Phase 2 of dengue control study kicks off in April

More Wolbachia-carrying male mosquitoes to be released into Tampines West, Nee Soon East

There will be an added buzz in Tampines West and Nee Soon East come April with the release of male Wolbachia-carrying Aedes aegypti mosquitoes into the estates.

It is part of phase two of an ongoing field study into a novel method to curb dengue transmissions in Singapore which has delivered promising results.

The Aedes aegypti mosquitoes, which neither bite humans nor transmit disease, have been artificially infected with Wolbachia bacteria. When they mate with uninfected females, the resulting eggs will not hatch.

Phase two will involve more housing blocks in the two sites and even X-ray treatment. About one to six mosquitoes will be released per person each week, up from one to three mosquitoes previously.

Braddell Heights, which was part of the first phase, will not be involved this time as it does not have high-rise buildings - a focus of phase two.

These estates represent a cross-section of typical housing estates and have seen dengue outbreaks previously.

The National Environment Agency (NEA) has been monitoring the mosquito population in these sites for years, providing a baseline for comparative studies.

In phase one conducted from October 2016 to December last year, NEA found mosquito populations in the study sites were reduced by half.

The phase two study will run till January next year and aims to overcome challenges that cropped up in the earlier study, NEA said yesterday.

The agency said the plan was to embark on a larger suppression trial after phase one. But the first trial threw up unexpected hurdles.

The first trial threw up unexpected hurdles.

Only 6 per cent of the adult male Wolbachia-carrying mosquitoes released on the ground floor were later found on the ninth-floor level and higher. So in phase two, the mosquitoes will be released on higher floors, in addition to being released on the ground floor. They will also be released twice a week instead of once a week previously, in order to keep the population up for longer.

Only 6 per cent of the adult male Wolbachia-carrying mosquitoes released on the ground floor were later found on the ninth-floor level and higher. So in phase two, the mosquitoes will be released on higher floors, in addition to being released on the ground floor.

They will also be released twice a week instead of once a week previously, in order to keep the population up for longer. The earlier study had found that only half of the mosquitoes released lived up to four days.

Containers holding male Wolbachia-carrying mosquito pupae will also be placed at the study sites this time, as the NEA wants to study if they will adapt better to the site conditions.

During phase one, the NEA noted that a small percentage of female Wolbachia-carrying Aedes aegypti mosquitoes were inadvertently released, as they had slipped through the sorting process which is about 99.7 per cent accurate.

While normal female Aedes aegypti mosquitoes that mate with male Wolbachia-carrying mosquitoes lay eggs which do not hatch, female Wolbachia-carrying mosquitoes still go on to have offspring.

These offspring cannot transmit dengue, chikungunya and Zika, but in the long term, they would affect the ability of male Wolbachia-carrying mosquitoes to suppress the urban mosquito population, said NEA.

Hence all batches of Wolbachia-carrying mosquitoes, adult and pupae, will undergo X-ray treatment to ensure that any females present will be made infertile.

Studies show the X-ray treatment does not affect the virility of males.

The chairman of the Dengue Expert Advisory Panel, Professor Duane Gubler of Duke-NUS Medical School, said:"The phase one studies were very successful in helping us understand this ecology. Phase two will build on this knowledge and, hopefully, increase the efficacy of the male release method."

As for the use of X-ray, NEA said it does not harm humans or the environment, and is currently used in a field study in Guangzhou, China.

"Irradiation has been successfully used to sterile other insect species and should increase the efficacy of the trial," added Prof Gubler.

A version of this article appeared in the print edition of The Straits Times on February 27, 2018, with the headline 'Phase 2 of dengue control study kicks off in April'. Print Edition | Subscribe