SIAU MING EN Today Online 26 Feb 18;
SINGAPORE — A fresh round of field studies taking male Wolbachia-carrying Aedes aegypti mosquitoes to greater heights – literally – will kick off in April.
This, after the previous study found that insufficient numbers of them reached the higher floors of housing blocks, hampering efforts to control the population.
In Phase Two of the field study, the authorities will release the male mosquitoes – both adults and pupae – from high floors in addition to the ground floor, the National Environment Agency (NEA) said on Monday (Feb 26).
The NEA will also use X-ray or other technologies to render infertile the 0.3 per cent of small female pupae that get mistaken for male pupae when they are sorted by size. Female pupae are generally bigger and 99.7 per cent of them are successfully sifted out by a device.
Only male Wolbachia-carrying mosquitoes are selected for release at study sites because when they mate with females, the eggs do not hatch. This leads to a smaller population of Aedes aegypti mosquitoes over time. Male mosquitoes also do not bite.
If female Wolbachia-carrying mosquitoes are inadvertently released and mate with the males, the eggs hatch, which may hamper population suppression efforts.
The Phase Two field study will be conducted over nine months, said the NEA.
Wolbachia is a naturally occurring bacterium found in more than 60 per cent of insect species, but not the Aedes aegypti mosquito, which spreads dreaded diseases like dengue and chikungunya. In addition to longstanding measures such as the destruction of breeding sites, it is a potential way to curb the mosquito population such that dengue transmission cannot be sustained.
The earlier field study, conducted between October 2016 and last December, took place at three sites: Braddell Heights, Tampines West and Nee Soon East.
This time round, the mosquitoes will be let loose in the latter two sites, but with redrawn boundaries. A total of 76 blocks with about 7,000 households will be covered – 40 more blocks than before.
Both the adult mosquitoes and and pupae will be released twice a week at the test sites, more frequently than the earlier study. More male Wolbachia mosquitoes will be released this time – one to six mosquitoes per person, instead of the one to three mosquitoes per human in the earlier study.
This is to counter the general increase in the Aedes aegypti population and compensate for any possible reduced virility of the male Wolbachia-Aedes from the X-ray treatment, said the NEA.
The mosquitoes are given low doses of X-ray and research shows they pose no harm to humans or the environment. Animals that eat or come into contact with these mosquitoes will not be affected.
They are not radioactive as they do not come into physical contact with a radioactive source, said the NEA.
X-ray technology has been used elsewhere for years without adverse effects, said chairman of the Dengue Expert Advisory Panel, Professor Duane Gubler of Duke-NUS Medical School.
For example, it is used to control or eradicate agricultural pests such as the melon fly in parts of Japan, said the NEA.
Professor Gubler does not think the Wolbachia study has been taking too long. “It is best to get this right the first time rather than rush ahead without adequate data,” he said.
A larger suppression trial was supposed to have started last year, but no date has now been set.
The new field study was called after Phase One threw up unexpected challenges. The impact of release of male Wolbachia-carrying mosquitoes was found to be limited by Aedes aegypti mosquitoes from surrounding areas moving into the release sites.
Half of the Aedes mosquito eggs collected from the release sites did not hatch, but a larger reduction of hatched eggs and the adult population is needed.
And to better distribute the Wolbachia-carrying mosquitoes, they will need to be released from higher floors of apartment blocks.
The NEA appealed for support from residents and other stakeholders at the study sites and said it will provide more information to them. It urged residents to continue mosquito-control measures.
Phase 2 of dengue control study kicks off in April
More Wolbachia-carrying male mosquitoes to be released into Tampines West, Nee Soon East
Samantha Boh Straits Times 27 Feb 18;
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.
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.
