Epidemiology of tick-borne encephalitis (TBE) and phylogenetic analysis of TBE viruses in Japan and Far Eastern Russia.

In Oshima, the southern part of Hokkaido, a tick-borne encephalitis (TBE) patient was found in 1993; in addition TBE virus was isolated from the blood samples of sentinel dogs, ticks pools, and rodents spleens in 1995 and 1996 by suckling mice. To identify when these TBE viruses emerged in Hokkaido, the times of divergence of TBE virus strains isolated in Oshima and Far Eastern Russia were estimated. TBE virus was isolated in Khabarovsk in 1998, and the nucleotide sequences of viral envelope protein genes of isolates from Oshima and Khabarovsk were compared. Based on the synonymous substitution rates of these virus E-protein genes, the lineage-divergence times of these TBE virus strains were predicted phylogenetically to be approximately 260-430 years ago. Furthermore, the virulence of TBE virus isolates from Oshima and Khabarovsk were compared in a mouse model. The results showed that the isolates possessed very similar virulence in mice. European TBE vaccine was found to be effective in TBE virus, Hokkaido strain. This review provides evidence that the Oshima strains of TBE virus in Hokkaido emerged from the Far Eastern Russia a few hundred years ago, which explains why the virulence of these strains is similar to that of TBE viruses isolated in Russia. Practical application of the vaccine should be considered in Japan.