Outbreak of Japanese encephalitis on the island of Saipan, 1990.

This was a study of the possible causes of an outbreak of encephalitis on Saipan in October 1990. The virus was not isolated but patients seroconverted to Japanese encephalitis (JE) virus, suggesting the first known outbreak of the disease on American territory since 1947. Ten cases were found in a population of 40,000. In a survey after the outbreak, the prevalence of antibody to JE virus was 4.2% among 234 native Saipan residents. Risk factors for infection were age, crowded living conditions and lack of air conditioning. The seroprevalence in pigs who are important amplifying hosts of the JE virus was 96% (n=52). In 1984, 288 native Saipan residents sampled in 1984 but none of their stored serum specimens proved seropositive. The conclusions drawn were that JE virus was only recently introduced to the island and that peridomestic factors affected the risk of an individual getting the infection. The transmission of the JE virus probably stopped when the supply of susceptible amplifying hosts was exhausted. Observation continued during 1991 for human cases and seroconversions in pigs but no evidence was found of JE virus transmission.