Acute viral encephalitis: the recent progress.

In the new Japanese control law for infectious diseases, most varieties of acute viral encephalitis belong to Category IV requiring report of all cases at sentinel hospitals. Herpes simplex virus type 1 (HSV-1) encephalitis comprises the majority of cases. With the increased prevalence of diagnostic procedures such as polymerase chain reaction (PCR), several forms of HSV-1, and -2 central nervous system (CNS) infections, including acute disseminated encephalomyelitis, brainstem encephalitis, and myelitis, have been clarified. Since 1990 we have conducted a survey of HSV CNS infections in the Kyushu and Okinawa regions, and the data are reviewed here. Trends include an increase in a new subtype of non-herpetic acute limbic encephalitis. In contrast, the incidence of Japanese encephalitis (JE) in Japan has dramatically decreased to a few patients per year; however, JE remains a threat for those with decreased or absent immunity to the JE virus. Imported emerging and reemerging CNS infections such as Murray Valley and West Nile encephalitis can occur in Japan. Influenza-associated encephalitis/encephalopathy is also described as a threat for adults as well as young children.