Phylogenetic and virulence analysis of tick-borne encephalitis viruses from Japan and far-eastern Russia

We have previously reported that tick-borne encephalitis (TBE) is endemic i n a specific area of Hokkaido, Japan. In Oshima, the southern part of Hokka ido, TEE virus was isolated from sentinel dogs, ticks and rodents in 1995 a nd 1996. To identify when these TEE viruses emerged in Hokkaido, the times of divergence of TEE virus strains isolated in Oshima and far-eastern Russi a were estimated. TEE virus was isolated in Khabarovsk in 1998 and the nucl eotide sequences of viral envelope protein genes of isolates from Oshima an d Khabarovsk were compared. From the synonymous substitution rate of these virus strains, the lineage divergence time of these TEE virus strains was p redicted phylogenetically to be about 260-430 years ago. Furthermore, the v irulence of TEE virus isolates from Oshima and Khabarovsk were compared in a mouse model. The results showed that the isolates possessed very similar virulence in mice. This report provides evidence that the Oshima strains of TEE virus in Hokkaido emerged from far-eastern Russia a few hundred years ago and this explains why these strains possess virulence similar to the TE E viruses isolated in Russia.