Tick-borne encephalitis virus (TBEV) is a zoonotic agent causing severe encephalitis in humans. A recent epizootiological survey indicated that endemic foci of TBEV have been maintained in the southern part of Hokkaido until recently. In this study, we sought to isolate TBEV from wild rodents in the area. One virus, designated Oshima 08-As, was isolated from an Apodemus speciosus captured in Hokuto in 2008. Oshima 08-As was classified as the Far Eastern subtype of TBEV and formed a cluster with the other strains isolated in Hokkaido from 1995 to 1996. Thirty-six nucleotide differences resulted in 12 amino acid changes between Oshima 08-As and Oshima 5-10 isolated in 1995. Oshima 08-As caused high mortality and morbidity in a mouse model compared with Oshima 5-10. Although similar transient viral multiplication in the spleen was observed in the mice infected with Oshima 08-As and Oshima 5-10, greater viral multiplication with an inflammatory response was noted in the brains of mice infected with Oshima 08-As than those infected with Oshima 5-10. These data indicate that a few naturally occurring mutations affect the pathogenicity of the Oshima strains endemic in the southern part of Hokkaido.

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