Dengue prevention strategies: Q&A with Associate Professor Ng Lee Ching


Roshini Claire Anthony spoke to Associate Professor Ng Lee Ching, Director of the Environmental Health Institute (EHI) at the National Environment Agency (NEA), Singapore, on the role of primary care physicians and government agencies in preventing dengue epidemics.


How can primary care physicians play a role in reducing the incidence of dengue in Singapore?

Early diagnosis and notification of dengue cases will enable the NEA to promptly identify locations with dengue transmission and initiate vector control measures on the ground. This includes the search for breeding habitats and indoor space spraying with insecticides to kill adult mosquitoes, which could potentially be carrying the dengue virus. Since 2008, the EHI has advocated the use of the NS1 test, which can diagnose dengue cases as early as day 1 from onset of fever.

Physicians should advise dengue patients to avoid being bitten by mosquitoes so as not to pass the dengue virus to mosquitoes in the environment. Such prevention measures include the use of repellent and staying in mosquito-free rooms during the febrile period.

Physicians could also advise patients to eliminate mosquitoes (potentially infected) at home by using aerosol sprays in dark corners to kill adult mosquitoes, and to remove stagnant water.


What other steps would you propose the NEA or Ministry of Health (MOH) take to reduce the likelihood of another dengue epidemic? What steps are currently being taken?

The NEA takes a systematic and holistic approach to stem dengue transmission in Singapore. Multipronged surveillance monitors the temporal trend and spatial distribution of cases, the virus serotypes in circulation, and Aedes vectors. Together with environmental factors such as temperature, humidity, and age of building, these data have allowed the NEA to develop risk stratified maps for prioritization of resources for vector control activities.  

The NEA also works with various agencies, through the Inter-Agency Dengue Task Force (IADTF), to reduce mosquito population in our environment. Strategies undertaken include:

·         Removing mosquito breeding sources to prevent the emergence of adult mosquitoes and reduce disease transmission. This is achieved through environmental management, intensive search-and-destroy operations (both indoors and outdoors) to remove mosquito breeding, and applying of larvicides.

·         Killing adult mosquitoes, including uninfected ones, to prevent transmission of the dengue virus. The most common measures are insecticidal fogging and misting for outdoor areas, as well as space-spraying indoors. These control methods are applied judiciously in situations where there is active transmission. Since 2013, the NEA has also been placing Gravitraps in dengue cluster areas, to complement these conventional measures.

·         Rallying the community to help sustain efforts for dengue prevention and control. The nationwide ‘Do the Mozzie Wipeout’ awareness campaign rallies the community to be vigilant and take collective action to prevent mosquito breeding and break the dengue transmission cycle. Through the Dengue Community Alert system, the NEA keeps the community up-to-date with the latest dengue situation via alert banners at strategic locations. 

Moving forward, the NEA has also been looking into the use of male Wolbachia-carrying Aedes aegypti mosquitoes to help suppress the Aedes mosquito population. When male Wolbachia-carrying Aedes aegypti mosquitoes mate with female urban Aedes aegypti mosquitoes (without Wolbachia), their resulting eggs do not hatch. This form of ‘sterility’ is due to a phenomenon known as ‘cytoplasmic incompatibility’. The strategy is based on the fact that the urban Aedes aegypti does not carry Wolbachia. The release of male Wolbachia-carrying Aedes aegypti in the field, to compete with urban males for urban females, can theoretically lead to a reduction in the urban Aedes aegypti mosquito population over time. The ultimate goal is suppression of the urban Aedes aegypti population to a level that cannot sustain dengue transmission. 

Male mosquitoes do not bite or transmit disease, and Wolbachia is a naturally occurring and nonharmful bacterium found in more than 60 percent of insect species. The use of male Wolbachia-Aedes is thus safe, and not harmful to humans, animals, or ecology. Since October 2016, the NEA has been conducting a small-scale field study to understand how male Wolbachia-carrying Aedes aegypti mosquitoes behave in our urban built-up environment – how far and high they fly, how long they live, and how well they compete in the field. Data collected from this study will support the design of a subsequent suppression trial, in which releases of male Wolbachia-carrying Aedes aegypti mosquitoes will be optimized for effective suppression of the mosquito population. Ultimately, if the study and trial are successful, the NEA hopes to use Wolbachia technology to complement NEA’s existing vector control efforts and reduce the spread of viruses such as dengue and Zika.


What advice would you give to other countries in the region on the implementation of vector control strategies?

Singapore’s dengue control programme has evolved through the years in response to increasing challenges due to low herd immunity and increasing global exchanges of viruses. Based on the fundamental strategy of removing the sources of Aedes breeding, and encouraging the ownership by various stakeholders, our programme is organized around the following key approaches:

Inter-epidemic surveillance and control

Considering that vector control during an epidemic would have very limited impact on curtailing disease transmission, attention should be focused on reducing the Aedes mosquito population and the number of human reservoir (cases) before the traditional dengue season. Besides regular preventive checks and community engagement, this requires surveillance of case, virus, and vector, which could provide alerts of potential epidemics.


Risk-based prevention and intervention

Epidemiological knowledge of dengue (with data from surveillance) and analytics can be used to develop a risk-based and risk-targeted approach in vector control. This could help in decision making, such as when there is a need to prioritize resources, and when to activate pre-emptive mitigation, social mobilization, and logistic and supplies preparedness to deal with an impending outbreak.


Coordinated inter-sectoral cooperation

Inter-sector partnerships are important for effective vector control strategies. Singapore advocates inter-sectoral partnerships in dengue control, and has the IADTF that comprises stakeholders from the public (government), people (community), and private (industry and professional bodies) sectors, working in tandem against the dengue threat. This coordinated approach leverages each agency’s expertise and pools resources together, with the aim of applying holistic treatment and amplifying dengue control efforts.


How would the introduction of the dengue vaccine affect the risk of future dengue epidemics?

This would depend on the efficacy and long-term safety of the dengue vaccine. An efficient vaccine could help raise the herd immunity and prevent dengue outbreaks. However, the sole commercially available vaccine is known to have low efficacy among sero-negative populations and its mid- to long-term safety for such populations remains unknown. As the seroprevalence of Singapore’s population is generally low, the vaccine will have limited impact in preventing future dengue epidemics, though it may offer protection to sero-positive individuals. Suppression of vector population through source reduction is expected to remain the mainstay of dengue control in Singapore. A low vector population will also reduce the risk of Chikungunya and Zika virus transmission.

Prof Ng Lee Ching