Challenges of dengue control.

We refer to Dr Ong Siew Chey’s letter “Dengue Fever Outbreak: Prevention Possible”, published in the recent September issue of Annals. According to the World Health Organisation, dengue has become the fastest growing mosquito-borne disease1 in an expanding list of countries. Our status as a travel hub results in a continual infl ux of genetically diverse dengue virus strains into Singapore.2 Our vector control efforts have kept our mosquito population relatively low and our population has low herd immunity. This makes us especially susceptible to viral strains with higher epidemic potential. A virus capable of replicating faster in the mosquito vector or human host will be selectively amplifi ed over other strains. Singapore’s current outbreak is driven by a new strain of DENV1, which was fi rst detected last year. This has replaced DENV2 as the predominant serotype since early this year. A serotype change has been a typical lead indicator of past epidemics.3 As a result of the strain replacement, there has been a record number of confi rmed dengue cases this year (19,553 as at 11 November 2013). Nevertheless, taking into account our low herd immunity, we are likely to continue to face the dengue threat for an extended period of time. The long-term solution is the development of a safe vaccine that is simultaneously effective against all dengue serotypes. Until this is achieved, the key strategy for disease control remains vector suppression. We share Dr Ong’s hope that a small highly urbanised island could theoretically be able to eliminate all mosquito breeding sites. However, the challenge of controlling Aedes aegypti, the primary dengue vector, cannot be underestimated.4 The mosquito is well adapted to our urban environment. It breeds predominantly in artifi cial containers found in human habitats, rests in cool and dark places within indoor environments, has a fl ight range as far as 300 metres and can traverse high-rise buildings.5 The eggs withstand desiccation for up to a year and are usually laid in small numbers in each container, which enhances the survivability of the mosquito.6 It takes only 7 days to complete a breeding cycle.