DiseaseVirus Hard Tick
Rocky Mountain
Spotted Fever
RickettsiaHard Tick
Colorado tick fever Virus Hard Tick
Lyme disease Spirochaete Hard Tick
Human Granulocytic
Anaplasmosis
Bacteria Hard Tick
Babesiosis Parasite Hard Tick
Relapsing fever Bacteria Soft Tick
Western Equine
Encephalitis
Virus Mosquito
Cutaneous LeishmaniasisParasite Sandfly
Rattus norvegicus - (Sewer rat) : It generally frequents the
sewers. It is a burrowing rat and is easily identified by its heavy
body, coarse brownish to reddish grey fur with greyish belly. It
is a good climber, nocturnal in habit and eats anything which is
edible. It has poor vision but keen senses of smell, touch, taste
and hearing and usually lives within 150 feet of food and water
source. It feeds on familiar food, preferring meats and grains
and has been found to be cautious of new items or food.
Table - 1 : Diseases transmitted directly by rodents or through contaminated water, food or animal.
Disease Agent Mode of spread
Rat bite fever Bacteria (Spirillum minus)
Bite or scratch from an infected rodent or contact with a dead rodent,
eating or drinking food or water that is contaminated by rat faeces
Leptospirosis
Spirochaete (Leptospira
icterohaemorrhagica)
Water & food contaminated with urine from infected animals
Salmonellosis Bacteria (Salmonella spp ) Water & food contaminated with faeces from infected animals
Tularaemia
Bacteria (Francisella
tularensis)
Handling infected animal carcasses, eating or drinking
contaminated food or water, breathing in the bacteria
Haemorrhagic fever
with renal syndrome
Virus
Breathing in dust that is contaminated with rodent urine or
droppings, direct contact with rodents or their urine and droppings
Lymphocytic Chorio
Meningitis (LCM)
Virus
Breathing in dust that is contaminated with rodent urine or
droppings, direct contact with rodents or their urine and droppings
Plague Bacteria (Yersinia pestis) Direct contact with infected animal

61. • 961 •
Rattus rattus : It is a slender rat and is a very agile climber
thus earning its common name - the roof rat. It usually nests
indoors in roof and wall cavities. It exhibits a strong fear of
new things (neophobic). It is nocturnal and an omnivorous
feeder with preference for fruits and propensity to hoard food.
Mus musculus : It resembles the roof rat, though smaller in
size. It can be distinguished by its dusky grey fur and smaller
feet. It commonly inhabits man made structures and therefore
called house mouse. It has poor vision and is colour-blind, but
has a well developed sense of smell which is used to locate food
items and recognize other mice and an acute sense of hearing to
respond to unusual noises as a means of detecting and escaping
danger. Mice are omnivorous and known nibblers, eating
small amounts of food at a time with preference for cereals
or foods rich in fat and protein. They are poor swimmers and
their normal activity range is usually within 10-30 feet from
nest. Mice are habitual gnawers, curious and avid collectors
of nesting materials; a habit which can be exploited while
trapping them by attaching nesting material to traps.
The morphological differences amongst the three commensal
or urban rodents are depicted in Fig. - 1.
Fig. - 1 : Morphological differences amongst the three
urban rodent species.
ROOFRAT
Rattusrattus
YOUNGRAT
longerthanhead
+body
TAIL
shorterthanhead
+body
light,slender
BODY
heavy,thick
large
EAR
small
large
EYE
small
pointed
MUZZLE
blunt
NORWAYRAT
Rattusnorvegicus
HOUSEMOUSE
Musmusculus
large
small
FEET
large
small
HEAD
Detection of Rodent Infestation
It is important to ascertain presence and level of rodent
infestation prior to undertaking control measures. Following
methods of detection may be employed for assessment :
Rub marks : During movement of rodents along rat runs, the
oil and dust from the body of the rodents leaves rub marks
especially in case of roof rats.
Gnawing marks : Due to the continuous gnawing habit of
rodents, wood scrapings around doors, windows and frames
may be the obvious visible signs of rodent activity.
Droppings : By seeing the droppings, it is possible to identify
the type of rodent infesting a given structure. The same has
been presented in Fig. - 2. Droppings are ideally visible along
rat runs, near rodent burrows and at the feeding sites.
Run Ways : Rodents generally use the same path for travelling;
the common runways or rat runs are seen along walls, behind
stored objects and similar places.
On inspection, when no signs are visible, it is presumed that
there is no rodent infestation. In low to medium infested areas,
no daytime rodent sightings or activity is noticed however,
their presence is detected by the above mentioned methods of
detection. The high-density infestation levels are detected by
daytime sightings, active gnawing, droppings and heightened
night time sightings.
Fig. - 2 : Rodent droppings of the three commensal
rodents
Norway Rat Droppings
Avg. Length - 3/4” Or 19 mm
Shape - Blunt
Roof Rat Droppings
Avg. Length - 1/2” Or 13 mm
Shape - Pointed
House Mouse Droppings
Avg. Length-1/4” or 6mm
Shape - Pointed
Prevention and Control
The best method of prevention is to deny rodents a place to
live / nest and food to eat. This is achieved by the following
methods :
Eliminating the Hiding and Nesting Sites : The rodents prefer
to breed in dark and undisturbed areas, which are available in
plenty by the debris / garbage/ clutter present in the houses/
surroundings. The following measures will ensure denial of
breeding / hiding place for the rodents.
Keep surroundings clean and free from debris to prevent ●●
rodents entry and breeding in the premises.
Good housekeeping will deny hiding and nesting places to ●●
rodents.
Deny dark, sheltered and undisturbed places in the house ●●
by routinely displacing furniture.
Make all openings to constructions rodent proof through ●●
engineering methods. The following measures can be
adopted by individuals to make their houses rodent proof:
Install doors to fit tightly.--
Rodent proof a door by placing sheet metal channel at --
bottom and cuffs at sides, over channel.
Reinforce doors with thick aluminium sheets of 22 --
gauze or thicker.
Steel wool and copper mesh can be packed tightly into --
holes to close openings or protect other areas from
gnawing.
Make the drainage points rodent proof by using sieves --
secured with cement.
Use materials such as mortar for sealing holes in --
concrete buildings.
Fill all gaps in basements/ building or holes in walls --
with cement or steel wool.
Rodent proof utility wires to limit access to buildings --
using rolling plastic tubes made from rectangular
sheets of plastic. The tube rolls when the rodent tries
to walk over it.
Rodent proof air vents and chimneys using 1/4” --
hardware cloth.

62. • 962 •
Eliminating Food Sources
Spillage should be cleaned daily.●●
Garbage should not be stored outside in plastic bags as ●●
plastic garbage bags are not rodent-proof, instead metal
bins or heavy duty plastic bins with tight-fitting lids
should be used.
Fallen fruits and nuts from the ground should be promptly ●●
disposed of.
Pet food dishes and leftovers should also be promptly ●●
removed after feeding.
Animal waste should be cleaned frequently.●●
Food scraps should not be placed in compost piles.●●
Food grains or other food materials should be stored in ●●
rodent proof containers and wherever feasible be placed
in cupboards.
In situations where preventive measures have failed to address
the problem of rodent nuisance effectively, control of rodent
infestation is achieved through the following three methods :
Trapping●●
Baiting●●
Fumigation●●
Trapping : Trapping can be an effective method of controlling
rodents, but it requires more skill and labor than most other
methods. Trapping is recommended where poisons seem
inadvisable and is the preferred method to try first in homes,
garages and other small structures, where only a few rodents
may be present. Traps should be laid out at or before dusk
along rat runs, near their feeding areas, areas of high activity
or potential entry points. Traps should be placed at 3-10 ft
apart for mice and about 20 ft for rats. It is also pertinent to
use the correct trap for the rodents; larger rodents may require
larger traps as compared to traps used for mice. For successful
trapping, it is advisable that baited traps should be placed.
Types of Traps : There are various types of traps available in
the market for use. The traps either trap live rats or kill the
rats. Commonly available traps are :
Cage Traps (Fig. - 3)
Used for capturing rodents live.●●
These traps are designed to catch and hold one or more ●●
animals by means of a falling or sliding door, triggered
when the rodent enters or nibbles at a bait.
Sherman Traps (Fig. - 4)
Are foldable box traps made of Aluminium.●●
Used for trapping rodents for research purposes or during ●●
epidemic investigation (Procedure of trapping is presented
in Box - 1).
Traps live rodents.●●
Have an added advantage of ensuring that ectoparasites ●●
are not lost, even if the trapped rodent dies.
Generally baited with freshly made baits rich in fat e.g. ●●
pakoras (fried snack of onion mixed with gram flour) or
fried fish, chicken etc.
Box - 1 : Trapping Procedure for Rodents using Sherman Traps
Number the Sherman traps with marking pens starting from 1-40.
Prepare flags bearing the same numbers i.e. 1-40 with cardboard pieces.
Carry freshly prepared pakoras for the traps.
The laying of traps should be carried out in the late afternoons and should be completed before dusk.
Personnel involved in laying/ collecting of rodent traps should wear trousers tucked in boots/anklets and full sleeved shirt. Wear
heavy duty gloves while laying/ collecting traps and use repellents on exposed skin.
Identify suspected mite islands (a patch of ground with vegetation, temp in the range of 27± 5°C, Relative Humidity in the
range of 80-100% and which provides an ideal place for rodents to hide).
Check the working condition of Sherman trap before placement.
Place pakora in traps; ensure it is placed at the closed end of the trap.
Place the trap in the bushes or where rodent is likely to frequent. Place the trap with its opening facing the likely direction of
rodent entry. Ensure that the trap opening is not obstructed. The trap should be placed on flat ground so that it does not tumble
if approached by rodent or any other movement in the area.
Place flag bearing the same number as the trap, close to the trap so that it is visible from a distance.
Tie a bandage to the nearby bush to indicate the site of trap placement to enable retrieval in the poor light conditions of dawn
- the time for trap collection.
Make a spot map of trapping area with the trap numbers indicated on the map.
Collect the trap before dawn next day.
Use heavy duty gloves and a torch for collection of traps at dawn. (All protective measures as followed earlier should be
ensured). If a trap is closed, do not try to open the trap, put the trap in a cardboard carton for transportation to the lab.
If a trap is open, fold it and place in carton/ plastic bags.
Once the traps are brought to the laboratory, the rats are transferred in to large polythene bags, anaesthetized, blood collected
for testing for presence of rodent borne diseases by conventional serology/ PCR, rodent identified and thereafter subjected to
ectoparasite screening.

63. • 963 •
Trigger or Snap Traps
These traps are generally more effective than cage traps.●●
Simple, inexpensive, wood-based snap traps are readily ●●
available.
For rats, bait the traps with fried food items like chicken, ●●
pakoras etc. tied securely to the trigger.
Break Back Traps
These traps are similar to snap traps but have a quicker ●●
killing action and hence there are no misses.
These traps are considered more humane as the rats die ●●
instantaneously.
They are generally used in fields by farmers.●●
Not preferred indoors.●●
Traps need to be baited with food similar to that used for ●●
other traps.
Advantages : Trapping is an effective rodent control option
when permanent rodent control measures are not in place or
inappropriately implemented. It is one of the most preferred
and safer rodent control options in domestic and peri-
domestic environments. It has an advantage that it allows
user satisfaction in terms of success of the trapping procedure
being obvious. Trapping as a rodent control option also ensures
proper disposal of carcasses or live trapped rodents thereby
preventing odour nuisance from the dead rodents. Traps are
especially effective indoors where use of rodenticides may pose
threat to the non target organisms viz. people or pets and in
kitchens, pantries or food serving areas due to possibility of
contamination of food.
Limitations : This is a temporary control option as after initial
success, the rodents become ‘trap wise’ (a phenomenon wherein
rodents avoid traps as they are able to associate traps and their
getting trapped in them) and the efficacy of the trapping process
is then compromised. The trapping process should be stopped
when rodents are sighted but are not being trapped.
Glue Boards : Glue boards are an alternative to rodent traps
and may control rodents in low infestation areas. They are
effective options for rodent control in areas where food is
commercially prepared, where the use of rodenticides is unsafe
and for monitoring rodent population. A combination of traps
and glue boards may prove more advantageous than a single
method. The mode of action of rat glue boards is the same as
that of fly papers. It entangles mice and rats in much the same
manner as flypaper catches houseflies. Like traps, glue boards
need to be placed along walls where mice and rats travel. It is
however important to place the glue boards in dust free areas
and away from direct sunlight or extremes of temperature so
that the tackiness of the glue is maintained.
Baiting : In the event of emergency rodent control or at times
when there is a failure to achieve effective rodent control
through non chemical strategies mentioned in the preceding
paragraphs, baiting emerges as an attractive alternative to
address the problem. The process of baiting utilises special
chemicals called rodenticides for killing the rodents. The use
of rodenticides or the process of baiting should be undertaken
by trained individuals and requires utmost care as they pose
threat to non target organisms, as well as hazards of accidental/
deliberate poisoning.
Factors Influencing Baiting : The adaptive behavioural
responses of rodents for selecting palatable and nutritious
food, while rejecting palatable though harmful food poses one
of the biggest challenges in making the rodents consume the
toxic baits. This is further challenged by the phenomenon of
‘neophobia’ exhibited by rats and their learned food aversions.
It is thus essential to undertake ‘prebaiting’ (the food material
without the bait is placed for 3-5 days to remove fear of new
food amongst the rodents) prior to actual baiting. It is further
important that the rodenticide should preferably be tasteless
and odourless in lethal concentrations and have a delayed
effect.
Time & Place of Baiting : Baiting should be done in the
evenings along rat runs, feeding and activity areas etc which are
generally frequented by the rodents as evidenced by the signs
of infestation elicited through the procedure given earlier.
Types of Rodenticides : Rodenticides are classified as acute
rodenticides and anticoagulants. They are used mostly mixed
with food or as contact poisons in the form of dusts (tracking
powder).
Acute Rodenticides : These kill the rodents on ingestion of
a single dose of the toxic bait. There are a large number of
acute rodenticides, however, not all are considered ideal for use
in domestic environments due to safety concerns. The acute
rodenticides are Red squill, Norbromide, Sodium fluoroacetate,
Fig. - 3 : Cage Trap Fig. - 4 : Sherman Trap

64. • 964 •
Strychnine, ANTU etc. Some commonly available acute
rodenticides are Zinc phosphide and Barium carbonate.
Zinc Phosphide : It is a black powder with a garlic odour.
The garlic odor attracts rodents but has a repulsive effect on
other mammals, though, birds cannot perceive the smell and
unintentionally feed on them leading to unwarranted death.
It is mixed in the proportion of 1 : 8 or 1 : 10 with wheat flour
and made into pellets with a little edible oil. Upon ingestion,
Zinc phosphide reacts with dilute acids in the gastrointestinal
tract and produces phosphine which enters the blood stream
and causes death within four hours of consumption of a single
dose of the bait.
Barium Carbonate : It is a white powder mixed in the
proportion of 1 : 4 with wheat flour. A single dose consumption
of the product causes kill within eight to twenty four hours.
Advantages : Being highly toxic, these rodenticides bring
about a rapid kill with a single dose consumption of the bait,
which is one of the greatest advantages especially when the
rodent populations are very high.
Limitations : These rodenticides need to be used with care
as they may cause toxicity to non target organisms. The
consumption of a single dose of the bait leads to very painful
death of the rodent, which when sighted by the other rodents
is quickly registered as an association of the new bait with
these deaths, thus leading to avoidance of the bait - i.e.
development of the phenomenon termed as “bait shyness”.
Once this develops, baiting with acute rodenticides is rendered
ineffective and needs to be discontinued.
Anticoagulants : These substances kill by preventing normal
blood clotting and causing internal haemorrhage through
effective blocking of vitamin K cycle, which results in inability
to produce essential blood-clotting factors (mainly coagulation
factors II (prothrombin), VII (proconvertin), IX (Christmas
factor) and X (Stuart factor)). The antidote to anticoagulant
poisoning is therefore Vitamin K itself. Anticoagulants are of
two types, multiple dose and single dose anticoagulants.
(a) Multiple Dose Anticoagulants : These are the earliest known
anticoagulants also called the first generation anticoagulants.
Multiple dose consumption of the product kills the rodents,
which generally takes about 1-2 weeks post ingestion of lethal
dose. The common examples in this category are :
Warfarin●● : It is the earliest product used for rodent control
from this category. The product is not a very popular
product now for rodent control as it has a delayed action.
This product requires a higher concentration of the toxicant
(usually between 0.005 and 0.1%) and consecutive intake
over days in order to accumulate the lethal dose. The
product is mixed in the proportion of 1 : 19 with wheat
flour and made into baits.
Coumatetralyl●● : Is an anticoagulant of the Warfarin
type. The toxicity of the product is known to increase
with continued ingestion or exposure to the bait. For the
product to be toxic, it should constantly remain present
in the blood for more than 1 to 2 days. A single exposure,
even though relatively large, may not produce toxic
symptoms as the compound is quite rapidly metabolized.
It is marketed in India as tracking powder (0.0375%) which
can be mixed with wheat flour and made into baits as in
case of Warfarin.
Advantages : The greatest advantage of these products are that
they cause apparently painless deaths and hence do not result
in the development of ‘bait shyness’ and therefore can be used
wherever continuous rodenticiding is required e.g. granaries,
ships etc.
Limitations of Multiple Dose Rodenticides : As multiple dose
consumption of the bait is required for the desired kill; single
dose consumption of the bait will lead to sublethal dosing
thereby giving rise to development of resistance.
(b) Single Dose Anticoagulants : Are derivatives of
4-hydroxycoumarin and are referred to as second generation
anticoagulants or as “superwarfarins”. These are far more
toxic and lethal than their first generation counterparts and
are applied in lower concentrations in baits (0.001 - 0.005%).
The rodents die after ingestion of a single dose of the bait as
in case of the acute rodenticides, however, the cause remains
haemorrhage. Single dose anticoagulants commonly used are
Bromadialone, Difethialone, Brodifacoum etc. Bromadialone
is used in the form of ready-to-use baits of low concentration
containing 0.005% bromadialone. Single dose consumption of
the bait kills the rodents. Bromadialone kills all the commensal
rodents viz. sewer rats, roof rats, house mice and is also
effective against Warfarin resistant sewer rats.
Advantages : These single dose anticoagulants are effective
against strains of rodents that have developed resistance to
first generation anticoagulants. A single dose is lethal to cause
kill and hence development of resistance also may not take
place readily.
Limitation : As a single dose of these rodenticides is lethal to
rodents; the use of this class of products may pose threat to
non target organisms and hence needs to be used with care and
under supervision.
Fumigation : Fumigation is the choice method for killing
rodents in burrows, enclosed structures / places. It is however,
carried out by trained professionals due to the risk posed by
the noxious gases released during the process. The fumigants
commonly used are Aluminium phosphide and Calcium cyanide.
Sulphur dioxide, Carbon disulphide, Carbon monoxide and
Methyl bromide are also used especially in godowns, granaries
and aircrafts.
Fumigation with Aluminium phosphide and Calcium cyanide
involves identification of burrows, locating the exit and entry
points and then sealing one of the ends. The fumigants in the
form of tablet (Aluminium phosphide) or in the form of powder
(Calcium cyanide - 30 g /rodent burrow) are introduced in the
burrows and the burrow closed with wet soil to release the gas
- (Phosphine in case of Aluminium phosphide and Cyano gas
in case of Calcium cyanide). Fumigation with Cyano gas has
been extensively used in India for fumigating rat burrows in
fields during Plague outbreaks, whereas Aluminium phosphide
tablets are generally preferred for use in peri-domestic areas
and agricultural fields.
Ultrasonic Repellent Device : The ability of rats to perceive
ultrasonic sound has been exploited by using devices generating
ultrasonic sounds. These are electrical devices and are popular

65. • 965 •
gadgets claiming efficacy in repelling rodents from infested
rooms. Ultrasonic sounds, however, have very limited use in
rodent control as they are directional, don’t penetrate behind
objects and lose their intensity with distance. They can be used
in small rooms in a limited area.
Summary
Rodents are the largest order of mammals with over 2000
living species. ‘Rodent’ has been derived from “rodere”, which
means to gnaw. Rodents range in size from 5 gm to over 70 kg.
Rodents are capable of transmitting a large number of diseases
to man through transfer of ectoparasites, which are vectors of
diseases like murine typhus, plague, scrub typhus. Rodents
use their sense of vision, hearing, touch, smell and taste for
exploring new areas or food. The rodents also possess the
ability to perceive ultrasounds and use chemical, tactile, visual,
auditory cues and intraspecific chemicals like pheromones for
communication.
Rodents are broadly classified into domestic & wild rodents;
principal rodents encountered in peridomestic and domestic
environments are Mus musculus (house mouse), Rattus
norvegicus (Norway or brown rat) and Rattus rattus (roof
or black rat). Rattus norvegicus or the sewer rat generally
frequents the sewers. It is a burrowing rat and is easily
identified by its heavy body. Rattus rattus is a slender rat &
is a very agile climber. Mus musculus has dusky grey fur and
smaller feet. Detection of rodent infestation can be done by rub
marks, gnawing marks, droppings & runways. Rodents are
mostly omnivorous with preference for cereals or foods rich in
fat and protein.
Prevention is achieved by denying rodents a place to live/ nest
and food to eat, keeping surroundings clean and making all
openings to constructions rodent proof etc. Food source need to
be eliminated and garbage should not be stored outside, animal
waste should be cleaned and food grains / other food materials
should be stored in rodent proof containers.
Control of rodent infestation is achieved through trapping; The
traps used are Cage traps, Sherman traps, Snap traps & Break
back traps. Traps should be laid out at or before dusk along rat
runs, near their feeding areas, areas of high activity or potential
entry points. Traps are the most preferred and safe rodent
control options in domestic and peri-domestic environments as
success of the trapping procedure being obvious, they however
have the limitation of being temporary control options as the
rats become trap wise.
Other alternative are use of glue boards, baiting & use of
rodenticides. The acute rodenticides are Red squill, ANTU, Zinc
phosphide & Barium carbonate (advantage of rapid kill but can
lead to development of bait shyness.), the other rodenticides
are the anticoagulants. Multiple dose anticoagulants are
Warfarin & Coumatetrayl; They have the advantage of causing
painless deaths but multiple dose consumption is required for
the desired effect. Single dose anticoagulants (bromadiolone)
have the advantage of killing the rodents by consumption of
a single dose with the added advantage of anticoagulants
i.e. no development of bait shyness takes place, thus making
them ideal for continuous use. Fumigation can be done using
Aluminium phosphide and Calcium cyanide.
Study Exercises
MCQs & Exercises
1) Which of these is known as roof rat (a) Mus musculus
(b) Rattus norvegicus (c) Rattus rattus (d) All of these.
2) Rodents are capable of transmitting which of these
diseases to man through vectors (a) Murine typhus
(b) Leptospirosis (c) Rat bite fever (d) All of these.
3) Detection of rodent infestation prior to deciding control
measures can be done by (a) Rub marks (b) Gnawing marks
(c) Droppings (d) All of these.
4) This trap is generally used for trapping rodents for
research purposes / outbreak investigation (a) Cage traps
(b) Sherman traps (c) Trigger trap (d) Break back traps.
5) Acute rodenticides are all except (a) Red squill (b) ANTU (c)
Zinc phosphide (d) Warfarin.
Fill in the Blanks
6) Burrowing rat which is easily identified by its heavy body,
coarse brownish to reddish grey fur with greyish belly &
frequents sewers is _______________
7) These traps have a quick killing action & are generally
used in fields_______________
True or False
8) Rodents range in size from 5 gm (pygmy mice) to over
70 Kg.
9) Mice are omnivorous and known nibblers, eating small
amounts of food at a time with preference for cereals or
foods rich in fat and protein.
10) Traps should be placed at 3-10 ft apart for mice and about
20 ft for rats.
Answers : (1) c; (2) a; (3) d; (4) b; (5) d; (6) Rattus norvegicus;
(7) Break back traps; (8) True; (9) True; (10)True.

66. • 966 •
166Snakes
Rina Tilak
Snakes are one of the most widely distributed animals spanning
across continents (with the exception of Antarctica), inhabiting
the sea and present as high as 16,000 feet (4900m) in the
Himalayan Mountains, though conspicuous by their absence
from Ireland and Iceland (1). There are over 3000 species of
snakes distributed world wide and about 275 species are found
in the Indian sub-continent (2).
Morphology & Biology
Snakes, which belong to the class Reptilia, are limbless reptiles
characterized by elongated body which is divided into head,
body and tail. The whole body of the snake is covered with
scales which are an important tool for their identification. A
thin skin, in turn covers the scales which is periodically shed
or cast off during the process of moulting; during this period
the snake is blind (as the skin covers the eyes as well) and
lethargic.
Snakes locate their prey by their senses of vision, smell or
thermo-sensitivity. The remarkable vision of some snakes
enables them to detect movement; snakes smell by using their
forked tongue which collects airborne particles and then passes
them to the Jacobson’s organ or the vomeronasal organ in the
mouth for examination. The body of the snakes which is in
direct contact with the ground is also very sensitive to vibration,
thus enabling snakes to sense other approaching animals by
detecting vibrations on the ground. Some snakes like pit vipers
and pythons have infrared-sensitive receptors present between
the nostril and eye or have labial pits on their upper lip just
below the nostrils (common in pythons) which allow them to
“see” the radiated heat and thus locate prey especially warm-
blooded mammals.
Of the roughly 725 species of venomous snakes worldwide,
only 250 are able to kill a human with one bite. The official
records state about 2, 50,000 snakes bite cases occur in India
every year, of which, over 50,000 die due to inadequate first aid
or unscientific treatment methods (3). The Indian snake bite
statistics are alarming as the figures are highest in the world,
though India does not host the largest number of snakes in the
world.
Venom Apparatus
The Venom apparatus of a snake (Fig. - 1) comprises a venom
gland which opens through the venom duct into fangs located
in front of the upper jaw which have a venom canal along which
venom is introduced while biting into the tissues of the prey. If
a human is bitten, venom is injected either subcutaneously or
intramuscularly.
Classification of snakes
The venomous snakes of the Indian continent may be broadly
classified into two important families i.e. Elapidae and
Viperidae. Snakes belonging to family Elapidae are the Cobras,
Kraits and Coral snakes. This group is characterized by the
presence of short permanently erect fangs. Sea snakes belong
to family Hydrophilidae. The members of family Viperidae are
divided into two subgroups viz. the pit vipers and the typical
vipers; they possess long fangs which are normally folded up
against the upper jaw and are erected while striking.
Fig. - 1 : Venom apparatus of a snake
Venomgland
Venomduct
Venomcanal
Venomous Indian Snakes : The important venomous snakes
in India are Cobra (Naja naja ), Common Krait (Bungarus
caeruleus) and three types of vipers i.e. Russell’s (Daboia
russelii), Saw scaled (Echis carinatus) and the Hump nosed
viper (Hypnale hypnale). The polyvalent antivenom available
as of now in India, however, has the antivenom targeted
against poisoning by only the first four types of snakes. The
identification of snakes is a very specialized task and is best left
to the herpetologists, however, certain common identification
marks and distribution of the venomous snakes of India is
presented below:
a) Cobra: Found all over India, are active during day and
night. Raises a hood which may have no mark e.g. king cobra
(Ophiophagus hannah), or hood may be spectacled or bicellate
(Naja naja) or monocled (monocellate i.e. single circular mark)
- Naja kaouthia (Fig. - 2).
b) Krait: Found all over India, are generally encountered at
night. It is the most venomous of all land snakes. The common
krait or Bungarus caeruleus is identified by paired white lines
on the body, whereas the other variety is the banded krait
Bungarus fasciatus (Fig. - 3).
c) Saw Scaled Viper: Found all over India and prefers dry scrub
or desert. Echis carinatus is identified by the White loop marks
along body and arrow or birds foot mark on head (Fig. - 4)
d) Russell’s Viper: Found throughout India up to 3000 m
altitude. Daboia russeli is identified by the black edged chain
like marks and white V on head (Fig. - 5)
e) Hump Nosed Viper: Hypnale hypnale, the hump nosed
viper is found in the Western ghats. It is identified by its
characteristic hump nose, whereas the other features resemble
Echis carinatus (Fig. - 6).
Snake Venom
Snake venom contains more than 20 different constituents
which evoke varied responses in the body. These constituents
are: procoagulant enzymes, haemorrhagins, cytolytic or

67. • 967 •
necrotic toxins, haemolytic and myolytic phospholipases A2,
pre-synaptic neurotoxins and post-synaptic neurotoxins. It is
important to note that not all venomous snake bites lead to
clinical effects as most of the time insufficient or no venom
enters the wound.
Envenomation Symptoms : The local symptoms and signs of
snake bite are fang marks, local pain and bleeding, bruising,
lymphangitis, lymph node enlargement, inflammation (swelling,
redness and heat), blistering, local infection, abscess formation
and necrosis. The systemic action of snake venom is either on
the nervous system (Neurotoxic) or on the haematological
system (anti-haemostatic). It may also cause local damage
or reaction as a result of the constituents of venom which
primarily function to spread the venom throughout the body.
Symptoms seen in neurotoxic type (due to elapid bites) are
drooping eyelids, vision disturbances, difficulty in breathing,
Fig. - 2 : Hoods of Cobra
Fig. - 3 : Morphology of Kraits
Common Krait
(Bungarus caeruleus)
Banded Krait
(Bungarus fasciatus)
Fig. - 4 : Saw scaled Viper
Photo courtesy Ian Simpson
Fig. - 5 : Russell’s viper
Photo courtesy Ian Simpson
Fig. - 6 : Hump nosed Viper

68. • 968 •
difficulty in speaking, opening of mouth or protruding the
tongue, difficulty supporting the neck and head and difficulty
in swallowing. The presentation in haematoxic type (due to
vipers) is continuous bleeding from bite site, bleeding from
gums or nose, appearance of bruises and dark urine.
Syndromic Approach as Guidelines
A “syndromic approach” as laid down by WHO may be useful
in prompt and effective management of snake bite especially
when the snake has not been identified; the envenomation
symptoms provide the relevant guidelines as presented in
Table - 1.
Table - 1 : Snake identification based on Envenomation
Syndrome
Envenomation Syndrome Snake species
Local swelling with bleeding/
clotting disturbance
Above + Shock or renal failure●●
Ptosis, external ophthalmo-●●
plegia, facial paralysis and
dark brown urine
Paralysis with dark brown urine ●●
and renal failure along with
bleeding/clotting disturbance
All Viper species

Russell’s & Saw ●●
scaled viper
Russell’s viper ●●
- reported from
South India
Russell’s viper ●●
- reported from
South India
Paralysis with minimal or no
local envenoming
Krait
Local envenoming (swelling etc)
with paralysis
Cobra or King cobra
Prevention of Snake Bites
Snake bites can be effectively prevented by following certain
simple preventive measures; the following do’s and don’ts
should be followed to prevent snake bites:
Do not try to kill a snake. Many people are bitten because ●●
they try to kill a snake or get too close to it- leave snakes
alone.
While trekking/ hiking, stay out of tall grass unless you are ●●
wearing thick leather boots and remain on hiking paths as
much as possible.
Keep hands and feet out of areas you cannot see. Do not ●●
pick up rocks or firewood unless you are out of a snake’s
striking distance.
Be cautious and alert when climbing rocks.●●
Wear boots and avoid moving barefoot in snake infested ●●
areas.
Always carry a torch while moving in darkness.●●
Remove garbage or any other junk material lying in and ●●
around the house - keep surroundings rubble free.
In snake infested areas, shake shoes well before wearing.●●
Dust bedding before sleeping, use mosquito nets while ●●
sleeping in camp sited areas and preferably avoid sleeping
on ground.
First Aid
Whenever dealing with a snake bite case, it is pertinent to
remember that a substantial proportion of all snake bites are
actually due to non-poisonous snakes; secondly, almost 50% of
bites by venomous snakes do not inject enough poison and hence
it is important to reassure the person. The following measures
(Do’s & Don’ts) can save lives even in case of venomous snake
bites, if timely action is taken (See Box - 1).
Box - 1 : Snake Bites - Do’s and Dont’s
Do’s
Calm the patient down to slow down blood circulation and retard the spread of venom.
Have the victim lie down with the affected limb lower than the heart.
Immobilize the bitten limb, using a splint if possible and position it below the level of the heart.
Get the victims to hospital urgently, lying flat, if possible - do not wait for the symptoms to develop.
Start Artificial Respiration, this can be life saving in Cobra and Krait bites as the victim may stop breathing, however they are
not dead.
Remove any rings, bracelets, boots, or other restricting items from the bitten extremity as it may swell.
Start antivenin in hospital after assessment of envenomation symptoms by a qualified medical person.
Don’ts
Do not apply any tourniquet or compression bandage (4).
Do not give incision or attempt to suck out venom as it is ineffective at removing venom; and in Viper bites will cause serious
bleeding.
Do not give the victim alcoholic drinks or Aspirin.
Do not wash the bitten area as it may increase the venom flow.
Do not cool the area of the bite.
Do not try to catch the snake; we do not need another victim. If the snake has been killed, take it to the hospital.
Do not give antivenin in any other place other than in a hospital.

69. • 969 •
Acknowledgement: The author gratefully acknowledges the
inputs by Ashok Captain, a well known Indian Herpetologist
and Ian Simpson for the photographs.
Summary
Snakes are one of the most widely distributed animals. There
are over 3000 species of snakes distributed worldwide and
about 275 species are found in the Indian sub-continent. Snakes
belong to the class Reptilia, are limbless and characterized by
elongated body which is divided into head, body and tail. The
whole body is covered with scales, which are covered by skin.
The skin is shed or cast off during the process of moulting.
Snakes locate their prey by their senses of vision, smell or
thermo-sensitivity, body is also very sensitive to vibration. Of
the roughly 725 species of venomous snakes worldwide, only
250 are able to kill a human with one bite.
The snakes are classified into families: Elapidae and Viperidae.
Elapids are the Cobras, Kraits and Coral snakes. Viperidae
are further divided into two subgroups- the pit vipers and the
typical vipers. The important venomous snakes in India are
Cobra, Common Krait and the three types of vipers i.e. Russell’s,
Saw scaled and the Hump nosed viper.
Not all venomous snake bites lead to clinical effects as most of
the time insufficient or no venom enters the wound. The local
symptoms and signs of snake bite are fang marks, local pain
and bleeding, bruising, lymphangitis, lymph node enlargement,
blistering, local infection, abscess formation and necrosis. The
systemic action of snake venom is either on the nervous system
or on the haematological system. Neurotoxic symptoms due to
elapid bites are drooping eyelids, vision disturbances, difficulty
in breathing, difficulty in speaking and opening the mouth.
Haematoxic symptoms (due to vipers) are continuous bleeding
from bite site, bleeding from gums or nose, appearance of
bruises and dark urine.
Prevention of snake bite is by avoiding snake infested areas,
staying out of tall grass, while hiking-keeping hands and
feet out of areas one cannot see, wearing boots, keeping
surroundings rubble free & dusting the beds before sleeping.
First aid involves reassurance, making the victim lie down with
the affected limb lower than the heart. It is important to start
artificial respiration and remove any rings, bracelets, boots etc.
Antivenom should only be given in a hospital after assessment
of envenomation. It is not advisable to apply any tourniquet or
compression bandage or give incision. The victim should not
be given any alcoholic drinks or Aspirin nor should the bitten
area be washed.
Study Exercises
MCQs & Exercises
1) The common poisonous snakes of India are (a) Cobra
(b) Common Krait (c) Russell’s vipers (d) All of these.
2) Which of these should not be done in case of snake bite
(a) Victim should be made to lie down with affected limb
lower than the heart. (b) Give incision (c) Start artificial
respiration (d) Start antivenom
Fill in the Blanks:
3) Snakes belong to the class __________
4) The most venomous of all land snakes, identified by paired
white lines on the body is__________
5) The skin which covers the scales of snakes is periodically
shed of & the process is called as __________
True or false:
6) The body of the snakes which is in direct contact with the
ground is not very sensitive to vibration.
7) Drooping eyelids, vision disturbances, difficulty in
breathing, difficulty in speaking are symptoms seen in bite
due to elapids.
Answers: (1) d; (2) b; (3) Reptilia; (4) Krait; (5) Moulting;
(6) False; (7) True.
References & Further Suggested Reading
Conant, Roger. A Field Guide to Reptiles and Amphibians Eastern/Central 1.
North America. Boston, Massachusetts: Houghton Mifflin Company. 1991;
pp.143.
Whitaker Romulus, Captain Ashok. Snakes of India: The Field Guide (2004) 2.
pp 11 -13.
Sinha, Kounteya (25 Jul 2006), “No more the land of snake charmers...”, 3.
The Times of India.
Simpson Ian D. Modern Snakebite First Aid for the Wilderness Environment. 4.
W.H.O. Snakebite Treatment Group. Tamil Nadu Government Snakebite
Taskforce.
Simpson ID, Norris RL. Snakes of medical importance in India: is the concept 5.
of the ‘Big 4’ still relevant and useful? Wilderness Environ Med. 2007; 18
(1): 2 - 9.

70. • 970 •
Anopheles Mosquitoes Identification : Group - I
Costauniformlydarki.e.notinterruptedbyanypalespot.
Alsonowhitespotonthewingfield
Anteriorforkedcellmuchlargerthantheposterior
A.aitkeni
Anteriorforkedcellofnearlythesamesize
asposterior
Distinctwhitebandingatthedistalendofhind
femur;frontalwhitescaletuftpresent.
A.barianensis Bandingabsent.
Scaletuftabsent
A.culiciformis

71. • 971 •
Anopheles Mosquitoes Identification : Group - II
Lessthanfourdarkspots,involvingthecosta,
subcostaandvein1
Palpiwithdistinctpalebanding PalpiUnbanded
Aprominenttuftof
scalesblackabove&
whitebelow,about
femoro-tibialjointof
hindlegs
A.annandalei
Nosuchtuftof
scalespresent
Hindfemurwitha
conspicuouswhiteband
A.lindesayi
Hindfemurwithout
anywhiteband
a)Innerquarterofcosta
withmarkedpale
interruptions.
b)Wingfringebetween
V5.2and6white.A
brightlycoloured
mosquito
A.gigas varA.hyrcanus
nigerrimus
A.barbirostris
a)Innerquarterof
costadark.
b)Tipofwinggolden.
Adarkmosquito
Palpishaggy.Fringespot
atV5.2present.Presence
ofbothdarkandwhite
scalesonwingveins.
Acommonspecies
Palpithinner.Fringespot
atV5.2absent.Onlydark
scalesonthewingfield.
ArarespeciesinIndia.
A.umbrosus
GroupIIItoVIhaveatleast4 darkspots
involvingthecosta,subcostaandvein1.

72. • 972 •
Anopheles Mosquitoes Identification : Group - III
Dark footed series (of hind legs only).
Femorae & Tibiae not speckled
Tips of palpi dark Tips of palpi pale
Wingveinsexcept on costa
& V1 containonlydarkscales
A. dthali
All wing veins contain
both dark & white
scales
V3 mostly dark V3 mostly pale
2 indefinite dark spot on
V6 the distal one being
very long
A. turkhudi 3 definite dark spots
on V6
A. multicolor
Only 2 fringe spots
V 4.2 and 5.1 present
(palpi of normal
length)
A. culicifacies
Fringe spots at all
veins except V6 .
(palpi very long)
A. sergenti
The two apicalpalebands are of equalor
nearlyequallengthand the intervening dark
area issmall
The two pale apical bands are
definitely unequal
No fringe spots
at V6
Fringe spots at V6
A. aconitus
The intervening dark
area on the palp is
much larger than
either of them
The intervening dark area
is either of the same size
as the apical pale band,
or very much smaller.
Broad white bands at the
tarsal joints of the front
legs.
Basal area of the costa
dark without any
pale interruption
A. varuna Basalarea of the costa
with one pale interruption
A. minimus
Inner third of costa
uninterruptedly dark
Inner third of costa
interrupted
The dark area is of
the same size as
the apical area.
A. subpictus
The dark area is
very much smaller.
A. vagus
2 dark spots on
V6. Tibiotarsal
jointsdark.
A. fluviatilis 3 dark spots on
V6. Tibiotarsal
and tarsal joints
of front legs
narrowly banded.
A. moghulensis Tarsal joints of leg 1
banded. Fringe
spot at V6 .
A. jeyporiensis
Tarsal banding of
leg 1 absent.
No fringe
spot at V6
A. superpictus

73. • 973 •
Anopheles Mosquitoes Identification : Group - IV
Darkfootedseries(Hindlegsonly).
Femorae& Tibiaespeckled
3palebandsonthepalp 4palebandsonthepalp
Theapicalandthesub
apicalbandsare
equallybroad
A.stephensi
Theapicalandthesub
apicalbandsare
unequaltheformer
beingbroaderthan
thelatter
A.sundaicus
Thethreedistalbands
aremuchbroader
thentheproximal
bandwhichisnarrow
A.tessellatus
Broadtibio-tarsal
whitebandsonthe
hindleg
A.leucosphyrus

74. • 974 •
Anopheles Mosquitoes Identification : Group - V
Whitefootedseries(Hindlegsonly)
Femorae& Tibiaenotspeckled
Onlytarsus5a nd1/3rdoftarsus4
ofhindlegscompletelywhite Atleasttarsalsegments3,4and5of
hindlegscompletelywhite
3palebandsonthepalp
A.majidi
4palebandsonthepalp
A.karwari
Vein5mainlydarkwitha
darkspotatitsbifurcation
A.annularis
Vein5extensivelypaleand
nodarkspotatitsbifurcation
Conspicuouswhitescaleson
thedorsumoftheabdomen
&thorax
A.pulcherrimus
Distalendoftarsus1o f
hindlegsconspicuously
markedwhite
A.philippinensis
Distalendoftarsus1o f
hindlegsdark
A.pallidus

75. • 975 •
Anopheles Mosquitoes Identification : Group - VI
Whitefootedseries(Hindlegsonly).
Femorae& Tibiaespeckled
Halfoftarsus5o fhind
legswhite.Prominent
scaletuftsontheventral
surfaceofeach
abdominalsegment.
A.kochi
Wholeoftarsus5 and
1/3oftarsus4o fhindlegs
whitewitha darkband
ontarsus4.
A.maculatus
A.maculatesvarwillmori
Wholeoftarsus5 and4
ofhindlegswhite
A.theobaldi
Atleasttarsi3,4and5
ofhindlegscompletely
white.
Twoequallybroadpale
apicalbands,palpi
speckled
A.splendidus
Atleastthelasttwo
segmentsofthedorsum
oftheabdomencovered
withgoldenscales
A.jamesi
Conspicuouswhitescalesonthe
dorsumofthethoraxandabdomen
(Femoraeandtibiaemaybespeckled
ornotspeckled)
A.pulcherrimus
Darkscalesontheabdomen
A.ramsayi
Thetwopaleapical
bandsareunequal