SINGAPORE - The third phase field study of Singapore's stealth weapon against dengue has been a success, achieving 90 per cent suppression of the urban Aedes aegypti mosquito population at test sites, paving the way for an expanded fourth phase study next month.

This will involve the release of mosquitoes via drones, said the National Environment Agency (NEA) on Monday (Oct 7).

Called Project Wolbachia, it is a programme where male mosquitoes carrying the Wolbachia bacterium mate with females, causing them to lay eggs that do not hatch.

NEA has been studying the programme since 2012.

The project's third phase, which began in February, has achieved more than 90 per cent suppression of the urban Aedes aegypti mosquito population at its Nee Soon East and Tampines West study sites.

This is an improvement from the 70 to 80 per cent suppression achieved by the project's second phase, which concluded in December last year, and 50 to 57 per cent suppression in its the first phase.

This demonstrates the effectiveness of Wolbachia-Aedes technology in Singapore's high-rise and high-density urban landscape, said the NEA.

Singapore's Dengue Expert Advisory Panel chairman Duane Gubler, who is also founding director of the Emerging Infectious Diseases Programme at Duke-NUS Medical School, said: "We are optimistic that this method, when fully implemented, will tip the balance in favour of dengue control."

The NEA added that continued releases have kept the Aedes aegypti mosquito population at levels that pose low dengue risk, with the average number of such mosquitoes caught in Gravitraps falling to 0.015 in some areas.

A total of 12,572 dengue cases have been reported from the start of this year until 3pm last Friday.

The number of reported cases per week has been falling steadily, hitting 203 in the week ending Oct 4, down from 327 in the first week of September.

As part of the project's fourth phase, NEA plans to release infected mosquitoes to 163 and 121 residential blocks in Nee Soon East and Tampines West respectively, an area nearly twice the size of the previous phase.

To this end, it will scale up production with a new mosquito production facility and test using drones to release infected mosquitoes efficiently at some areas of the study sites.

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Associate Professor Ng Lee Ching, director of NEA's Environmental Health Institute, said the larger release area would help the agency demonstrate better efficacy of the programme, showing that the same success rate could be achieved with a smaller number of mosquitoes released per block.

She said: "Our systematic approach is needed to derive good comparative results, and a consistent and comprehensive dataset, to ensure robustness of the study before scaling up to more areas beyond Yishun and Tampines."

The NEA said it will continue to work with companies to enhance efficiency and quality in the production and release of mosquitoes.

The agency cautioned, however, that Project Wolbachia is "not a silver bullet" in the fight against dengue.

"Breeding of mosquitoes in the community will cancel out the positive impact of the technology. It is therefore important that the community continues to prevent and remove mosquito breeding habitats, so that fewer male Wolbachia-Aedes mosquitoes have to be released for the technology to be successful," said the NEA.

If successful, the Wolbachia technology will form part of Singapore's dengue control programme, alongside existing methods such as comprehensive mosquito surveillance, eradicating mosquito breeding habitats, and spraying insecticide where necessary to control the adult mosquito population.