| Abstract
| - Human serum was collected from a sample cohort of 494 persons residing in the five study areas in the Chiangmai Valley, Thailand. The sample was bled before the start of the 1970 Japanese encephalitis epidemic, and three times during the epidemic year. The purpose was to define the degree of past and present human exposure to Japanese encephalitis virus (JEV), and to study the seroepidemiologic interrelationships existing between dengue and JEV. Analysis of village and urban sera revealed that JEV transmission had probably been occurring for many years. JEV antibody prevalence rose with age; by age 30 nearly everyone of both sexes had been infected. Serologic and entomologic data further revealed that dengue transmission before 1970 had been occurring more in and near Chiangmai City than in rural districts, but there was no clinical or serologic evidence of dengue infections occurring during 1970. Inapparent JEV infections occurred in 6% of the sample cohort. Fevers and respiratory illnesses occurred only slightly more frequently in persons having JEV infections than in those not infected. The valley residence, age, sex, and antibody response patterns of persons experiencing inapparent JEV infections were very similar to the 1970 encephalitis cases. Inapparent JEV infections occurred throughout the study year. The ratio of inapparent to apparent JEV infections was estimated to be 300: 1 under age 40. Primary or secondary-type hemagglutination-inhibition antibody responses after JEV infection occurred in individuals in each age group depending upon the absence or presence, respectively, of pre-existing dengue antibody. Evidence is presented which suggests that the incidence rate of JEV infections in persons with previous dengue infections) was at least as great as that in persons without previous dengue exposure.
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