This tiny worm is barely visible to the naked eye. Its life cycle is very similar to the hookworm in that it has an infective larval stage where it can penetrate the skin, a stage when it migrates from the skin through the bloodstream to the lungs and up the trachea where it is swallowed, and a phase where it matures to the adult worm, buries into the intestinal wall and begins producing eggs (2).

Roll mouse over picture below to see the life cycle of Strongyloides stercoralis.

Two types of cycles exist:

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 in the lumen 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", below), 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 both cases the filariform larvae 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. (Autoinfection is a feature unique to Strongyloides among helminthic infections in humans, and explains the possibility of persistent infections and of hyperinfections in immunodepressed individuals.(3))

The autoinfection characteristic, or its capacity to replicate within the host, separates S. stercoralis from almost all other worms that infect humans. With the result that infection persists for many years: the current record appears to be 65 years. How this happens is still a mystery (4).

Free-living cycle: The rhabditiform larvae passed in the stool can either molt twice and become infective (filariform) larvae (direct development) or molt four times and become free living adult males and females which mate and produce rhabditiform larvae. These larvae produced can either develop into new free-living adults, or into infective (filariform) larvae. The filariform larvae penetrate the human host skin to initiate the parasitic cycle (3).

The incubation period from penetration of the skin by filariform larvae until rhabditiform larvae appear in the stool is 2 to 4 weeks. The period until symptoms appear is indefinite and variable.

Humans are the principal reservoir of S. stercoralis, with only occasional transmission of dog and cat strains to man (5).