Opisthorchiasis in Western Siberia: Epidemiology and distribution in human, fish, snail, and animal populations.

Opisthorchiasis is a widespread helminth infection in Russia. The largest opisthorchiasis endemic focus in the world is the Ob river watershed in Western Siberia. The main causative agent of this condition is the liver fluke, Opisthorchis felineus. In addition, another liver fluke species in the Opisthorchiidae family, Metorchis bilis, causes a symptomatically similar disease, metorchiasis. Despite a long research history going back to 1927, opisthorchiasis remains a serious problem in Russia, and numerous questions related to the epidemiology of these liver fluke infections and their patterns of distribution in Western Siberia, the causes of high prevalence in different populations, and the prognosis of the epidemiological situation remain to be answered. In this review, we first briefly describe the life cycle of O. felineus and then summarize the available published data on the epidemiological aspects of O. felineus infection among populations in Western Siberia. Additionally, the geographical distribution and rates of infection with the two major small liver flukes, O. felineus and M. bilis, in the intermediate (Bithyniidae snails and cyprinid fish) and definitive (humans, wild and domestic carnivorous animals and birds) hosts are described to assess their role in the transmission cycle. Moreover, species in the genus Opisthorchis and the genus Metorchis that have been reported in carnivorous mammals and birds in Western Siberia are listed and their potential to serve as the agents of opisthorchiasis transmission is discussed.

Parasite-induced changes in nitrogen isotope signatures of host tissues.

To estimate isotopic changes caused by trematode parasites within a host, we investigated changes in the carbon and nitrogen isotope ratios of the freshwater snail Lymnaea stagnalis infected by trematode larvae. We measured carbon and nitrogen stable isotopes within the foot, gonad, and hepatopancreas of both infected and uninfected snails. There was no significant difference in the delta13C and delta15N values of foot and gonad between infected and uninfected snails; thus, trematode parasite infections may not cause changes in snail diets. However, in the hepatopancreas, delta15N values were significantly higher in infected than in uninfected snails. The 15N enrichment in the hepatopancreas of infected snails is caused by the higher 15N ratio in parasite tissues. Using an isotope-mixing model, we roughly estimated that the parasites in the hepatopancreas represented from 0.8 to 3.4% of the total snail biomass, including the shell.