Recent Epidemics of Hemorrhagic Fever in Thailand: Observations Related to Pathogenesis of a “New” Dengue Disease

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Abstract
Linear prospective virologic and epidemiologic studies of mosquito-borne hemorrhagic fever (HF) in Bangkok (pop. 2,000,000) during 1962-63 have established that the disease is endemic throughout the year with epidemic surges occurring biannually during the rainy season. Hospitalizations totaled 10,367 for the period 1958-63. Cases were primarily restricted to children under 15; modal admission rates occurred in 3-5 year olds. Deaths, comprising 5-8% of hospitalized cases, were proportional to the age and sex distribution of all patients. Dengue viruses identical or similar to types 1, 2 & 3 were recovered from Aedes aegypti and from patients admitted with a clinical diagnosis HF. Chikungunya virus, a group A arbovirus also transmitted by A. aegypti, has been recovered repeatedly from patients with febrile diseases occurring during HF outbreaks. Present evidence suggests that chikungunya is not responsible for severe or fatal illnesses. HF patients in Bangkok typically have high titered CF antibody in early convalescent phase serums which reacts with several different dengue antigens (secondary dengue virus infections?) Between June-August 1964, 94 patients were admitted to the provincial hospital of Ubon (pop. 27,000) with a diagnosis of hemorrhagic fever; 8 died. The age distribution of cases and deaths resembled that observed in Bangkok. Twelve dengue type 1 viruses were recovered from 34 studied patients. In most patients CF antibody levels in early convalescent phase sera were absent or low titered reacting with only the homologous virus (primary dengue virus infection?) Conclusive evidence is not available from these or other studies to explain why variant dengue virus syndromes exist. Although the described epidemics behave as though due to virulant mutant dengue viruses, Caucasians contracting dengue infections during HF outbreaks in endemic areas, thus far, have developed only benign classical dengue fever. For public health practise it would be wise to regard dengue viruses in Southeast Asia as lethal mutants taking necessary precautions to prevent their spread to other A. aegypti infested areas.