Opisthorchiasis (liver fluke infection): causes, symptoms and prevention

Opisthorchiasis is not acquired in the UK, and no dedicated UK patient information page on it (from the NHS or a UK charity) was found while preparing this guide. This article is therefore based on World Health Organization material, which is international rather than UK-specific. If you are in the UK and think you may have been exposed, a GP or an infectious diseases or tropical medicine service can advise on testing and treatment.

WHO explains that opisthorchiasis belongs to the group of foodborne trematode infections and is a zoonosis, meaning an animal infection that can be passed to humans. After larvae in raw or undercooked freshwater fish are eaten, they mature into adult flukes in the bile ducts of the liver, where they lay eggs that leave the body in stool.

Two species cause human disease: Opisthorchis viverrini and Opisthorchis felineus. WHO says cats, dogs and other fish-eating mammals can act as reservoir hosts, shedding eggs in their faeces and sustaining the cycle in water systems. The related infection clonorchiasis, caused by Clonorchis, is described alongside it in WHO material.

WHO says opisthorchiasis is endemic in Southeast Asia, particularly Thailand, Lao PDR, Cambodia and Vietnam, as well as parts of Eastern Europe and Russia. High prevalence is linked to cultural practices of eating raw or undercooked freshwater fish, and transmission is strongly influenced by poor sanitation and limited access to clean water, which contaminate water bodies with parasite eggs.

WHO notes in its clonorchiasis questions and answers that migrants or travellers from endemic regions may carry asymptomatic infections, and that doctors in non-endemic areas may lack awareness and stool tests for parasite eggs are not routine unless specifically requested. For people living in the UK, the relevant history is eating raw, undercooked or fermented freshwater fish in an endemic region, which is worth mentioning to a doctor if you have unexplained symptoms.

WHO says that in humans both opisthorchiasis viverrini and opisthorchiasis felinea may be asymptomatic or scarcely symptomatic in light infections, but when worm numbers are high, fever and right upper-quadrant pain are not infrequent and may be associated with intermittent colic pain caused by worms obstructing the gallbladder. Opisthorchiasis felinea is typically associated with a more pronounced and severe acute phase.

Chronic infection is the greater concern. WHO says the presence of worms in the smaller bile ducts causes chronic inflammation of their walls, resulting in fibrosis of the ducts and destruction of the adjacent liver tissue, and that recurrent pyogenic cholangitis may be frequent. Chronic opisthorchiasis viverrini, which results from repeated reinfection over time, may be most severe and is strongly associated with cholangiocarcinoma. The International Agency for Research on Cancer classifies O. viverrini as carcinogenic to humans, whereas WHO says there is insufficient evidence to link opisthorchiasis felinea with cholangiocarcinoma.

The WHO pages used for this guide do not describe the clinical diagnostic pathway in detail. WHO does say that adult flukes lay eggs that are passed in stool, and, in its clonorchiasis material, that stool tests for parasite eggs are not routine unless specifically requested. In practice that means telling the doctor about your travel and diet so that appropriate tests can be considered.

WHO says treatment of foodborne trematodiases is important to prevent progression of clinical disease and reduce morbidity, and that praziquantel is recommended for clonorchiasis and opisthorchiasis. It explains the limits of the drug: it is effective at killing adult flukes but cannot reverse existing bile duct damage such as scarring, inflammation or an increased risk of bile duct cancer, it does not kill the younger stages of the parasite, and it does not prevent reinfection if raw or undercooked fish continues to be eaten.

WHO describes early treatment as critical to prevent complications, because bile duct damage accumulates over decades in people with long-standing infection.

WHO says the best way to prevent infection is to avoid eating raw or undercooked freshwater fish, and gives freezing or thoroughly cooking fish as safer preparation methods. It notes that dishes such as koi pla in Thailand are deeply rooted in tradition, which is why education focuses on safer preparation rather than abandoning cultural foods. Contaminated utensils do not spread clonorchiasis, according to WHO, so the main route is the fish itself.

For travellers the practical message is to be cautious with raw, pickled, fermented or lightly cooked freshwater fish dishes in endemic areas. TravelHealthPro, the UK service for travel health advice, is the place to check destination-specific guidance before you travel; it was not read for this page.

In the UK, suspected parasitic infections are usually assessed by a GP and, if needed, an infectious diseases or tropical medicine service on the NHS, which also provide testing and treatment. Private travel and tropical medicine clinics also exist.

Whichever route you use, tell the clinician exactly where you travelled or lived and what you ate, as this determines the tests and the treatment.