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

During October 1990, an outbreak of encephalitis occurred on Saipan. Although no virus was isolated, patients seroconverted to Japanese encephalitis (JE) virus, indicating the first known occurrence of JE on US territory since 1947. Ten cases occurred among a population of 40,000. The prevalence of antibody to JE virus among 234 lifelong Saipan residents surveyed after the outbreak was 4.2%. Age, household crowding, and lack of air conditioning were risk factors for infection. The seroprevalence in pigs, which are important amplifying hosts of JE virus, was 96% (n = 52). None of 288 stored serum specimens from lifelong Saipan residents sampled in 1984 were seropositive. These data suggest that JE virus was recently introduced onto Saipan and that peridomestic factors affected the risk of human infection. Transmission of JE virus probably ended with exhaustion of the supply of susceptible amplifying hosts. Surveillance for human cases and seroconversions in pigs during 1991 revealed no evidence of ongoing JE virus transmission.