The first human cases of Opisthorchis viverrini infection
were reported in Thailand nearly 100 years ago.1 O. viverrini
is endemic in the Lower Mekong Basin, including Thailand,
Lao People’s Democratic Republic (Lao PDR), Cambodia,
and central Vietnam.1 Specifically, O. viverrini and other
fluke infections (i.e., minute intestinal flukes [MIFs] and the
blood fluke Schistosoma mekongi) and soil-transmitted helminths
are endemic.
The adult worm of O. viverrini inhabits the intra- and extrahepatic
biliary system. The embryonated eggs containing a
miracidium released from the gravid worm are discharged
with the bile and eventually passed into environment in the
fecal stream. After reaching freshwater, the egg is ingested by
snails of the genus Bithynia. The snails are abundant in water
reservoirs with shallow water, rice fields, and wetlands.
Within the snail, eggs hatch to release miracidia, which transform
to sporocysts. Then, sporocysts undergo asexual reproduction
to give rise to rediae and finally, cercariae. After the
cercariae are released from snails, they attach to and penetrate
the skin of freshwater fish (belonging to 1 of 20 susceptible
species of cyprinids) from infective stage, the metacercaria.
Humans are the major definitive host. People become
infected when they ingest raw or inadequately cooked fish,
such as koi pla (fish salad) and pla som (short fermented fish),
particularly in Lao PDR. After ingestion of fish flesh, the
encysted metacercariae are digested by gastric and intestinal
juices, and excysted juvenile flukes migrate through the ampulla
of Vater into the common bile duct and the intrahepatic bile
duct, where they mature and sexually reproduce. Some flukes
can be found in the common bile duct, cystic duct, and even
the gallbladder.