Centers for Disease Control and Prevention Website
A cycle of human infection begins when the embryonated eggs of the
parasites O. viverrini and O. felineus are passed through
the feces of an infected mammal and enter the water habitat of the first
intermediate host, a freshwater snail of the genus Bithynia.
Inside their snail host, the eggs hatch in the alimentary canal of the
snail and are now call miracidia. The miracidia penetrate the intestine
and develop into sporocysts. The sporocysts develop into radiae and
mature into cercariae in the digestive gland of the snail.
The cercariae leave the snail and penetrate the second intermediate
host, a freshwater fish. They encyst as metacercariae in the muscles
or under the scales of the fish. The definitive host of the parasite
is the mammal that ingests an undercooked fish containing the metacercariae.
Once ingested, the metacercariae excyst in the duodenum and travel to
the biliary ducts where they attach and develop into adults that lay
eggs after 3-4 weeks. This adult "liver fluke" will live in
the biliary and pancreatic ducts of the host and attach to the mucosa.
Life cycle of both O. viverrini and O. felineus in
Image from the Centers for Disease Control and Prevention website
Dogs, cats, and other fish eating mammals are the reservoir for the
parasites that cause human infections of Opisthorchiasis. There is not
a vector of this parasite.
Centers for Disease Control and Prevention Website, Mehlorn (2001)
Humans become infected through ingestion of raw, pickled, or poorly
cooked fish that contain the metacercariae of the parasite. Infected
humans and other mammals excrete the eggs of the parasite through their
feces. The first intermediate host, a freshwater snail, which is often
Bithynia leachi, ingests these eggs. The eggs hatch into free-swimming
cercariae and implant themselves into their second intermediate host,
a freshwater fish. The cycle of infection continues once these fish
are eaten without proper preparation to kill the parasite