Causal Agent:
The nematode (roundworm) Gnathostoma spinigerum and Gnathostoma hispidum, which infects vertebrate animals.  Human gnathostomiasis is due to migrating immature worms.

Life Cycle:

Adapted from a drawing provided by Dr. Sylvia Paz Díaz Camacho, Universidade Autónoma de Sinaloa, Mexico.

In the natural definitive host (pigs, cats, dogs, wild animals) the adult worms reside in a tumor which they induce in the gastric wall.  They deposit eggs that are unembryonated when passed in the feces .  Eggs become embryonated in water, and eggs 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 (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 .

Geographic Distribution:
Asia, especially Thailand and Japan; recently emerged as an important human parasite in Mexico.