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The nematode (roundworm) Angiostrongylus cantonensis, the Rat Lungworm, is the most common cause of human eosinophilic meningitis, a meningoencephalitis characterized by eosinophils in the cerebrospinal fluid. Humans are accidental hosts for these parasites. Most cases of eosinophilic meningitis have been reported from Southeast Asia and the Pacific Basin, although the infection is spreading to many other areas of the world, including Africa and the Caribbean.
Adult A. cantonensis live in the pulmonary arteries of rats. The females lay eggs that hatch, yielding first-stage larvae, in the terminal branches of the pulmonary arteries. The first-stage larvae migrate to the pharynx, are swallowed, and pass in the feces. They penetrate, or are ingested by, an intermediate host (snail or slug). After two molts, third-stage larvae are produced, which are infective to mammalian hosts. When the mollusk is ingested by the definitive host, the third-stage larvae migrate to the brain where they develop into young adults. The young adults return to the venous system and then to the pulmonary arteries, where they become sexually mature. A variety of animals can act as paratenic (transport) hosts: after ingesting the infected snails, they carry the third-stage larvae, which can resume their development when the paratenic host is ingested by a definitive host. Humans can acquire the infection by eating raw or undercooked snails or slugs infected with the parasite; they may also acquire the infection by eating raw produce that contains a small snail or slug, or part of one. There is some question regarding whether or not larvae can exit the infected mollusks in slime (which might be infective to humans if ingested, for example, on produce). The disease can also be acquired by ingestion of contaminated or infected paratenic animals (crabs, freshwater shrimps). In humans, juvenile worms migrate to the brain, or rarely to the lungs, where they ultimately die.