Dengue Haemorrhagic Fever in the South of Vietnam during 1975-1992 and Its Control Strategy

The incidence of dengue (DEN) virus infection in Vietnam has increased dramatically in the past 30 years. From 1960 up to now, the number of dengue haemorrhagic fever (DHF) cases has been continuing to increase and recorded as the greatest one in Southeast Asia and the Western Pacific Regions. DHF was endemic in South Vietnam and 30-380 cases/100,000 population were reported annually. Major epidemics have occurred in a 3-4 years frequency, as in 1975, 1978-1979, 1983 and 1987. The 1987 outbreak was the largest one with 83,905 cases and 904 deaths. DHF epidemics usually took place from June to November every year. The peak transmission was recorded in July-August and September, coinciding with the rainy season and the breeding period of Aedes aegypti. The majority of confirmed cases was children of 5-6 year-old-group, in which there was no sex difference. In the virological surveillance in Ho Chi Minh City and in some surrounding provinces, several DEN virus strains were isolated from patients' blood as well as from the mosquitoes. In the 1987 DHF outbreak, DEN-2 was the dominant sero-type (90.5%). But from 1990 the epidemic sero-type has changed, DEN-1 was introduced and was continuing to grow-up (58%). Then in 1992 outbreak, the DEN-2 was reintroduced, from 26.3% in 1991 has increased to 41.4%. Serological investigation of healthy persons in 12/17 southern provinces had demonstrated that there were relatively wide circulations of DEN, Japanese encephalitis (JE) and chikungunya (Chik) viruses and the endemic strains of DEN were found different in some studied areas. There had been augmentation of primary infection rate and decrease of the susceptibility rate in children prior to the DHF epidemic. Seven JE virus strains were isolated from dengue fever (DF) patients' blood. This fact has introduced as a new concept in the isolation of JE virus. In the final pant of the report, the authors stressed on the "Active surveillance" as a new strategy for controlling the DHF epidemic in South Vietnam. The goal of this active surveillance is to have early warning and predictive capability for epidemic dengue, based on serological and clinical surveillance. Because dengue cases were detected in January-February of every year, so the base line is: when first cases of DF/DHF were detected, it needs to be promptly implemented the larval control and insecticide must be sprayed around the patient's houses. But now almost Provincial Centers of Hygiene and Epidemiology often waited and when big DHF epidemic had already occurred, they started to implement its control measures, so it is too late to control the DHF outbreak