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Data collected from areas of Thailand outside Bangkok during 1962 through 1964 were analyzed with special reference to similarities or differences in the epidemiologic, clinical, or virologic features in hospitalized cases of hemorrhagic fever to disease observed in Bangkok. In 1964, major outbreaks of hemorrhagic fever were reported in Northeast Thailand in communities with no previous history of this disease. During the study, 8,184 patients were hospitalized, and 557 died. The sex distribution and seasonal incidence of cases were similar to those in Bangkok; mortality rates were higher. Hospitalization rates in rural communities were often more than 10 times higher than the average for the metropolitan area. The modal age of hospitalized hemorrhagic fever patients “up-country” averaged 2 years older than in Bangkok outbreaks. In nearly all studied outbreaks, most patients had evidence of a dengue infection with secondary antibody response. Dengue infections were documented in schoolchildren resident in three localities in Thailand in 1963 in the absence of overt dengue hemorrhagic fever. In a 1962 survey the proportion of young adult males in all sections of the country except North Thailand with dengue type 1 HI antibody was similar to that in a matched group of Bangkok residents. The “new” outbreaks of dengue hemorrhagic fever reported in 1964, therefore, may not have been related to introduction of novel “hemorrhagenic” dengue viruses in susceptible populations, but probably were due to a dengue epidemic in a population with previous “silent” dengue experience.
Formerly Chief, Virology Department. Present address: Section of Medical Microbiology and Tropical Medicine, University of Hawaii School of Medicine, Leahi Hospital, 3675 Kilauea Avenue, Honolulu, Hawaii 96816.
Formerly Chief, Entomology Department. Present address: Department of Medical Entomology, School of Public Health, University of Texas, Houston, Texas 77025.