[The characteristics of the circulation of the causative agent of opisthorchiasis in the tributaries of the Ob].

Opisthorchiasis morbidity is reviewed in the area of the Vasyugan River, an Ob tributary. Intermediate hosts have been found in 3 settlements situated at various distances from the ostium. The infestation of intermediate and additional hosts with Opisthorchis larvae is investigated. The above data and the results of previous population surveys (establishing infestation and contamination) served the basis for elucidating the nature of helminth circulation on the territory along the Vasyugan River. The role of infected fish migration is discussed as a source of human infection in areas where helminth circulation is altogether absent or is at an extremely low level.