Foodborne trematode infections
Key facts
- At least 56 million people suffer from one or more foodborne trematode infections.
- People become infected through the consumption of raw fish, crustaceans or vegetables that harbour the parasite larvae.
- Foodborne trematode infections are most prevalent in South-East Asia and South America.
- Foodborne trematode infections result in severe liver and lung morbidity.
- Safe and effective medicines are available to prevent and treat foodborne trematode infections.
Foodborne trematode infections affect more than 56 million people throughout the world. They are caused by trematode worms ("flukes"), of which the most common species affecting people are Clonorchis, Opisthorchis, Fasciola and Paragonimus.
People become infected through the consumption of raw or poorly cooked fish, crustaceans and vegetables that harbour the minute larval stages of the parasites (see table 1).
Transmission
Foodborne trematode infections are zoonoses, i.e. they are naturally transmissible from vertebrate animals to people and vice versa. They have complex life-cycles that usually involve two intermediate hosts. The first intermediate host in all cases is a freshwater snail, while the second host differs: in clonorchiasis and opisthorchiasis it is a freshwater fish, while in paragonimiasis it is a crustacean. The final host is always a mammal.
People become infected when they ingest the second intermediate host that is infected with larval forms of the parasite. Fascioliasis does not require a second intermediate host and people become infected when the larvae are ingested together with the aquatic vegetables to which they are attached (see table 1 for details).
Table 1. Epidemiological characteristics of foodborne trematode infections
| Disease | Infectious agent | Acquired through consumption of | Natural final hosts of the infection |
| Clonorchiasis | Clonorchis sinensis | Fish | Dogs and other fish-eating carnivores |
| Opisthorchiasis | Opisthorchis viverrini | Fish | Cats and other fish-eating carnivores |
| Fascioliasis | Fasciola hepatica, F. gigantica |
Aquatic vegetables | Sheep, cattle and other herbivores |
| Paragonimiasis | Paragonimus spp. | Crustaceans (crabs and crayfish) | Cats, dogs and other crustacean-eating carnivores |
Epidemiology
In 2005, more than 56 million people were infected with foodborne trematodes and over 7 000 people died.
Cases of foodborne trematode infections have been reported from over 70 countries worldwide; however South-East Asia and South America are the most affected areas. In these regions, foodborne trematode infections represent a significant public health problem.
Within countries, transmission is often restricted to limited areas and reflects behavioural and ecological patterns, such as people’s food habits, methods of food production and preparation, and the distribution of the intermediate hosts. Information on the epidemiological status of foodborne trematode infections in Africa is largely missing.
The economic impact of foodborne trematode infections is significant, and is mainly linked to the expanding aquaculture industry.
Symptoms
The public health burden attributable to foodborne trematode infections is predominantly due to morbidity rather than mortality.
Early and light infections often pass unnoticed, as they are asymptomatic or only scarcely symptomatic. Conversely, if the worm load is high, general malaise is common and severe pain can occur, especially in the abdominal region, and most frequently in the case of fascioliasis.
Chronic infections are invariably associated with severe morbidity. Symptoms are mainly organ-specific and reflect the final location of the adult worms in the body.
Clonorchiasis and opisthorchiasis: the adult worms lodge in the smaller bile ducts of the liver, causing inflammation and fibrosis of the adjacent tissues and eventually cholangiocarcinoma, a severe and fatal form of bile cancer. Both C. sinensis and O. viverrini, but not O. felineus, are classified as carcinogenic agents.
Fascioliasis: the adult worms lodge in the larger bile ducts and the gall bladder, where they cause inflammation, fibrosis, blockage, colic pain and jaundice. Liver fibrosis and anaemia are also frequent.
Paragonimiasis: the final location of the worms is the lung tissue. They cause symptoms that can be confounded with tuberculosis: chronic cough with blood-stained sputum, chest pain, dyspnoea (shortness of breath) and fever. Migration of the worms is possible: cerebral locations are the most severe.
Prevention and control
Control of foodborne trematode infections aims at reducing the risk of infection and at controlling associated morbidity.
Veterinary public health measures and food safety practices, are recommended to reduce the risk of infection, while, to control morbidity, WHO recommends preventive chemotherapy and improved access to treatment using safe and effective anthelminthic medicines (drugs that expel the worms).
Preventive chemotherapy involves a population-based approach whereby everyone in a given region or area is given medicines, irrespective of their infection status. Individual case-management involves the treatment of people with confirmed or suspected infection (see table 2).
Table 2. Recommended treatments and strategies
| Disease | Recommended drug and dosage | Recommended strategy |
| Clonorchiasis and opisthorchiasis | Praziquantel: – 40 mg/kg in single administration, or – 25 mg/kg three times daily for 2–3 consecutive days |
Preventive chemotherapy: – In districts where the prevalence of infection is ≥ 20%, treat all residents every 12 months – In districts where the prevalence of infection is < 20%, treat all residences every 24 months, or treat only those individuals reporting the habit of eating raw fish, every 12 months |
| Fascioliasis | Triclabendazole: – 10 mg/kg in single administration |
Individual case-management: – Treat all confirmed cases – In endemic areas: treat all suspect cases Preventive chemotherapy: – In sub-districts, villages or communities where cases of fascioliasis appear to be clustered: treat all school-age children (5–14 years) or all residents, every 12 months |
| Paragonimiasis | Triclabendazole: – 2 x 10 mg/kg in the same day (individual case-management), or – 20 mg/kg in single administration (preventive chemotherapy), or Praziquantel: – 25 mg/kg three times daily for three days (individual case-management) |
Individual case-management: – Treat all confirmed cases – In endemic areas: treat all suspect cases Preventive chemotherapy: – In sub-districts, villages or communities where cases of paragonimiasis appear to be clustered: treat all residents every 12 months |
WHO response
WHO’s work on foodborne trematode infections is part of an integrated approach to the control of neglected tropical diseases, and includes:
- development of strategic directions and recommendations
- support for mapping in endemic countries
- support for pilot interventions and control programmes in endemic countries
- support for monitoring and evaluation of implemented activities
- documentation of the burden of foodborne trematode infections and the impact of implemented interventions.
WHO is working to include foodborne trematode infections in its mainstream preventive chemotherapy strategy and ensure that their worst consequences (cancers of the bile duct) are fully prevented.
WHO has also negotiated an agreement with Novartis Pharma AG whereby this company will donate triclabendazole for the treatment of human fascioliasis and paragonimiasis. The medicines are shipped free of charge upon application from ministries of health. WHO invites all endemic countries to take advantage of this donation programme.