SINGAPORE: The National Environment Agency (NEA) will release more Wolbachia-infected mosquitoes at three sites between April this year and January 2019.
They are Tampines Street 81, Tampines Avenue 4 and Nee Soon East.
This will mark the second phase of studying the use of Wolbachia-infected mosquitoes to suppress the Aedes aegypti mosquito population in Singapore, the agency said.
Depending on the need, it said more mosquitoes may end up being released compared to the first phase of the study, which began in October 2016.
During the first phase of the study, NEA released thousands of Wolbachia-infected male mosquitoes over a period of six months in Tampines West, Nee Soon East and Braddell Heights.
NEA said it would be making improvements to its strategies and tactics for release in the second phase, based on the results from Phase 1, which it said yielded “valuable ecological information”.
MOSQUITOES RELEASED FROM HIGHER GROUND
The first phase of the study showed that there was 50 per cent reduction of the urban Aedes Aegypti mosquito population at the study sites compared to areas where no infected mosquitoes were released, NEA said.
Both had a similar Aedes mosquito population prior to the release.
The mosquito population in the study sites fell because the eggs that are produced when the Wolbachia-infected male mates with uninfected urban Aedes females do not hatch, NEA said.
But the agency added that the first phase of the study also threw up some challenges.
For example, Singapore’s densely-packed housing environment saw mosquitoes easily "migrating" from surrounding areas into the release sites where the population had fallen.
In fact, data showed that about 20 per cent of Wolbachia-infected mosquitoes were found outside release sites, suggesting that a similar number of urban Aedes mosquitoes would move into the release sites.
NEA also found that there were mosquito breeding incidents at higher floors of apartment blocks, while the Wolbachia-infected mosquitoes released at these sites only flew up to the eighth floor on average.
Hence, there were not enough infected mosquitoes to suppress the mosquito population on these higher floors.
NEA said the second phase of the study would involve releasing infected mosquitoes from ground floors as well as high floors of apartment blocks.
Going forward, it will also limit the release to Tampines West and Nee Soon East, as the Braddell Heights area is mostly made up of landed property.
To address the movement of mosquitoes, it will extend the release to bigger areas within Tampines West and Nee Soon.
RELEASED MOSQUITOES WILL BE X-RAYED TO MITIGATE IMPACT OF INFECTED FEMALE MOSQUITOES IN ENVIRONMENT
NEA said its current methods of separating the mosquitoes to release only male mosquitoes can also be improved.
Typically, male pupae are smaller than females, and physically separating infected male mosquitoes from female ones is laborious.
NEA said its current methodology for large scale sex-sorting results in about 0.3 per cent of infected female mosquitoes being released.
The agency’s Director-General of Public Health, Derek Ho, said while the presence of infected females will not have much of an impact in the short-run, it could limit NEA’s overall approach of using Wolbachia-infected mosquitoes to suppress the population.
“The Wolbachia itself will have some blocking of the dengue transmission so there is no danger of dengue transmission but in the long-term, we do not want the population of female Wolbachia mosquitoes out there because it just means that in the future, we will not be able to continue to use the release of male Wolbachia mosquitoes to suppress the population,” Mr Ho said.
That’s because the males will eventually be able to mate with the infected female mosquitoes and produce eggs that hatch into offspring, while NEA's strategy is to reduce the population of mosquitoes altogether.
Going forward, he said mosquitoes released as part of the study will undergo a low-dose X-ray treatment to sterilise any female mosquitoes that are not eliminated through the sex-sorting process.
“This very low dosage of X-ray that we are using will sterilise the females but will not affect the virility of the males so that even if this very small number of females are released, they will not be able to produce any progeny,” Mr Ho said.
He added that the sex-sorting is done at the pupae stage, after which the x-ray treatment would be applied.
Emeritus Professor at Duke-NUS’ Emerging Infectious Diseases Programme, Duane Gubler said the method is tried and tested with positive results. Prof Gubler is also Chair of NEA’s Dengue Expert Advisory Panel.
“Irradiation has been used for years to successfully sterilise insects for use in sterile male release control trials,” he said.
“We anticipate that this method will increase the efficacy of the Wolbachia sterilised male approach to mosquito control.”
MORE MOSQUITOES RELEASED WITH GREATER FREQUENCY
NEA said it may release a larger population of infected mosquitoes compared to the first phase of study. This is due to a general increase in the mosquito population in 2016.
The potential increase will also address the movement of both infected and urban mosquitoes in and out of the field sites.
As an example, about one to six male mosquitoes per person could be released each week, as opposed to one in three mosquitoes per person in the first phase of study.
NEA found that the Wolbachia-infected mosquitoes also survived an average of four days in the field. Therefore, to sustain their population, it said it would need to release the required number more frequently.
The agency said it will now release the mosquitoes twice a week compared to once weekly during the first phase. It would also release mosquitoes at an earlier stage of their life cycle to give them more time to survive at the study sites.
Results from phase two of the study would “strengthen” NEA’s planning for a larger suppression trial, which was delayed due to the challenges that surfaced in the initial phase.
NEA said researchers will set up mosquito traps and pupal containers at various locations within the study sites, and urged members of the public not to remove or tamper with these traps.
It said it would provide residents with more information to residents at the study sites before April.