البوم لبويضات ال Strongyloidiasis

Strongyloides stercoralis
first-stage rhabditoid (L1) larvae.

The first-stage rhabditoid larvae (L1) of
Strongyloides stercoralis are 180-380 µm long, with a short
buccal canal, a rhabditoid esophagus and a prominent genital primordium.
These L1 larvae are usually found in stool, as the eggs embryonate and
hatch in the mucosa of the small intestine of the host. They may
also be found in soil and cultured feces.


A	B

A, B: Rhabdititoid
larvae of S. stercoralis in unstained wet mounts of stool.
Notice the short buccal canal and the genital primordium (red arrows).


C	D

C: Close-up of
the anterior end of a rhabditoid larva of S. stercoralis, showing the
short buccal canal (red arrow) and the rhabditoid esophagus (blue arrow).
Image taken at 1000x oil magnification.
D: Rhabditoid larva of S.
stercoralis in an unstained wet mount of stool. Notice the short
buccal canal and the genital primordium (red arrow).


E	F

E: Rhabditoid
larva of S. stercoralis in an unstained wet mount of stool.
Notice the rhabditoid esophagus (blue arrow) and prominent genital
primordium (red arrow).
F: Rhabditoid larva of S. stercoralis in an unstained
wet mount of stool. Notice the prominent genital primordium (blue
arrow), rhabditoid esophagus (red arrow) and short buccal canal (green
arrow).

Life cycle of Strongyloides
stercoralis.


[Strongyloides stercoralis]

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The Strongyloides life cycle is complex among helminths with its
alternation between free-living
and parasitic cycles, and its potential for autoinfection and multiplication within the host.
Two types of cycles exist:

Free-living cycle: The rhabditiform larvae passed in the stool
(see "Parasitic cycle" below) can either molt twice and become infective filariform larvae
(direct development)

or molt four times and become free living adult males and females

that mate and produce eggs

from which rhabditiform larvae hatch
.
The latter in turn can either develop

into a new generation of free-living adults (as represented in
), or into infective filariform larvae
.
The filariform larvae
penetrate the human host skin to initiate the parasitic cycle (see below)
.

Parasitic cycle: Filariform larvae in contaminated soil penetrate the human skin
, and are transported to the lungs where they penetrate the alveolar spaces;
they are carried through the bronchial tree to the pharynx, are swallowed and then reach the small intestine
.
In the small intestine they molt twice and become
adult female worms
.
The females live threaded in the epithelium of the small intestine and by parthenogenesis produce eggs
, which yield rhabditiform larvae.
The rhabditiform larvae can either be passed in the stool

(see "Free-living cycle" above), or can cause autoinfection
.

In autoinfection, the rhabditiform larvae become infective filariform
larvae, which can penetrate either the intestinal mucosa (internal
autoinfection) or the skin of the perianal area (external
autoinfection); in either case, the filariform larvae may follow the
previously described route, being carried successively to the lungs,
the bronchial tree, the pharynx, and the small intestine where they
mature into adults; or they may disseminate widely in the body. To
date, occurrence of autoinfection in humans with helminthic infections
is recognized only in Strongyloides stercoralis and Capillaria philippinensis infections.
In the case of Strongyloides,
autoinfection may explain the possibility of persistent infections for
many years in persons who have not been in an endemic area and of
hyperinfections in immunodepressed individuals.

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