Ohira et al.
First evidence of tick-borne encephalitis (TBE) outside of Hokkaido Island in Japan
Emerg Microbes & Infect. 2023;12(2):2278898. doi:10.1080/22221751.2023.2278898

Since 1993, five cases of TBE (caused by the Far Eastern subtype of the virus) have been identified in Japan, all on the island of Hokkaido. Outside of this island, TBE virus-specific antibodies have been found in rodents, but no TBE case has so far been recorded, although TBE is a nationally notifiable disease in Japan.

It is assumed that TBE virus infections have been missed in Japan, and therefore a retrospective analysis has been carried out in 15 hospitals including 13 hospitals outside of Hokkaido. They were dealing with the review of medical records to identify patients with encephalitis or meningitis of unknown etiology for whom a standard-of-care serum or CSF specimen was collected from 2010 to 2021. TBE virus infections were detected by using a commercial ELISA (based on antigen from the European virus subtype) and neutralizing antibodies (using a Far Eastern subtype in the assay).

A total of 761 residual specimens were tested for infections by Borrelia burgdorferi, Japanese encephalitis virus or TBE virus. TBE virus infection was identified in 3 of 520 previously hospitalized patients. These 3 cases were identified outside of the island of Hokkaido. Combined with previous studies that identified TBE antibodies in rodents outside of Hokkaido, these results indicate that TBE virus transmission occurs in Japan outside of Hokkaido.

TBE virus infections may be undiagnosed in Japan, apparently due to the lack of availability of TBE diagnostic tests and low awareness for TBE, supporting the need for enhanced efforts to detect and prevent this disease in Japan.

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