.
Eggs are ingested by a suitable snail intermediate host
; there
are more than 100 species of snails that can serve as intermediate hosts.
Each egg releases a miracidia
, which
go through several developmental stages (sporocysts
, rediae
, and
cercariae
). The
cercariae are released from the snail and after a short period of
free-swimming time in water, they come in contact and penetrate the flesh of
freshwater fish, where they encyst as metacercariae
.
Infection of humans occurs by ingestion of undercooked, salted, pickled, or
smoked freshwater fish
. After
ingestion, the metacercariae excyst in the duodenum
and ascend the biliary tract through the ampulla of Vater
.
Maturation takes approximately 1 month. The adult flukes (measuring 10 to
25 mm by 3 to 5 mm) reside in small and medium sized biliary ducts. In
addition to humans, carnivorous animals can serve as reservoir hosts.