Liver Flukes

Synonyms: Opisthorchis sinensis=Chlonorchis sinensis, Opisthorchis felineus=Opisthorchis tenuicollis, Trematode infestation

Trematodes or flukes (members of the flatworm family) cause a range of clinically relevant infestations in humans. They are classified into blood flukes (Schistosoma sp.), lung flukes (Paragonimus sp.), intestinal flukes (Fasciolopsis buski and others) and the liver flukes, depending on their predominant mammalian parasitic habitat.

Liver flukes have a complex life cycle involving a definitive mammalian host and two intermediate aquatic hosts.¹

• Humans or other mammals eat the encysted metacercariae in inadequately cooked/raw/pickled fish or on wild watercress.
• After digestion of the cyst in the duodenum the larva enters the biliary duct where it matures into the adult worm, which lives off mucosal secretions.
• The worm lays eggs which pass into the stool and enter freshwater where they form a miracidium either before or after colonising various species of aquatic snail.
• Asexual reproduction leads to the formation of metacercariae. These pass to either fish or wild watercress/grass plants where they become encysted and complete their life cycle by being eaten by humans/other mammals.

• Opisthorchiasis is infection with 1 of 2 species of the liver fluke Opisthorchis, which is acquired by eating raw or undercooked freshwater fish that contains infectious metacercariae.
• The life cycle of Opisthorchis requires both snails and fish.
• Opisthorchisiasis is often asymptomatic but can cause a serum sickness-like illness with high fever, eosinophilia and rash.
• There may be tender hepatomegaly, ascites, signs of cholecystitis and oedema.
• Opisthorchisiasis can lead to recurrent pyogenic cholangitis, jaundice and cholangiocarcinoma.
• Praziquantel is the treatment of choice.
• Opisthorchis (Clonorchis) sinensis
  • This organism is endemic in South-East Asia, Japan, Korea, Taiwan and most of China.
  • There are an estimated 20-30 million cases of human infestation in this region, of which this species counts for the majority.
  • It is relatively small (10-25mm X 3mm).
  • It infects humans, dogs, pigs, cats, rats and a variety of wild animals.
• Opisthorchis felineus/tenuicollis
  • This is normally an intestinal parasite of cats dogs, foxes, pigs and cetaceans (whales/porpoises/dolphins) in Eastern Europe, Siberia and other parts of Asia.
  • It is morphologically very similar to sinensis.
• Opisthorchis viverrini
  • Found in domesticated and wild dogs and cats in South-East Asia.
  • It is a very common human infection in North-East Thailand where it is said to infect up to half of the population.
  • It is morphologically very similar to sinensis/felineus.

There is a separate article that discusses Fasciola Hepatica in more detail.

• Fasciola hepatica is distributed widely throughout temperate and tropical regions of the world. It is relatively large, being 20-30mm long X 8-15mm wide.
• F. hepatica requires an aquatic snail as an intermediate host.
• Infection occurs by eating cercariae on submerged watercress/other plants.
  In the United Kingdom, Fasciola hepatica is a frequent cause of disease in ruminants.
• Cattle and sheep are infected when they consume the infectious stage of the parasite from low-lying, marshy pasture.
• Acute illness: fascioliasis is often asymptomatic/subclinical. Lethargy, fever, night sweats and pain/tenderness in the right subcostal region are features.
• Chronic illness: fascioliasis tends to cause sporadic fever, anaemia, colicky abdominal pain, jaundice and may predispose to secondary bacterial infection.
• Black disease is a complication which is usually fatal, caused by secondary infection with Clostridium oedematiens, which proliferates in necrotic lesions produced by the young larvae migrating in the liver.
• Liver fluke is diagnosed by yellow-white eggs in the faeces.
• The drug of choice for fascioliasis is bithionol, but praziquantel, triclabendazole and metronidazole are also effective.

• Viral hepatitis
• Pancreatitis
• Inflammatory bowel disease
• Leishmaniasis
• Tuberculosis
• Typhoid
• Carcinoma of head of pancreas
• Cholangiocarcinoma

• Microscopic stool examination may reveal the eggs.
• Duodenal aspiration is more sensitive for this purpose than examination of 2 stool specimens.
• An ELISA method for detecting antigen in the stool may be used to diagnose opisthorchisiasis. Serological tests are sometimes used to diagnose acute fascioliasis as eggs may not be released for several months. However, previous infection which is cleared can cause this result to be falsely positive. PCR techniques are being developed to improve diagnosis.
• Full blood count may reveal anaemia and eosinophilia.
• Ultrasound of liver can reveal shadows in the biliary tract due to fascioliasis. Cholangiography or ERCP may also be used to make the diagnosis.

• Praziquantel is used to treat all trematode infestations, except fascioliasis which is usually treated with bithionol, but will respond to praziquantel.
• Triclabendazole is increasingly being used for fascioliasis and metronidazole has also been used with some success.
• Intercurrent bacterial infections are treated with appropriate antibiotics.
• Surgery may be needed to treat biliary tract complications.

• Anaemia, eosinophilia
• Intercurrent bacterial infection
• Pancreatitis
• Pyogenic cholangitis
• Cholangiocarcinoma

• Early, moderate infestations are likely to be cured without complications.
• Chronic or severe infestations tend to lead to complications and death is not uncommon.

• Proper cooking of fish and avoidance of wild watercress in diet.
• Freezing fish intended for raw consumption, -20ºC for 7 days, or -35ºC for 15 hours kills parasites, unlike brining and pickling.
• Encysted metacercariae can be killed by washing water-grown vegetables in 6% vinegar or potassium permanganate for 5-10 minutes. This approach is more successful than attempts to halt the consumption of raw vegetables.
• Avoid sewage contamination of growing areas.
• Application of HACCP (Hazard Analysis and Critical Control Points) principles to the growing fresh water aquaculture programmes has been recommended by the WHO, to control digenetic trematode infections in humans.
• Molluscicides is the most frequent public health intervention, as it prevents the transmission of many other trematodes, including Schistosoma species.
• Treatment of animals to reduce the reservoir and stock losses has been used. Until the introduction of single-dose triclabendazole, bithionol was the only available treatment, limited by expense and treatment duration.