Strongyloidiasis is caused by the nematode (roundworm) Strongyloides stercoralis. Other Strongyloides species include S. fülleborni, which infects chimpanzees and baboons and may produce limited infections in humans.
life cycle is more complex than that of most nematodes because it alternates between free-living and parasitic life cycles, and it has a potential for autoinfection and multiplication within the host. Free-living cycle:
The rhabditiform larvae that pass in the stool
(see “Parasitic cycle” below) can either become infective filariform larvae (direct development)
or free living adult males and females
that mate and produce eggs
from which rhabditiform larvae hatch
and eventually become 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 migrate into the small intestine
by various, often random routes. Historically it was believed that the L3 larvae migrate via the bloodstream to the lungs, where they are eventually coughed up and swallowed. However, there is also evidence that L3 larvae can migrate directly to the intestine via connective tissues. 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 disseminate throughout 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 immunosuppressed individuals.
Life cycle image and information courtesy of DPDx.