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Strongyloides is also known as Strongyloides stercoralis and, rather confusingly, is sometimes called a 'threadworm' in America. It is more common in the tropical and subtropical countries of West Africa, the Caribbean and Southeast Asia. Although it is uncommon in the United States it can be found in Appalachia and the southeast. It is a parasitic nematode that causes strongyloidiasis and has two different types of life cycle. Some of the Strongyloides larvae will develop into "parasitic" larvae and some will develop into "free-living" larvae that develop outside of a human host.

An infection occurs when the larvae of strongyloides, contained in contaminated faeces in the soil, is able to penetrate human skin. This life cycle is similar to that of the hookworm. It follows the pattern of "tracheal migration" and thrives in the same unsanitary conditions and climates as the hookworm. After the egg matures in the soil the larvae hatch and become infective. The larvae then penetrate any human skin that they come in contact with, usually through bare feet, entering the bloodstream and passes through the right side of the heart into the lungs. The strongyloides then migrate from the lungs and crawl up the windpipe. They are then swallowed and are transported to the upper area of the small intestines where they will develop into mature worms. The worms found in the small intestine are the mature females as male strongyloides worms are not parasitic. Ingestion of the infective strongyloides larvae may also result in infection via the faecal-oral route.

In certain conditions, a more indirect life cycle develops in a "free-living" external environment when mature adult male and female worms develop in the soil and produce a new generation of larva. The female worm produces larvae parthenogenically, meaning without fertilization. This life cycle is unique to strongyloides among helminths, as is the process of "auto-infection".

Due to a cycle of internal "auto-infection" this type of worm can persist in a human host for decades. Some patients have remained infested for as long as 40 years, even after they moved to areas not normally infected with Strongyloides. An example of this was the American soldiers who contracted strongyloides in Burma during the Second World War and suffered from periodic abdominal pain until the infestation was diagnosed and treated decades later. "Auto-infection" occurs when the strongyloides larvae develops rapidly into an infective state within the superficial tissues of the small intestines of the human host. Instead of passing out of the body in the faeces, as would normally occur, they penetrate the host's intestinal mucosa or the skin of the perianal area. This type of "auto-infection" can produce very heavy infections and severe disease. At its most severe, an infestation may cause a life-threatening infection in a human host whose immune system has been immunocompromised by disseminated strongyloidiasis. Hyper-infection syndrome can result from allowing strongyloides to infect most parts of the body.

There is often no symptom of an infestation of strongyloides, especially if it is only a mild one. With a moderate infection the symptoms observed can include a recurrent rash at the site of larval penetration. Coughing and wheezing may be the result of the migration of the larvae to the lungs. Other symptoms include abdominal pain that may be aggravated by food, nausea and/or vomiting, alternating diarrhoea and constipation, epigastric pain and gastrointestinal bleeding. If the infection is a severe one the human host may experience chronic colitis, anaemia caused by blood-loss, chronic diarrhoea, and weight loss.

Because the symptoms are hard to detect, it requires a doctor, with reference to a pathology laboratory, to make a diagnosis of strongyloides infestation. The laboratory examination will include an examination of the faeces and duodenal contents for strongyloides larvae. The presence of nematode larvae in a faecal sample is characteristic of strongylodiasis so the larvae, and not the egg, is the diagnostic stage.

Strongyloides is usually treated with thiabendazole.