Clonorchiasis: Chinese liver fluke infection from raw freshwater fish

Clonorchiasis is a foodborne parasitic disease caused by the liver fluke Clonorchis sinensis, sometimes called the Chinese or oriental liver fluke. WHO groups it with other foodborne trematode infections, which it says cause 2 million life years lost to disability and death worldwide every year. Adult worms live in the bile ducts of the liver, where they can cause inflammation.

The parasite has a complex life cycle. WHO explains that the first intermediate host is a freshwater snail and, for Clonorchis, the second host is a freshwater fish. The final host is a mammal such as a dog or other fish-eating carnivore, and people become infected when they eat the infected fish raw or undercooked.

WHO says clonorchiasis and opisthorchiasis are confined largely to Asia, with many countries endemic. Transmission is often restricted to focal areas, and inadequate sanitation and food hygiene, limited access to safe drinking water and cultural food preferences are associated with increased risk. Family and community clusters may occur because of food-sharing habits.

Because the infection is largely confined to Asia, in the UK it is mainly relevant to people who have lived in or travelled to endemic areas and have eaten raw or undercooked freshwater fish. The sources read for this page do not give UK case numbers, and no UK public-health page dedicated to clonorchiasis was found when this page was written, so this page relies on WHO for the disease and on the NHS for the link with bile duct cancer.

Many people have no symptoms. WHO says the public health burden is predominantly due to morbidity rather than mortality, and that early and light infections often go unnoticed. Acute infection with Clonorchis sinensis may be asymptomatic in light infections, while fever and right upper-quadrant abdominal pain may be seen with high parasite burdens because of obstruction of the gallbladder by the worm.

Chronic infection, resulting from repeated re-infection over time, may be most severe. WHO says it causes chronic inflammation, fibrosis of the bile ducts and destruction of adjacent liver tissue. These changes can result in cholangiocarcinoma, a bile duct cancer that is often fatal, which is why Clonorchis sinensis is classified as a carcinogen. The NHS lists a parasite in the liver (liver flukes) among conditions that may make bile duct cancer more likely, and says it is important to get any symptoms of bile duct cancer checked by a GP.

WHO says the diagnosis is made on the clinical picture, a history of eating raw fish, detection of eosinophilia (raised levels of a type of white blood cell) and typical findings on ultrasound, CT or MRI scans. Confirmation relies on finding Clonorchis eggs in stool samples, which can only be done in the chronic phase and has low sensitivity in light infections. WHO recommends preparing 2 Kato-Katz smears.

Other tests exist. Immunological tests detect parasite-specific antibodies or antigens and may be more sensitive, but antibody detection does not distinguish between current, recent and past infection. Molecular tests such as PCR are described by WHO as experimental. Because of these limits, a travel and food history is an important part of the assessment, and anyone who has eaten raw freshwater fish in an endemic area and has unexplained symptoms should tell their doctor.

WHO says praziquantel is the only medicine it recommends for treatment of clonorchiasis, and gives dosing schedules for use in public health programmes and individual case management. Dosing in an individual patient is decided by the treating doctor, usually an infectious diseases or tropical medicine specialist, and this page does not reproduce it.

Treatment aims to clear the worms and reduce long-term damage to the bile ducts. WHO notes that reinfection after treatment is common in endemic areas because of poor sanitation and food hygiene, animal reservoirs and cultural eating habits. For a person who has left an endemic area and no longer eats the fish, the chance of reinfection is far lower.

The infection is caught from food, so prevention rests on food safety. WHO describes interventions including information, education and communication on safe food practices, improved sanitation and veterinary public health measures. For individual travellers, this means avoiding raw or undercooked freshwater fish in areas where the parasite is found, and using the advice of a travel health service before you go.

• Avoid raw or undercooked freshwater fish in endemic areas, since WHO says eating raw fish is the route of infection.

• Seek travel health advice before visiting affected parts of Asia.

• Tell your GP about any raw-fish exposure abroad if you develop unexplained upper abdominal pain, fever or jaundice.

See a GP if you have persistent upper abdominal pain, unexplained weight loss, or yellowing of the skin or eyes, and mention any time spent in Asia and any raw fish eaten. The NHS says it is important to get any symptoms of bile duct cancer checked by a GP. If you feel very unwell with high fever, severe pain or confusion, use NHS 111 or call 999.

In the UK, suspected parasitic infection acquired abroad is usually assessed by a GP who may refer you to an infectious diseases or tropical medicine service. Private clinics also offer travel medicine and parasite testing. Because treatment is specialist-led and uses a prescription-only medicine, ask for referral to a clinician with experience in tropical infections if you are concerned.