Synonyms: Opisthorchis sinensis = Chlonorchis sinensis or Chinese or oriental liver fluke, Opisthorchis felineus = Opisthorchis tenuicollis or cat liver fluke, Opisthorchis viverrini = Southeast Asian liver fluke

Opisthorchiasis is a trematode (fluke) infection caused by infection with one of the species of the liver fluke Opisthorchis, which is acquired by eating raw or undercooked freshwater fish containing infectious metacercariae. The three species are: O. sinensis (still widely known as Clonorchis sinensis), O. felineus/tenuicollis and O. viverrini.

Life cycle

Opisthorchis spp. 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.
• 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 fish where they become encysted in the muscles/scales and complete their life cycle by being eaten by humans/other mammals.

Epidemiology

Approximately 30 million people are infected by liver flukes.¹

• Opisthorchis (Clonorchis) sinensis:
  • This organism is endemic in Southeast Asia, Japan, Korea, Taiwan and most of China.
  • There are an estimated 19 million cases of human infestation.¹
  • It is relatively small (10-25 mm x 3 mm).
  • 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.
  • There are an estimated 1.2 million cases worldwide.¹
• 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. Worldwide it is estimated at 9 million cases.¹
  • It is morphologically very similar to O. sinensis/O. felineus.

Presentation

• Opisthorchiasis 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.
• Opisthorchiasis can lead to recurrent pyogenic cholangitis, jaundice and cholangiocarcinoma.

Differential diagnosis

• Viral hepatitis.
• Pancreatitis.
• Inflammatory bowel disease.
• Leishmaniasis.
• Primary biliary cirrhosis.²
• Tuberculosis.
• Typhoid.
• Carcinoma of head of pancreas.
• Cholangiocarcinoma.

Investigations

• Microscopic stool examination may reveal the eggs.
• Duodenal aspiration is more sensitive for this purpose than examination of two stool specimens.
• An enzyme-linked immunosorbant assay method for detecting antigen in the stool may be used to diagnose opisthorchiasis. Polymerase chain reaction techniques are being developed to improve diagnosis.^3,4
• FBC may reveal anaemia and eosinophilia.

Management

• Praziquantel is the treatment of choice.^5,6
• Intercurrent bacterial infections are treated with appropriate antibiotics.
• Surgery may be needed to treat biliary tract complications.

Complications

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

Prognosis

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

Prevention

• Proper cooking of fish..
• Freezing fish intended for raw consumption, -20ºC for 7 days, or -35ºC for 15 hours kills parasites, unlike brining and pickling.
• Application of Hazard Analysis and Critical Control Points (HACCP) principles to the growing freshwater aquaculture programmes has been recommended by the World Health Organization to control digenetic trematode infections in humans.
• Use of molluscicides is the most frequent public health intervention, as it prevents the transmission of many other trematodes, including Schistosoma spp.
• 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.

Document references

1. Parija SC et al; Trematode infection, eMedicine, Nov 2009
2. Melling N, Hohenberger W, Yedibela S; Opisthorchiasis mimicking primary biliary cirrhosis as an indication for liver J Hepatol. 2009 May;50(5):1057-9. Epub 2009 Mar 14. [abstract]
3. Umesha KR, Kumar S, Parvathi A, et al; Opisthorchis viverrini: detection by polymerase chain reaction (PCR) in human Exp Parasitol. 2008 Dec;120(4):353-6. Epub 2008 Sep 9. [abstract]
4. Intapan PM, Thanchomnang T, Lulitanond V, et al; Rapid molecular detection of Opisthorchis viverrini in human fecal samples by Am J Trop Med Hyg. 2009 Nov;81(5):917-20. [abstract]
5. Clonorchiasis, DPDx
6. Opisthorchiasis (O. felineus/O. viverrini), DPDx

Internet and further reading

• Opisthorchis Infection, Centers for Disease Control and Prevention

Acknowledgements