Characterization of tick-borne encephalitis virus isolated from a tick in central Hokkaido in 2017

Tick-borne encephalitis virus (TBEV) is a zoonotic virus in the genus Flavivirus, family Flaviviridae. TBEV is widely distributed in northern regions of the Eurasian continent, including Japan, and causes severe encephalitis in humans. Tick-borne encephalitis (TBE) was recently reported in central Hokkaido, and wild animals with anti-TBEV antibodies were detected over a wide area of Hokkaido, although TBEV was only isolated in southern Hokkaido. In this study, we conducted a survey of ticks to isolate TBEV in central Hokkaido. One strain, designated Sapporo-17-Io1, was isolated from ticks (Ixodes ovatus) collected in Sapporo city. Sequence analysis revealed that the isolated strain belonged to the Far Eastern subtype of TBEV and was classified in a different subcluster from Oshima 5-10, which had previously been isolated in southern Hokkaido. Sapporo-17-Io1 showed similar growth properties to those of Oshima 5-10 in cultured cells and mouse brains. The mortality rate of mice infected intracerebrally with each virus was similar, but the survival time of mice inoculated with Sapporo-17-Io1 was significantly longer than that of mice inoculated with Oshima 5-10. These results indicate that the neurovirulence of Sapporo-17-Io1 was lower than that of Oshima 5-10. Using an infectious cDNA clone, the replacement of genes encoding non-structural genes from Oshima 5-10 with those from Sapporo-17-Io1 attenuated the neuropathogenicity of the cloned viruses. This result indicated that the non-structural proteins determine the neurovirulence of these two strains. Our results provide important insights for evaluating epidemiological risk in TBE-endemic areas of Hokkaido.