The liver fluke Opisthorchis felineus as a group III or group I carcinogen

Opisthorchiasis caused by the liver fluke Opisthorchis felineus is one of the most common helminthic infections in the Russian Federation. The largest area affected by opisthorchiasis felinea occupies almost the entire territory of Western Siberia and extends to northern Kazakhstan and a part of the Ural region. Natural endemic regions of opisthorchiasis also exist in the European part of Russia, and in the regions of Western and Eastern Europe. According to the official statistics of the Russian Federation, up to 40 000 patients with opisthorchiasis are registered annually in the country. Opisthorchiasis felinea affects the hepatobiliary system and causes serious liver disorders, including cancer of the biliary tract. Other parasitoses, opisthorchiasis viverrini and clonorchiasis, are widespread in the Southeast Asia and China. The causative agents of these diseases, liver flukes O. viverrini and Clonorchis sinensis, are officially recognized as Group 1 biological carcinogens and are classified as the main risk factors for cholangiocarcinoma. O. felineus is included in Group 3 of biological carcinogens and is not officially considered carcinogenic to humans. Studies on the carcinogenic potential of this liver fluke and the epidemiology of cholangiocarcinoma in the Russian Federation have started in earnest quite recently. Nevertheless, we have some evidence that infection with O. felineus leads to a precancerous state of the bile duct epithelium. This state, combined with additional risk factors, poses a real risk of cholangiocarcinoma. In our opinion, taking into consideration the accumulated facts, the classification of the carcinogenic potential of O. felineus requires revision. In this review, we focus on the relevant characteristics of the biology and epidemiology of this helminth as well as experimental data on opisthorchiasis felinea; this information might clarify the carcinogenicity of O. felineus to humans.