SINGAPORE - 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 and even X-ray treatment.

The mosquitoes will be released at 10 more blocks in Tampines West and 30 more blocks in Nee Soon East compared to phase one. A total of 76 blocks, comprising about 7,000 households, will be involved.

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 several years, providing a baseline for comparative studies.

In the earlier study conducted from October 2016 to December 2017, 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 on Monday (Feb 26).

The agency said the plan was to embark on a larger suppression trial after phase one.

But the trial threw up unexpected hurdles.

Only 6 per cent of the adult male Wolbachia-carrying mosquitoes released at the ground floor were later found on the ninth floor level and higher.

So in phase two, the mosquitoes will be released at higher floors, in addition to being released at 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 to four days.

Containers containing 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.

And the NEA will be employing X-ray treatment in the field study for the first time, to render female Wolbachia-carrying Aedes aegypti mosquitoes infertile.

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, the NEA said.

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, before they are released into the study site.

Studies show the X-ray treatment does not affect the virility of males. But the NEA will be keeping their eye out for changes to their physical abilities, for instance, how well they fly.

Dr Raman Velayudhan, coordinator of the department of control of neglected tropical disease at the World Health Organisation, said the results from phase one were encouraging.

"The phase one (study) has clearly showed the ability of Wolbachia infected males to suppress the population," he said.

However, the NEA wants to push the numbers down further.

Although the larger suppression trial has been delayed, chairman of the Dengue Expert Advisory Panel, Professor Duane Gubler of Duke-NUS Medical School, said "it was not a setback".

"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," he said.

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 make sterile other insect species and should increase the efficacy of the trial," said Prof Gubler.