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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
.
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