Dengue--a re-emerging infectious disease in Singapore

Despite its well-established integrated nationwide Aedes mosquito control programme which incorporates source reduction, public health education and law enforcement, Singapore has not been spared from the regional resurgence of dengue. The disease incidence has been increasing from 9.3 per 100,000 in 1988 to 102.7 per 100,000 in 1996 at the time when the Aedes house index (HI) has dropped to around 1% from > 25% in the 1960s. Majority of the cases reported from 1990 to 1996 were dengue fever (DF); dengue haemorrhagic fever (DHF) constituted only 6.7%. The case-fatality rate was 0.1% with 13 (81.3%) of 16 serologically confirmed deaths above 19 years of age. The median age has shifted from 14 years in 1973 to 27 years in 1996. The proportion of primary infections also increased from about one-third in 1990 to nearly half in 1996. All four dengue serotypes have been detected from infected persons with dengue 2 predominating in 1990, 1991 and 1993, dengue 3 in 1992 and 1994 and dengue 1 in 1995 and 1996. The disease incidence was significantly correlated with Aedes aegypti HI and residents of compound houses had a significantly higher rate of infection as well as a higher morbidity rate compared with dwellers of high-rise public housing estates. Seroprevalence surveys confirmed the low level of dengue transmission. The immunity level of the general population has been declining with only 6.4% of children and young adults below 25 years of age possessing haemagglutination-inhibition antibody to dengue 2. It would appear that the successful vector control programme over the last two decades has brought about a paradoxical situation in that outbreaks tend to occur more frequently and with even greater intensity because of the low herd immunity of the population. Until the dengue vaccine is commercially available for mass immunisation of the population, community-based integrated control of Aedes aegypti remains the key to the prevention and control of DF/DHF.