[Rivers of the Orenburg Region: maintenance of the life cycle of Opisthorchis felineus].

Specific hydrological and climatic features contribute to the functioning of the parasitic system egg--mollusk--fish--man parasitic in the Orenburg Region. Four types of areas, such as floodplain, suprafloodplain-terrace, valley-ravine, and near-lake ones, are adequate for the formation of a natural opisthorchiasis focus. The periods of active functioning of a natural opisthorchiasis focus have been established in the Orenburg Region. These include the emergence of the first cercariae and, therefore, the first (in this epidemic season) infection of the supplementary host (fish) occur in mid-June to early July and then 6 weeks later metacercariae achieve invasiveness in the fish muscles and subcutaneous fat, as a result the first infection of the final host with Opisthorchis occurs in early August.