[Evolution of tick-borne encephalitis and a problem of evolution of its causative agent].

The evolution of tick-borne encephalitis (TBE) is marked by the expanded nosological area, the transformation of landscapes, the formation of anthropurgic foci, the change of environmental systems, the increase of mortality rate mainly among urban dwellers, as well as pathomorphism. The evolution of natural TBE virus (TBEV) populations was studied in Eastern and Western Siberia, Middle Urals, and the European part of the nosological area. The paper first describes the types of evolutionary transformations of viral populations under the conditions of a varying environmental and epidemiological situation. These include: 1) the change of TBEV subtypes over 50-60 years; substitution of the Far-Eastern subtype for its Siberian subtype (the Sverdlovsk and Kemerovo regions); 2) the steady-state circulation of one Siberian subtype with mutanttypes being accumulated (the Vologda region); 3) co-existence of the Far-Eastern and Siberian subtypes with the common vector Ixodes persulcatus (the Yaroslavl and Irkutsk regions, etc.); 4) original mixed TBEV strains including the gene sites of proteins E and NSI of two subtypes. There is new evidence that the Siberian subtype is able to induce focal TBE forms, leading to death.