A seroepidemiological study of Japanese encephalitis and dengue virus infections in the Chiang Mai area, Thailand.

As part of a virological and epidemiological survey of encephalitis in the Chiang Mai area, the neutralizing (N) antibody levels of healthy persons to Japanese encephalitis (JE) and dengue (DEN) type 1-4 viruses were examined. A total of 985 blood samples was collected by the filter paper method from subjects of nine age groups in five districts, four (Pasang, Sarapee, Doi Saket and Mae Taeng) in the Chiang Mai Valley and one (Fang) in another valley separated by several ranges of mountains from the Chiang Mai Valley. From analyses of the results of N tests on the specimens, the following conclusions were drawn about the prevalences of JE and DEN viruses in the Chiang Mai area: (1) In the Chiang Mai Valley, the percentage incidences of N antibodies to JE and DEN viruses increased with age and by the age of 15, two thirds or more of the residents had been infected with JE and all DEN viruses except DEN type 2 virus, which showed the lowest prevalence. (2) In the Fang district, the percentage incidence of N antibody to JE virus increased with age, but those to DEN viruses did not, indicating much lower prevalences in the past of all four serotypes of DEN viruses in this district than in the Chiang Mai Valley. (3) At present, most infants in the Chiang Mai area, including the Fang district, seem to be exposed to DEN viruses first and later to JE virus.