Serological survey of dengue infections among individuals in Rayong, Thailand

Abstract

Background: Dengue diseases have been a major public health problem in Thailand over the past 50 years. Clinical dengue has traditionally affected children with rare cases among adults. Even though the incident number of cases does not seem to have decreased, a shift towards older age groups has been observed over the past years. The reasons for this shift have not been elucidated.
Methods: The age-stratified serological study conducted among school aged children living in the district of Mueang district in Rayong, Thailand. Schools and classrooms were sampled probabilistically from all schools serving the district. Samples were analyzed using single dilution neutralization testing. We estimated and compared the age-specific seroprevalence for the two serosurveys in 1980 and 2010. To explore the factors associated with seropositivity in 2010, we fit a mixed-effects model with a random intercept for school.
Results: A total of 1,811 children, from 90 classes in 25 schools participated in the study. The overall response rate for the study was 53%. Overall, 69% of the sampled children showed evidence of prior exposure to Dengue virus and 46% of the samples showed immunological evidence of exposure to Japanese encephalitis. The agespecific seroprevalence to dengue virus in the 2010 serosurvey as compared to the 1980 serosurvey while according to the survey conducted in 1980, 96% of the population had been exposed to dengue by age 11 years, the 2010 survey suggests that only 74% of children aged 11 years a have been exposed. By age 18 years, 16% of the population remains susceptible to dengue virus. As expected, age was the factor most strongly associated with seropositivity in all analyses (OR 1.16, 95%CI 1.10, 1.23). Attending one of the schools in the single industrial subdistrict was also associated with a significantly greater risk of seropositivity (OR 3.17, 95%CI 1.51, 6.67). Although there was a trend towards a negative association between owning a car/motorbike and seropositivity (OR 0.48, 95%CI 0.22, 1.02).
Discussion: Our results, from two studies conducted in the same location, 30 years apart, and show that underlying this shift there has been a significant increase in the age at exposure to dengue virus. Multiple factors might be responsible for the changes in age at exposure. A decrease in the hazard of infection may indicate decreased contact with the mosquito, as a result of decreasing vector populations or to improved housing.