Strongyloidiasis is caused by the round worm Strongyloides stercoralis. This worm is fairly common in warm moist areas, but in North America it can be found as far north as Canada. 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.
Exposure occurs when skin has direct contact with contaminated soil.
The larvae migrate through the skin to the intestines. Unlike the hookworm life cycle where the eggs must pass in the feces and mature in damp earth and vegetation, the strongyloides eggs hatch and the larvae become infective even at the time they are passed in the feces. Because of this, the larvae may penetrate the skin near the anus and repeat the cycle. The individual becomes more heavily infected with each cycle. Areas near the anus where the larvae penetrate may become red, irritated and hive-like in appearance.