What are foodborne trematode infections?

Foodborne trematode infections are parasitic diseases caused by flatworms (trematodes/flukes) transmitted through consumption of raw or undercooked animal products. The main groups: liver flukes (Opisthorchis viverrini, O. felineus, Clonorchis sinensis — from raw freshwater fish; Fasciola hepatica/gigantica — from aquatic plants); lung flukes (Paragonimus species — from raw crabs or crayfish); intestinal flukes (multiple species — from fish, plants). All WHO Neglected Tropical Diseases.

Which foodborne trematodes cause cancer?

IARC classifies Opisthorchis viverrini and Clonorchis sinensis as Group 1 human carcinogens — both causing cholangiocarcinoma (bile duct cancer) after prolonged chronic infection. Northeast Thailand — where O. viverrini infection from eating raw fermented fish (koi plaa) is culturally embedded — has the world's highest rates of cholangiocarcinoma (approximately 85 per 100,000 vs 1-2 per 100,000 in most countries). Opisthorchiasis control in Thailand through targeted praziquantel treatment and food safety education has been a major public health campaign.

How do people get infected?

Liver flukes (Opisthorchis, Clonorchis): eating raw or undercooked freshwater fish containing encysted metacercariae (larvae). Freezing fish at -20°C for 24 hours or thorough cooking kills metacercariae. The practice of eating raw, marinated or lightly fermented freshwater fish is culturally entrenched in many endemic areas. Lung flukes (Paragonimus): eating raw/undercooked freshwater crabs, crayfish or shrimp. Fasciola: eating raw watercress, water plants or drinking contaminated water containing metacercariae. Prevention: cook fish, crustaceans and watercress thoroughly.

Why is paragonimiasis frequently misdiagnosed as TB?

Pulmonary paragonimiasis presents with: chronic cough (often productive); haemoptysis (blood-stained sputum); chest pain; and pleural effusion — a clinical presentation essentially identical to pulmonary tuberculosis. Chest X-ray findings (infiltrates, cavities, pleural effusion) also mimic TB. In endemic areas (Southeast Asia, West Africa, Latin America), many paragonimiasis patients are diagnosed with TB and given prolonged anti-TB therapy — with the parasite burden continuing to increase. The diagnostic clue: eosinophilia and lack of TB treatment response in someone with crustacean exposure. Stool or sputum microscopy identifies Paragonimus eggs.

What is the treatment?

Opisthorchiasis and clonorchiasis: praziquantel 75mg/kg/day in 3 divided doses for 2 days (high-dose, multiple-day regimen for liver flukes — not the standard single-dose for schistosomiasis). Cure rates approximately 90%. Paragonimiasis: praziquantel (75mg/kg/day in 3 doses for 2 days) — highly effective. Fascioliasis: triclabendazole — the only effective drug (praziquantel does not work against Fasciola). WHO recommends 10mg/kg orally as a single dose (can double dose for severe infection). On WHO Essential Medicines List.

What is the global burden?

WHO estimates approximately 40-50 million people are chronically infected with foodborne trematodes globally — predominantly in East and Southeast Asia (clonorchiasis, opisthorchiasis), Eastern Europe/Russia (O. felineus), parts of Africa, Latin America and the Pacific. An estimated 750 million people are at risk. The burden is underreported because: infections are often asymptomatic for years; diagnosis requires stool examination or serology not routinely available; and awareness among clinicians is low outside endemic areas. Cholangiocarcinoma from liver flukes may be responsible for tens of thousands of cancer deaths annually in endemic areas.

How does GMJ News cover foodborne trematodes?

GMJ News covers WHO foodborne trematode data, liver fluke-cholangiocarcinoma link (IARC Group 1 carcinogen), paragonimiasis-TB misdiagnosis, triclabendazole for Fasciola, praziquantel MDA and food safety prevention. Foodborne trematodes illustrate the intersection of cultural practices, food safety and cancer prevention — directly relevant to PHIG's food safety and NTD programme.

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Foodborne Trematode Infections

GMJ News knowledge hub · last reviewed September 2026 · Georgian Medical Journal

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Foodborne trematode infections — caused by liver flukes (opisthorchiasis: Opisthorchis viverrini, O. felineus, Clonorchis sinensis), intestinal flukes and lung flukes (Paragonimus spp.) — affect an estimated 750 million people at risk with approximately 40-50 million chronically infected, predominantly in Southeast Asia, East Asia and Eastern Europe through consumption of raw or undercooked fish, crustaceans and aquatic plants (WHO). Critically, opisthorchiasis and clonorchiasis cause cholangiocarcinoma (bile duct cancer) — a largely preventable malignancy in endemic areas. WHO recognises foodborne trematode infections as a major group of NTDs requiring integrated control.

Key messages

40-50 million chronically infected

Foodborne trematode infections — liver flukes (Opisthorchis, Clonorchis), lung flukes (Paragonimus) and intestinal flukes — affect an estimated 40-50 million people chronically worldwide, predominantly in Asia, Eastern Europe and Latin America (WHO).

Liver flukes cause cholangiocarcinoma

Opisthorchis viverrini and Clonorchis sinensis are classified as Group 1 carcinogens by IARC — causing cholangiocarcinoma (bile duct cancer). Northeast Thailand has the highest cholangiocarcinoma rates in the world — directly attributable to endemic O. viverrini infection from eating raw/undercooked fish.

Prevention through cooking — simple but culturally challenging

Foodborne trematode infections are entirely preventable by thoroughly cooking fish, crustaceans and aquatic vegetables. However, cultural practices of eating raw/undercooked fish and crayfish are deeply embedded in endemic communities, making behaviour change challenging.

Praziquantel treats most — triclabendazole for Fasciola

Praziquantel (standard dose) is effective for Opisthorchis, Clonorchis and Paragonimus. Fasciola hepatica and gigantica (liver flukes from watercress) are resistant to praziquantel — triclabendazole (WHO EML) is the treatment of choice.

WHO mass drug administration

WHO recommends preventive chemotherapy with praziquantel for populations at risk of opisthorchiasis, clonorchiasis and other foodborne trematode infections — combined with food safety education.

Paragonimiasis mimics tuberculosis

Pulmonary paragonimiasis — infection with Paragonimus lung fluke — causes chronic cough, haemoptysis and pleural effusion that clinically resembles tuberculosis, leading to frequent misdiagnosis and inappropriate TB treatment.

Key statistics

40-50M

people chronically infected with foodborne trematodes

WHO

~750M

people at risk from foodborne trematode infections

WHO

Group 1

carcinogen: Opisthorchis viverrini and Clonorchis sinensis (IARC)

IARC

NE Thailand

highest cholangiocarcinoma rate in world — due to O. viverrini

IARC/WHO

2.4-17M

people infected with Fasciola liver flukes

WHO

23M

people infected with Paragonimus lung flukes

WHO

Foodborne trematodes — main species, reservoirs and geographic distribution

Source: WHO. Multiple distinct trematode infections require different interventions and treatments.

Glossary of key terms

Opisthorchiasis
WHO/IARCInfection with Opisthorchis viverrini (Southeast Asia — Thailand, Lao PDR, Cambodia, Vietnam) or O. felineus (Eastern Europe, Russia — reservoir in fish-eating mammals). Acquired by eating raw/undercooked freshwater fish. Adult flukes in bile ducts cause cholangitis, recurrent pyogenic cholangitis and — with prolonged infection — cholangiocarcinoma (IARC Group 1 carcinogen).

Clonorchiasis
WHO/IARCInfection with Clonorchis sinensis ("Chinese liver fluke") — the most prevalent liver fluke globally (approximately 35 million infected in China, Korea, Vietnam, Russia). Transmitted by eating raw or undercooked freshwater fish. Causes biliary inflammation and — with chronic infection — cholangiocarcinoma (IARC Group 1 carcinogen). Treated with praziquantel.

Fascioliasis
WHOInfection with Fasciola hepatica or F. gigantica (liver flukes) — acquired by eating contaminated watercress, aquatic plants or water containing metacercariae. Affects livestock (cattle, sheep) and humans. Unlike other foodborne trematodes: NOT caused by eating fish — from aquatic plants. Treated with triclabendazole (praziquantel ineffective). 2.4-17 million infected globally.

Paragonimiasis
WHOInfection with Paragonimus lung flukes — acquired by eating raw or undercooked freshwater crabs or crayfish. Adult flukes encyst in the lungs, causing chronic cough, haemoptysis and pleural effusion. Frequently misdiagnosed as tuberculosis. Common in Asia, West Africa and Latin America. Treated with praziquantel or triclabendazole.

Cholangiocarcinoma
IARC/WHOBile duct cancer — a rare but aggressive cancer with poor prognosis in most settings. In Northeast Thailand and Lao PDR, cholangiocarcinoma rates are among the highest in the world — directly driven by endemic O. viverrini infection. O. viverrini causes cholangiocarcinoma through chronic biliary inflammation, cellular proliferation and possibly secretion of carcinogenic metabolites.

Praziquantel for trematodes
WHO EMLThe drug of choice for most foodborne trematode infections — opisthorchiasis, clonorchiasis, paragonimiasis, and intestinal trematodes. Single-dose (for most intestinal trematodes) or multi-dose (75mg/kg per day in 3 doses for 2 days for liver flukes). Does NOT work for Fasciola (requires triclabendazole).

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Foodborne Trematode Infections

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