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Tick-borne encephalitis (TBE) is a classic obligatorily transmissible viral infection with natural focality, which is widespread mainly in Eurasian forests of the temperate zone. Virus was discovered in the Russian Far East in 1937. Its main long-term reservoirs and vectors in natural foci are Ixodes persulcatus and I. ricinus ticks. Boundaries of the virus range and the location of natural foci within it are closely associated with the distribution pattern of these ticks. The largest categories of TBE range zoning are described. Each natural focus functions as a relatively autonomous parasitic system. The principal scheme of TBE virus circulation related to the development of three successive generations of the main vector is presented. The main parameter of epizootic activity and epidemic manifestation of natural foci and their long-term fluctuations in different regions are discussed. TBE morbidity in Russian and its dynamics during approximately 45 years are analysed.

In 1992-1993, a total of 29 isolates of Borrelia were obtained from small mammals captured in taiga forests of the Perm' region: 26 isolates were obtained from Clethrionomus glareolus and one each from C. rufocanus, Microtus oeconomus, and Apodemus sylvaticus. Isolation of Borrelia was performed by plating the animal material on BSK-II medium. Most isolates (86.2%) were obtained from the urinary bladder, the rest from the heart and spleen. According to the results of IFA with a set of monoclonal antibodies, isolates did not contain Borrelia burgdorferi sensu stricto. Preliminary data suggest that they can belong to B. garinii and B. afzelii. On the basis of the abundance of animals, it is suggested that C. glareolus, a background species among small mammals inhabiting the study region, is the main reservoir host for Borrelia.