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The nematodes (roundworms) Gnathostoma spinigerum and G. hispidum, infect vertebrate animals. Human gnathostomiasis is due to migrating immature worms. This disease is known from Asia, especially Thailand and Japan, and these worms have recently emerged as important human parasites in Mexico.
In the natural definitive host (pigs, cats, dogs, wild vertebrates), the adult worms reside in a tumor that they induce in the gastric wall. They deposit eggs that are unembryonated when passed in the feces. Eggs become embryonated in water and release first-stage larvae. If ingested by a small crustacean (Cyclops, first intermediate host), the first-stage larvae develop into second-stage larvae. Following ingestion of the Cyclops by a fish, frog, or snake (second intermediate host), the second-stage larvae migrate into the flesh and develop into third-stage larvae. When the second intermediate host is ingested by a definitive host, the third-stage larvae develop into adult parasites in the stomach wall. Alternatively, the second intermediate host may be ingested by the paratenic host, or "transport host" (animals such as birds, snakes, and frogs), in which the third-stage larvae do not develop further but remain infective to the next predator. Humans become infected by eating undercooked fish or poultry containing third-stage larvae, or reportedly by drinking water containing infective second-stage larvae in Cyclops.