Opisthorchiasis

Opisthorchiasis is a parasitic disease caused by certain species of genus Opisthorchis (specifically, Opisthorchis viverrini and Opisthorchis felineus). Chronic infection may lead to cholangiocarcinoma, a cancer of the bile ducts.

Medical care and loss of wages caused by Opisthorchis viverrini in Laos and in Thailand costs about $120 million annually. In Asia, infection by Opisthorchis viverrini and other liver flukes affects the poorest people. Along with other foodborne trematode infections such as clonorchiasis, fascioliasis and paragonimiasis, opisthorchiasis is listed among the World Health Organization's list of neglected tropical diseases.

Symptoms of opisthorchiasis are indistinguishable from clonorchiasis. About 80% of infected people have no symptoms, though they can have eosinophilia. Asymptomatic infection can occur when there are less than 1000 eggs in one gram of feces. Infection is considered heavy when there are 10,000-30,000 eggs in one gram of feces. Symptoms of heavier infections may include diarrhea, epigastric and right upper quadrant pain, lack of appetite, fatigue, yellowing of the eyes and skin and mild fever.

These parasites are long-lived and cause heavy chronic infections that may lead to accumulation of fluid in the legs (edema) and in the peritoneal cavity (ascites), enlarged non-functional gallbladder and also ascending cholangitis, which can lead to periductal fibrosis, cholecystitis and cholelithiasis, obstructive jaundice, hepatomegaly and/or portal hypertension.^{[citation needed]}

Both experimental and epidemiological evidence strongly implicates Opisthorchis viverrini infections in the etiology of a malignant cancer of the bile ducts (cholangiocarcinoma) in humans which has a very poor prognosis. Clonorchis sinensis and Opisthorchis viverrini are both categorized by the International Agency for Research on Cancer (IARC) as Group 1 carcinogens.

In humans, the onset of cholangiocarcinoma occurs with chronic opisthorchiasis, associated with hepatobiliary damage, inflammation, periductal fibrosis and/or cellular responses to antigens from the infecting fluke. These conditions predispose to cholangiocarcinoma, possibly through an enhanced susceptibility of DNA to damage by carcinogens. Chronic hepatobiliary damage is reported to be multi-factorial and considered to arise from a continued mechanical irritation of the epithelium by the flukes present, particularly via their suckers, metabolites and excreted/secreted antigens as well as immunopathological processes. In silico analyses using techniques of genomics and bioinformatics is unraveling information on molecular mechanisms that may be relevant to the development of cholangiocarcinoma.

In regions where Opisthorchis viverrini is highly endemic, the incidence of cholangiocarcinoma is unprecedented. For instance, cholangiocarcinoma represents 15% of primary liver cancer worldwide, but in Thailand's Khon Kaen province, this figure escalates to 90%, the highest recorded incidence of this cancer in the world. Of all cancers recorded worldwide in 2002, 0.02% were cholangiocarcinoma caused by Opisthorchis viverrini. Cancer of the bile ducts caused by opisthorchiasis occurs in the ages 25–44 years in Thailand. A few cases have appeared in later life among U.S. veterans of the Vietnam War, who consumed poorly cooked fish from streams in endemic areas near the border of Laos and Vietnam.

The medical diagnosis is usually established by finding eggs of Opisthorchis viverrini in feces using the Kato technique. Alternatively, an antigen of Opisthorchis viverrini can be detected by ELISA test. A polymerase chain reaction test that can be performed on faeces has been developed and evaluated in a rural community in central Thailand.