Dengue and Chikungunya Virus Infection in Man in Thailand, 1962–1964

An attempt at a comprehensive measurement of illness documented by physicians and infection in man caused by dengue and chikungunya viruses was made in Bangkok and Thonburi during all or portions of 1962–1964. Inpatient data were collected directly from all metropolitan hospitals. Outpatients were studied only at Children's Hospital. In 1962, a seroepidemiologic study of arbovirus infection was made in 7,093 Bangkok households in 19 separate areas, from five of which mosquitoes were collected for enumeration and attempts to isolate virus. During the study, 10,194 patients with hemorrhagic fever were admitted to Bangkok hospitals, and 559 died. At least 80% of these cases were caused by dengue virus. In 1962 alone, it was estimated that between 181,000 and 287,000 dengue illnesses and 44,000 to 70,000 chikungunya illnesses were seen by physicians in outpatient children. Agespecific hospitalization rates peaked at less than 1 year and again at 3 or 4 years, few cases occurring after the age of 15. Hospitalizations were rare in infants during the first 3 months of life; rates then rose abruptly, peaking at 8 months then declining markedly. Hospitalization rates for girls over the age of 1 were consistently higher than for boys. Dengue- and chikungunya-infection rates measured in 1,887 persons bled before and after the rainy season in 1962 were 41% and 31%, respectively. Hospitalization rates varied directly with dengue-virus infection rates but not with income or ethnic group. Hospitalizations occurred within families at almost four times the expected rate. Disease progressed throughout the city and within study areas in a "brush-fire" manner. In contrast to the observations of others, we isolated dengue and chikungunya virus almost exclusively from Aedes aegypti. Although Culex tritaeniorhynchus and Culex gelidus were numerous in light-trap collections, they were rarely attracted to man. These observations suggest a low level of anthropophilia in these species in Thailand and the possibility of a biological barrier to transmission of arboviruses by them to man.