[The isolation of tick-borne encephalitis virus strains and the determination of the scope of immune stratification among the population and the domestic animals on the northern Russian plain].

Between 1986 and 1987 the authors conducted virological survey of 689 Ixodes persulcatus and 420 mouse-like and insectivorous rodents at the territory of Arkhangelsk Province and the Republic of Komi. A total of 8 strains of tick-borne encephalitis virus were isolated. Out of them 3 strains were isolated from small mammalians (2 from Clethrionomys glareolus, 1 from Microtus oeconomus) at the southeastern areas of Arkhangelsk Province within the middle taiga subarea, 5 strains were isolated from I. persulcatus (the rate of infection was 1.25 per cent) gathered in the southeastern Komi within the southern margins of the middle taiga subarea. PHAT investigation of the sera (2064 human, 656 cow and 171 dear specimens) evidenced almost the absence of immunity among the studied populations of the tundra and forest tundra zones. Certain positive samples observed in the northern taiga region could be explained by the infection gained in areas situated more to the south. Among the middle taiga human populations the immunity reached 4.9 per cent.