The nematode (roundworm) Strongyloides
stercoralis. Other Strongyloides include S. fülleborni, which
infects chimpanzees and baboons and may produce limited infections in humans.
The Strongyloides life cycle is
more complex than that of most nematodes 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
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.
subtropical areas, but cases also occur in temperate areas (including the South of the
United States). More frequently found in rural areas, institutional settings, and lower