The trematode Opisthorchis viverrini is one of a trio of closely-related medium-sized liver flukes that inhabit the bile ducts of fish-eating animals. Attaching to the lining of the bile duct, O. viverrini feeds on cells there and grows to a maximum length of about two and a half centimeters (one inch). An estimated six to seven million people harbour this parasite in Southeast Asia. The infection is called opisthorchiasis.
The Life Cycle of Opisthorchis viverrini
Infection with O. viverrini begins with the ingestion of raw freshwater fish, such as carp, that has metacercariae — the larval form of the parasite — encysted in its tissues. As the fish is digested, the metacercariae are freed.
- Juvenile flukes probably migrate directly from the small intestine up the common bile duct to the smaller bile ducts of the liver. They attach to the wall of the duct.
- Mature in less than three months, O. viverrini flukes produce eggs that are washed away with bile, mixed with bowel contents, and passed in the stool.
- Eggs contain a fully developed miracidium — the first larval stage — when passed. Those that enter fresh surface water, and are eaten by an aquatic snail, hatch.
- Each miracidium forms a sporocyst in the snail’s tissues and begins a process of asexual reproduction resulting in the formation of many cercariae.
- After between two and three weeks, cercariae leave the snail and invade the tissues of freshwater fish, developing to metacercariae in the fish muscle tissue.
- Humans and other fish-eating animals become infected when they ingest live metacercariae with a meal.
Symptoms of Opisthorchiasis
Most people who have O. viverrini flukes living in their bile ducts have no symptoms, but the more flukes there are in the liver, the more likely it is that symptoms will appear. Abdominal discomfort, gas, and fatigue are typical, and over time bile ducts become damaged and blocked, leading to a variety of liver abnormalities. An association between opisthorchiasis and cancer of the bile ducts — cholangiocarcinoma — has long been known.
Opisthorchis viverrini and Liver Cancer
The form of liver cancer known as cholangiocarcinoma accounts for more than two thirds of liver cancers in Khon Kaen, Thailand, a province where more than one in three residents are infected with O. viverrini. In the United States, where O. viverrini does not occur, cholangiocarcinoma accounts for just under a fifth of liver cancers (Sripa et al).The association between the parasite and liver cancer is so strong that the parasite has been accepted as a known carcinogen even though the mechanism isn’t fully understood.
Researchers and medical specialists have long speculated that O. viverrini, and several similar flukes, promote the development of cancer in the liver by damaging tissue and excreting carcinogenic substances. Recently, research has isolated a protein secreted by O. viverrini that causes bile duct cells to proliferate, and creates an environment where cholangiocarcinoma becomes much more likely (Smout et al).
Who’s at Risk for Opisthorchiasis?
Most of the world’s people needn’t worry about acquiring O. viverrini flukes. In order to develop this disease one would have to:
- Live or travel in Southeast Asia — Thailand, Laos, Vietnam, or Cambodia and
- Eat raw or undercooked freshwater fish such as carp.
Eating raw or undercooked freshwater fish in Eastern Europe and various parts of Asia can result in infection with a related fluke, Opisthorchis felineus, which is also associated with liver cancer.
Sources
"A Granulin-Like Growth Factor Secreted by the Carcinogenic Liver Fluke, Opisthorchis viverrini, Promotes Proliferation of Host Cells.” (2009) Smout MJ, Laha T, Mulvenna J, et al. PLoS Pathogens 5(10): e1000611. doi:10.1371/journal.ppat.1000611
Foundations of Parasitology 6th Ed. Roberts, Larry S. and John Janovy Jr. Boston: McGraw Hill, 2000.
“Liver Fluke Induces Cholangiocarcinoma.” Sripa, B., S. Kaewkes, P. Sithithaworn et al. PLoS Medicine. 2007 July; 4(7): e201. doi: 10.1371/journal.pmed.0040201.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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