Causal Agent:
Trematodes in the genus, Echinostoma.  The genus is worldwide, and about ten species have been recorded in humans, including E. hortense, E. macrorchis, E. revolutum, E. ilocanum and E. perfoliatum.

Life Cycle:

Many animals may serve as definitive hosts for various echinostome species, including aquatic birds, carnivores, rodents and humans.  Unembryonated eggs are passed in feces and develop in the water .  The miracidium takes on average 10 days to mature before hatching and penetrating the first intermediate host, a snail .  Several genera of snails may serve as the first intermediate host.  The intramolluscan stages include a sporocyst , one or two generations of rediae , and cercariae .  The cercariae may encyst as metacercariae within the same first intermediate host or leave the host and penetrate a new second intermediate host . Depending on the species, several animals may serve as the second intermediate host, including other snails, bivalves, fish, and tadpoles.  The definitive host becomes infected after eating infected second intermediate hosts .  Metacercariae excyst in the duodenum and adults reside in the small intestine .

Geographic Distribution:
Worldwide, but human cases are seen most-frequently in southeast Asia and in areas where undercooked or raw freshwater snails, clams and fish are eaten.