Tick-borne encephalitis virus isolates from natural foci of the Irkutsk region: clarification of the genotype landscape.

The Irkutsk region is the unique territory where all known subtypes of tick-borne encephalitis virus (TBEV) circulate. In the last years, the phenomenon of changes in TBEV subtypes (substitution of the Far-Eastern subtype by the Siberian one) was noted in some regions of the Russian Federation. The results of individual investigation of 11522 Ixodes persulcatus ticks and brain specimens from 81 small mammals collected in natural foci of the Irkutsk region during 2006-2014 are presented in the article. More than 60 TBEV strains have been isolated and studied by virological methods; E gene fragments (1193 b.p.) of 68 isolates have been typed. The majority of the strains (irrespective of subtype) were of high virulence for laboratory mice (LM) in case of both intracerebral and subcutaneous inoculation of virus. All isolates from warm-blooded small mammals and humans were of high virulence for LM, but placed in the same clusters of the phylogenetic tree with ticks collected in the same area. Tick-borne strains of different virulence also did not form separate clusters on the tree. Phylogenetic analysis showed that modern TBEV genotypic landscape of the studied territory is changing toward absolute predominance of the Siberian subtype (94.1%). This subtype is represented by two groups with prototype strains “Zausaev” and “Vasilchenko”. The “Vasilchenko” group of strains is spread on the whole territory under study; the strains of “Zausaev” group were isolated previously in the Irkutsk suburbs. The European subtype of TBEV circulates in natural foci of Pribaikalie permanently (at least 5% of the random sampling); the strains are of high virulence for LM. The Far-Eastern TBEV subtype was not found within the group of isolates collected in 20062014. The phylogenetic relationship of the strains under study had a higher correlation with the place of isolation than with the year or source.