88% fewer dengue cases in selected study sites under Project Wolbachia: NEA

From the fourth quarter of this year, more of the male mosquitoes carrying the Wolbachia bacterium will be released in selected residential estates. ST PHOTO: LIM YAOHUI

SINGAPORE - Project Wolbachia - Singapore's weapon against dengue fever - has managed to suppress the Aedes aegypti mosquito population by up to 98 per cent, with 88 per cent fewer dengue cases, at selected study sites.

The study sites were in Yishun, Tampines, Choa Chu Kang and Bukit Batok towns, where male mosquitoes carrying the Wolbachia bacterium were released in selected areas so that when female Aedes aegypti mosquitoes mate with them, their eggs do not hatch.

This can help in suppressing the mosquito population and reduce the number of dengue cases.

The National Environment Agency (NEA) said in a statement on Monday (July 5) that from the fourth quarter of this year, more of these male mosquitoes will be released in selected residential estates, including a landed estate in Marine Parade, and construction sites within the study sites.

"The aim is to test different strategies to determine the most effective and impactful approaches for future wider-scale deployment of Project Wolbachia in Singapore," it said.

As at last month, the release of male Aedes aegypti mosquitoes has been expanded from 39 Housing Board blocks in Yishun and Tampines to 860 blocks across both towns.

The release of the mosquitoes is ongoing at high-risk dengue neighbourhoods in Choa Chu Kang and Bukit Batok towns, covering 207 HDB blocks where there have been consistently high Aedes aegypti mosquito populations.

NEA noted that the positive outcome of the field studies in Tampines and Yishun had shown a 98 per cent suppression of dengue vector populations at the core of the study sites.

"Positive spillover effect was also observed at non-release areas adjacent to the release sites," it added.

In most areas within the study sites, NEA also noted that one to nine mosquitoes were caught per 100 traps a week after a few months of releasing the male mosquitoes at the sites, significantly below the 50 or more mosquitoes caught prior to the releases.

In addition, the core areas within the study sites where mosquitoes had been released for at least a year had seen up to 88 per cent fewer dengue cases than in areas without any mosquito releases.

Recent data from Choa Chu Kang and Bukit Batok towns had shown a similar reduction in the Aedes aegypti mosquito populations, with most areas having fewer than 10 mosquitoes caught per 100 traps each week, NEA said.

However, while most areas in the study sites had shown good suppression of the Aedes aegypti mosquito population within three to four months, dengue transmission was still seen in areas where it took longer to reduce the mosquito population.

In parts of Tampines East and Changkat, the beginning of mosquito releases coincided with a general spike in the Aedes aegypti mosquito population in May last year, suggesting that environmental factors could have a part to play in delaying suppression.

NEA noted that some of these areas were adjacent to non-release areas with persistently high Aedes aegypti populations or existing large dengue clusters, which contributed to dengue transmissions within the study sites from September to December last year.

The Aedes aegypti mosquito populations there were eventually suppressed to lower levels by February.

The study also showed that in areas with low Aedes aegypti mosquito populations, the presence of female Wolbachia-carrying Aedes mosquitoes could result in such mosquitoes propagating in the field, increasing their population.

This was observed at a small section of the Tampines West study area, where the Aedes aegypti mosquito population had previously been reduced to very low levels by Project Wolbachia.

Female Wolbachia-Aedes mosquitoes in the community are less harmful than urban Aedes aegypti mosquitoes, because they are partially resistant to dengue infection and do not transmit dengue well.

The male Wolbachia-Aedes mosquitoes were sterilised using a technique known as X-ray irradiation, which successfully reduced the urban Wolbachia-Aedes mosquito population, said NEA.

Associate Professor Ng Lee Ching, group director of the Environmental Health Institute at NEA, said: "The overall positive impact of Project Wolbachia - Singapore is very encouraging. This is the first time that the technology has been shown to be effective in suppressing Aedes mosquitoes and reducing dengue in a challenging tropical, highly urbanised and high-rise environment."

Senior research officer Caleb Lee of NEA's Environmental Health Institute releasing male Wolbachia-Aedes mosquitoes in Hong Kah North on June 2, 2020. PHOTO: ST FILE

The mosquito releases will be expanded to cover 686 HDB blocks in Tampines and 769 blocks in Yishun by the first quarter of next year, said NEA.

Up to 451 blocks in high-risk areas within Choa Chu Kang and Bukit Batok will be covered by the end of the year.

At construction sites within the study areas, NEA is working with stakeholders, including construction companies, to guide them on the surveillance and release of male Wolbachia-Aedes mosquitoes on their premises.

At the Marine Parade landed estate, NEA has engaged residents, and recruited volunteers to keep different mosquito traps in their homes for surveillance.

From the fourth quarter of this year, vans will release the male Wolbachia-Aedes mosquitoes in the estate.

The data gathered will help to guide deployment for larger-scale mosquito releases in the area, NEA added.

Join ST's WhatsApp Channel and get the latest news and must-reads.