Ascaris lumbricoides is the largest nematode (roundworm) parasitizing the human intestine (adult females are 20 to 35 cm and adult males 15 to 30 cm in length). Infection with this parasite is known as ascariasis and is the most common helminthic infection in the world ("helminth" is a functional term, rather than a meaningful taxonomic one, referring to any worm-like internal parasite and used mainly to refer to parasitic flatworms [phylum Platyhelminthes] and roundworms [phylum Nemata or Nematoda]).
Adult A. lumbricoides live in the lumen of the small intestine. A female may produce 200,000 eggs each day, which are passed with the feces of the host. Ingested unfertilized eggs are not infective, but fertile eggs begin to develop and become infective after 18 days to several weeks, depending on environmental conditions (an optimal environment being moist, warm, shaded soil). After infective eggs are swallowed, the larvae hatch, invade the intestinal mucosa, and are carried via first the portal and then the systemic circulation to the lungs. The larvae mature further in the lungs for 10 to 14 days, then penetrate the alveolar walls, ascend the bronchial tree to the throat, and are swallowed. Upon reaching the small intestine, they develop into adult worms. Between two and three months are required from ingestion of the infective eggs to oviposition (egg-laying) of the adult female. Adult worms can live one to two years.
Ascaris lumbricoides has a worldwide distribution, but is most prevalent in tropical and subtropical regions and areas with inadequate sanitation. In the United States, it occurs in rural regions of the southeastern U.S.
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