Etiology

etiology

1. Intrinsic and extrinsic
causes of disease
Dr. JenyK John
Assistant Professor, Clinical Pathology
Department of Veterinary Clinical Pathology
COVAS, SVPUAT, Meerut

2. Introduction
•Etiology: Study of the cause of disease
•It can be intrinsic or extrinsic
•Intrinsic factors act from within the individual
eg. Genus, breed, species etc
•Extrinsic factors are the influences from outside
•Eg. Physical, chemical, biological etc

3. Intrinsic causes of disease/Predisposing /Internal factors/endogenous
1.Geneticorinheritedcauses
Transmittedtotheoffspringthroughthegermplasm
a.Lethalcharacters:inheritedfromparentsandinvariablycausedeathofthe
animal,eitherin-uteroorafterbirth
Eg.Atresiacoliinhorses,parrotbeakincattle
b.Sublethalfactors:Inheritedandinterferewiththefunctionofthebodybut
donotcausedeathoftheanimal
Eg.Imperforateanus,infertilityincattle,scrotalherniainpigs,
deafnessinwhitecats
c.Errorsinstructureorfunction:inheritedbutnotlethalonlyaffectthestructure
andfunction
Eg.Cryptorchidism,webbeddigits

4. Conti….
2. Non genetic or non inherited defects
Abnormalities not transmitted via the germplasm
a.Anomalies : development defect affecting an organ or part of the body
These may be
1.Disturbance in development
a. Arrest of development
b. Excessive development

5. A. Arrest in development
1.Agenesis or aplasia
2.Hypoplasia
3.Atresia
4.Fissure
5.Fusion of paired organs: Horse shoe Kidneys
B. Excessive development
1.Congenital hypertrophy
2.Increase in number

6. Other intrinsic causes
1.Genus
Certaindiseasesarepeculiartocertaingeneraofanimalswhileothersarenotaffected
Eg.Swinefeveristhediseaseofpigsandnootheranimalareinfected
2.Breed
Certainbreedseitherduetoselectivebreedingorinherentcharacteristics,aremoresusceptibletosomediseasesthanothers
Eg.GermanShepherd,GreatDanesaremorepronetobonediseases,Bulldogbreedsandboxersuffermorebraintumors
3.Age:
Differentdiseasesaffectdifferentagegroups
Eg.Stranglesaffectsyoungerhorses,caninedistemperaffectspups,tumorsingeneralarefoundmorefrequentlyinolder
agegroups,deficiencydiseasemoreinyounganimals,caecalcoccidiosisinyoungbirds

7. Conti…
4.Sex:
Maleandfemaledifferinthesusceptibilitytodisease
Eg.Malearemoresusceptibletonephritis(dog)andincattlemore
commoninfemales,Reproductivediseasesaremorecommoninthe
femalesthaninthemale
5.Colour
Eg.Greyandwhitehorsessufferfrommalignantmelanomamore
frequentlythanothercolouredhorses
Animalwithwhiteskinornonpigmentedskinmoresusceptibleto
photodynamicdiseases

8. Extrinsic causes/Exciting causes/exogenous
1. Physical agents
1.Physical influences
1.Mechanical trauma: perforation, laceration, concussion, sprain, luxation or
dislocation, fracture
2.Injuries due to pressure: atropy, bed sores
3.Injuries due to obstruction
4.Injuries due to malposition

9. 4.Injuries due to malposition
Volvulus: Rotation of the small intestine around its mesenteric
attachment
Torsion: Twisting of the organ upon itself , seen in uterus, large
intestine
Intussusception: Telescoping or invagination of one portion of
the intestine into immediately posterior portion of the gut
Prolapse:Appearance of an organ or a portion of an organ at a
natural or artificial opening

10. Excess heat:
•Lesion produced from excessive application of heat is called a thermal burn-surface burns or
inhalation burn injuries
•Excessive heat retention results in heat stroke
Excess cold
•Local effects-Frostbite-Local freezing is usually confined to the extremities egfeet, teats,
scrotum, tails, fetlocks, pasterns, coronary bands, combs, wattle and toe of fowls
•Systemic effects
3. Light and UV rays-sunburn/photosensitation
4. Electricity: Electrical burns and electrocution
2. Thermal injury:

11. 4.Injuries from change in atmospheric pressure
a. Decreased atmosphere pressure: Brisket disease
Animal moved from lower altitudes to an altitude of 8000 feet or more, they suffer from brisket disease
b. Increased atmosphere pressure: Caisson Disease Also called bends
Sudden decrease in pressure, free gaseous bubbles ,may be released in the blood and act as emboli

12. 5. Those that injure by radiation
1.X-rays
2.Gamma rays
3.Particulate radiation
Important target in living cell is DNA

13. 2. Chemical causes:
•Most toxic chemicals are poisons
•Inorganic poisons: acids (sulphuricacid), base (potassium hydroxide) and salts
(mercuric chloride)
•Organic poisons are derived from plants, molds, fungi and bacteria and certain
parasites and venomous animals
•Toxic agents exert corrosive or caustic action or act as organ poisons or nerve
poisons or they alter the blood

14. •Bacteria
•Viruses
•Helminths
•Trematodes
•Cestodes
•Arthopodes
•Fungi
•Rickettsiae
•Protozoa etc
3.Biological causes

15. 4. Immunological reactions:
Although the immune system serves in the defense against infectious agents,
immune reactions may cause cell injury
Eg. Autoimmune reaction, Anaphylactic reaction to drugs or foreign protein,
immunodeficiency

16. 5. Nutritional imbalance
Nutritional deficiencies and excess of nutrition
•Excess-HypervitaminosisA, D
•Deficiency-Hypoproteinaemia, hypovitamonosis(e.g. Xerophthalmiain
vitamin A deficiency, star gazing in chicken in vitamin B 1 deficiency),
Hypocalcaemia (Milk fever in high yielding cows)

17. Thank you