Japanese encephalitis: surveillance and elimination effort in Japan from 1982 to 2004

Japanese encephalitis (JE) surveillance has been conducted since 1965 as a part of the National Epidemiological Surveillance of Vaccine Preventable Diseases in Japan. Over 1,000 JE cases were reported annually in the late 1960s. The number of JE cases has since markedly decreased, with less than 10 cases reported annually from 1992 to 2004. A total of 361 JE cases were reported between 1982 and 2004. Prognosis was available for 320 cases; 58 (18%) died, 160 (50%) recovered with neuropsychiatric sequelae, and 102 (32%) completely recovered. Seventy-eight percent of these cases were 40 years old or over with a peak age group of 60-69 years old. JE predominantly occurred in unvaccinated populations. A high seroconversion rate among sentinel pigs was recorded every year. This suggests the presence of JE virus-infected mosquitoes during the summer in most areas of Japan, including the northern districts where no JE cases were reported from 1982 to 2004. Although JE cases have been reported in single figures since 1992, the risk of JE virus infection is still present. Thus, high immunization rates of JE vaccine should be maintained in Japan.