Epidemiology of tick-borne encephalitis in Japan.

A tick-borne encephalitis (TBE) patient has not been reported for many years in Japan although a serological survey of domestic animal sera suggested the presence of TBE foci in Hokkaido, the northern island of Japan. In October 1993 in Hokkaido, a severe encephalitis case in a farm wife was diagnosed as tick-borne encephalitis. Serological examination of paired sera showed a rise of neutralization (NT) antibody titer to Russian spring summer encephalitis virus. A sero-epizootiological survey of dogs showed that the TBE-related virus was prevalent in the area. Three virus isolates were obtained from the blood of sentinel dogs and the antigenic analysis grouped the isolates into TBE-related viruses. Sequence analysis of envelope protein gene identified one of the isolates as the same subtype as Russian spring summer encephalitis (Far-Eastern TBE) virus. The results provide the evidence that TBE is endemic in a certain area of Japan.