Why S'pore's dengue fight differs from that in some countries

The use of only Wolbachia-infected male mosquitoes to help control dengue suits Singapore better than releasing female ones with the bacterium as well, as it means less chance for the dengue virus to evolve, a smaller mosquito population and fewer bites to put up with.

This was highlighted yesterday by Associate Professor Ng Lee Ching, group director of the National Environment Agency's Environmental Health Institute, when she explained why Singapore has decided on a different method than that used by neighbouring countries such as Malaysia, Indonesia and Vietnam.

Project Wolbachia will be expanded to cover about 31 per cent of Housing Board blocks here, it was announced yesterday.

Under Singapore's "suppression" method, male Aedes aegypti mosquitoes infected with Wolbachia are released at selected sites. When female Aedes mosquitoes which are not infected with the bacterium mate with the males, they produce eggs which do not hatch, thus reducing the population of Aedes aegypti mosquitoes. This method is used by some five other countries, including the United States, China and Thailand.

In contrast, about a dozen countries are using the "replacement" approach. In this method, Wolbachia-carrying males and females are released to mate with mosquitoes in the wild, breeding Wolbachia-carrying offspring. As Wolbachia hampers the reproduction of viruses such as dengue, the plan is to replace the existing wild mosquito population with one that carries Wolbachia and is therefore less likely to transmit dengue and other mosquito-borne illnesses.

The replacement approach is also used in Australia and Sri Lanka.

Speaking at the three-day Asia Dengue Summit, which was organised by Asia Dengue Voice and Action to discuss dengue management strategies for the region and which ended yesterday, Prof Ng cited five reasons why Singapore has chosen the suppression approach and not the replacement one.

First, suppressing the mosquito population leads to a lower risk of dengue transmission.

Singapore has a small mosquito population size, and the replacement strategy, which relies on mosquitoes breeding, may potentially increase this.

A larger mosquito population may mean the Republic is unable to control the ability of the mosquitoes to transmit dengue, said Prof Ng.

Second, the replacement approach carries the risk that viruses may be able to evolve and adapt to overcome Wolbachia's blocking effect in the long run. In contrast, the suppression approach means there is less opportunity for viruses to evolve.

Third, the suppression approach, which releases only male mosquitoes - which do not bite - is more acceptable to the public, said Prof Ng. This approach will also reduce the rate of people getting bitten.

Fourth, the suppression approach is consistent with Singapore's policies and public messaging on the importance of source reduction, which have been emphasised over the past six decades.

Fifth, it is still possible to switch to the replacement approach after carrying out the suppression method, whereas it is more difficult to suppress the population after using the replacement method, said Prof Ng.

She acknowledged that the suppression method is costlier, and requires a sustained effort to maintain.

Nevertheless, Prof Ng added that Project Wolbachia has been successful in Singapore so far.