Lowered herd immunity is, however, insufficient to account for the resurgence of dengue in Singapore. Dengue is predominantly a childhood disease in most parts of Southeast Asia, and more women than men are infected as adults. This disease pattern fits the behavior of A. aegypti
. This species of mosquito is highly domesticated, lives and breeds indoors, has a limited flight range, and feeds almost exclusively on humans. Consequently, persons who spend more time at home during the daytime, i.e., mothers and children, are more likely to be infected than those who leave the home for work. In Singapore, however, the incidence of DF/DHF is lower in children than in adults (14
). This finding could be due to a high proportion of subclinical infection in children or a lack of infection in the domestic environment.
To investigate this observation, a serologic survey of 1,068 children <15 years of age was conducted during an 18-month period in 1996 and 1997 (17
). All children who were born at or who visited outpatient clinics of the National University Hospital, which serves the entire country, for routine check ups and vaccinations were included in this study, with parental consent. This population would have grown up during dengue resurgence. The results of this survey showed that preschool children, 10 months to 5 years of age, had a seroprevalence ratio of 0.77%, children 6–10 years of age and 11–15 years of age has prevalence ratios of 6.7% and 6.5%, respectively (17
). School-age children were therefore 9× more likely to have antibodies to dengue than were preschool children (17
Preschool children spend most of their time either at home or at a nursery or kindergarten. Most of these facilities are run out of residences or shophouses in government-owned, high-rise accommodations. Formal half-day schooling starts at the age of 6 years, often with after-school extracurricular activities. The significant difference in seropositivity between preschool and school-age children suggests that the risk of acquiring dengue in Singapore is greater when a person spends more time away from home (17
This hypothesis is supported by the lower premises index in residences than nonresidences in 1997. Residential properties in 1997 had low premises indexes; 2.1% in landed premises and 0.6% in apartments compared to indexes in schools (27.0%), construction sites (8.3%), factories (7.8%), and vacant properties (14.6%) (20
). In contrast, the premises index in 1966 was highest in residences: slum housing (27.2%), shophouses (16.4%), and apartments (5.0%) (9
). Furthermore, women, who are more likely than men to care for children at home, have a lower incidence of dengue, as indicated by the male-to-female disease ratio of 1.6:1 (21
). Collectively, these findings suggest that substantial virus transmission occurs away from the home.