Brief description of nematode strongyloides stercoralis and its pathogenesis

Brief description of nematode strongyloides stercoralis and its pathogenesis

1. Brief description of nematode Strongyloides stercoralis and its pathogenesis
By Negash Alamin
Clinical laboratory scientist
December 7, 2017
E-mail: [email protected]

The general visage of a nematode looks like the figure presented in figure 1; but I believe understanding
the specific anatomy of Strongyloides stercoralis will give us an insight into its pathogenesis thus a detail
presentation has been presented in figure 2. The parasite is classified in the taxonomical order under
phylum Nemathelminthes in the class Nematoda, Sub class Phasmidia (with caudal chemoreceptors
1
) ,
suborder Rhabditina, superfamily rhabditoidea, family strongyloididae and species stercoralis.

General characteristics of nematodes where S. stercoralis belongs:
 Nematodes are elongated cylindrical worms having a cavity in which their organs lie.
 Their size vary from the smallest like Strongyloides to the longest like D. medinensis
 Their body is tough and has a resistant cuticle.
 The alimentary canal is complete possessing mouth
2
and anus at opposite proximal and distal
regions.
 They have no circulatory and respiratory organs.
 They are sexually differentiated and males are smaller than the females.

S.stercoralis is a parasite of the small intestine; which is prevalent and frequent in the tropics. It has a free
living and parasitic form (see its life cycle for details). The female lies buried in the mucosa of the
jejunum and the males are found free though rarely in the lumen of the small intestine. The infective
larvae which evolve from the free living stages are found in contaminated soil, water and latrines. They
need a warm and moist soil to evolve and complete their life cycle.

In the female the esophagus is like a cylinder and is longer than the male extending 1/3 of the worm.
They are colorless, slender and measure 2.2 mm by 0.075 to 0.3 mm with a posterior end straightened and
anterior (head) has a short buccal cavity and four indistinct lips.

1
Phasmid is term used in relation to nematodes which possess caudal chemoreceptors (special sensory organs)
situated on the papillae behind the anus. In contrast T. trichuria and T. spiralis are categprized under Subclass
Aphasmidia
2
In primitive free-living nematodes the mouth is surrounded by three lips, 1 dorsal and 2 latro-ventrals. Whereas,
in filaria form there is no lip.

2. Eggs
Primarily the eggs lie in the uterus oviduct about 6-8 eggs can be seen (see figure 2). The eggs are oval
covered by a thin transparent shell measuring about 50 microns by 30 microns. They have segmentations
(embryonated) in the central region of the cell; which eventually hatch out within a few hours into a
rhabditiform larva and this is what is discovered in stool.
The adult parasitic female as said above dwells in the tunnels of the mucosa of the small intestine and she
lays her eggs on the submucosa tissue of the small intestine. As they are excreted into the soil by a human
they feed on organic debris and evolve into a filariform larvae which is the infective stage
3
.
Figure 1 Digestive and reproductive system of a general nematode A- female B-male (T.K Dey; Medical parasitology: 2010)



3
T.K Dey; Medical parasitology: 2010

3. Development leading to pathology
Indirect development of the larvae is the development of the rhabditiform larva under favorable
conditions into free-living male and female rhabditiform which mate and produce eggs outside the host
and after a few days they become filariform and infect human hosts by skin penetration.
Direct development of S.stercoralis refers to its life cycle in which the rhabditiform undergo molting and
directly evolve within a few days into filariform larva without distinguishing into male and female. The
filaria forms can remain in the soil for several weeks.
Figure 2 Strongloides stercoralis morphology

Pathological route and processes
When the filariform enter the buccal cavity or the integumentary system of a human host they migrate via
the blood vessels to the right section of the heart. They break through the pulmonary capillaries and enter
the alveoli; where they develop into post filariform (adolescent forms). In the bronchi some of the
adolescent females oviposit; but, others molt two times move to the pharynx and are swallowed to reach
the intestine to begin the oviposition of eggs
4
.

4
T.K Dey; Medical parasitology: 2010

4. Figure 3 The pathological life cycle

Autoinfection or hyperinfection
This is a term to denote phenomena in which as it is obvious from the life cycle of the vermin; the
rhabditiform can develop into filariform in heavily parasitized weak patients in the bowel. Meaning there
is a recurrent infection within the host.

What are the major injuries incurred by the host during all these processes?

1- First the infection itself is termed as strongyloidiasis or strongloidosis and it is asymptomatic in people
with intact immune systems.
2- During penetration dermatitis, creeping eruptions, urticarial rash, and petechiae will manifest.
3- Massive amount of the pathogen might migrate to vital organs of the body like the brain which can be
fatal creating cerebral abscess and meningitis.
4- Bronchopneumonia and hemorrhage in the lung from the migration.

5. 5-Intestinal damage due to the female adults from burrowing through the tissues of the intestine and
diarrhea with blood and mucus is produced as a result.
6- Significant leukocytosis and eosinophilia.
7- The parasite may migrate to the skin travelling 10 mm/hr.
5
causing cutaneous strongloidosis or larva
currens via what has been termed autoinfection meaning by penetrating the anal skin areas and migrating
in the cutaneous tissue.

Important points to remember:
The free living (non parasitic) form of the vermin can remain viable in the soil indefinitely.
 The free living phase is also called heterogonic life cycle
6
.
 The number of the parasite can increase without infection from outside due to autoinfection.
 Typically larvae appear in about one month after about 28 days after skin penetration
7
.
 S. stercoralis has a vertical transmission.
 The parasite was not recognized until 1876 when its larvae was discovered by Dr. Louis Alexis
Normand at the French hospital in Toulon
8
.
 The first name given to Strongyloides stercoralis was Anguillula stercoralis by the colleague of
Dr. Norman — Mr. Arthur Rene Jean Baspitiste Bavay
9
.
 The alternation between free-living and parasitic form was determined in 1883 by German
parasitologist Karl Georg Friedrich Rudolf Leuckart.
 The parthenogenic nature of the female parasite in the intestine was noted by parasitologist Karl
in addition to other scientists Kreis and Faust.
 The mode of infection of the larva through direct infection was discovered by a Belgian
Physician Paul Van Durme.
 The mode of autoinfection was determined in dogs by German parasitologist Friedrich
Fulleborn and its migratory habit as well as life cycle of the pathogen in humans.

People are at high risk of infection if they are:
 They are under immunosuppressive therapy like corticosteriods.
 Organ recipients because the vermin may be in the organ for transplantation and it is rarely tested
which can be fatal.
 Senility (ageing)
 Malignancy like leukemia which derails the body’s immune system.
 Live in an endemic region


5
Parasite.org.au
6
emedscape.medscape.com
7
emedscape.medscape.com
8
Web.stanford.edu
9
Web.stanford.edu