Opisthorchis viverrini

General Description: This liver fluke is endemic to northeast Thailand and surrounding regions, where it is estimated that 7 million people are infected. Adult O. viverrini are 7-12mm long and 1.5-3 mm wide, transparent, and elongate. They typically have a rounded posterior, a tapered anterior, and oral and ventral suckers. The digestive system consists of the oral sucker that leads to a small pharynx and esophagus and then two ceca. O. viverrini is hermaphroditic and has an extensive reproductive system for its size. This is suggested by the name Opisthorchis (opis=behind, orchis=testis). The pair of testes is lobated and near the posterior end. The uterus is a highly coiled tubule and transverse compressed follicles lie laterally₁₁.

Eggs of O. viverrini

Prevention: There is a program in effect by the National Public Health Development Plans to attempt to control liver fluke infection. This program involves attempting to decrease the human host reservoir by testing stool and treating O. viverrini infections with praziquantel, education and promotion of cooking fish, and improvement of hygienic defecation. Between 1981 and 1991, infection rates in north and central regions increased while infections in the northeastern regions decreased₇.

Association with Disease and Malignancy: O. viverrini is the etiologic agent of opisthorchis and is strongly correlated with cholangio-carcinoma in Thailand. People, particularly males, with heavy liver fluke infection have a significantly increased risk of cholangiocarcinoma, a normally rare form of liver cancer. Diverse studies have been done about this relationship: case studies in hospitals, population-based ecologic studies, and case-control studies. A study by Bhamarapravati and Viranuvatti in the 1960s of 9694 autopsies showed the ratio of hepatocellular carcinoma to cholangiocarcinoma in uninfected patients was 5:1 and 1:2 in patients with liver fluke infection. Numerous other studies have supported the dramatic increase in the occurrence of cholangiocarcinoma associated with infection by O. viverrini. Bunyaratvej and his associates later found cholangiocarcinoma was 2.4 times more common in males. In the 1970s, Hitanant found 29% of O. viverrini infected patients had cholangiocarcinoma. Vatanasapt found in 1988 that clusters of cancer correlated with the prevalence and intensity of O. viverrini infection in Khon Kaen and a 1991 hospital study showed that infection with O. viverrini increased the risk of cholangiocarcinoma fivefold₁₁.

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