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Wolbachia-carrying male mozzies to get a leg-up in second field study

SINGAPORE — A fresh round of field studies taking male Wolbachia-carrying Aedes aegypti mosquitoes to greater heights – literally – will kick off in April.

File photo of Wolbachia mosquitoes. Photo: NEA

File photo of Wolbachia mosquitoes. Photo: NEA


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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.

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