NEA to release Wolbachia-carrying males in mosquito control trial

SINGAPORE – From October, male Aedes aegypti mosquitoes carrying the naturally-occurring Wolbachia bacteria will be released in three housing estates over the next six months, as part of a field study to determine their behaviour, with the objective of controlling the mosquito population.

One to three mosquitoes per person will be released regularly at public spaces – such as stairwells and void decks – around the blocks and houses in Nee Soon East, Tampines West and Braddell Heights.

This works out to about 1,500 male mosquitoes for 500 people living in 1 block of flats.

As male mosquitoes do not bite, they will not transmit any diseases. The National Environment Agency (NEA) has assured that these Wolbachia-carrying mosquitoes are safe and pose an insignificant risk to the ecology and no risk to human health.

The housing blocks in Yishun Street 21(Nee Soon East), Tampines Avenue 4, as well as landed homes in Jalan Riang and Jalan Sukachita (Bradell Heights), have been chosen because they are representative of the typical housing mix in Singapore.

The three areas also have had previous dengue outbreaks, and the NEA has been monitoring the mosquito population in these locations for up to three years to provide a baseline for comparative studies.

The Yishun Street 21 area will be used to study the mosquitoes’ lifespan, and how far and how high they can travel. The other two areas will be to determine how the male Wolbachia-carrying mosquitoes mate with the wild female mosquitoes, and how competitive they are against the wild male mosquitoes.

Gravitraps and ovitraps, which trap female Aedes mosquitoes and their eggs respectively, will be set up at public spaces in the selected release sites and their surrounding areas to determine the female Aedes population and the eggs’ hatch rates. Meanwhile fan-based mosquito traps will be placed in the homes of residents who volunteer to trap the male Wolbachia-carrying mosquitoes and monitor their flight range.

During a media briefing on Thursday (Aug 25), the NEA was asked why the study will be conducted after the traditional dengue peak season of June to October.

An NEA spokesperson said the timing would not affect the results, as they are not looking at how this particular field study would control dengue or suppress the mosquito population. A subsequent suppression trial could be conducted sometime next year.

The small-scale field study comes in the wake of a warning issued by the authorities in February that Singapore could face a historic high of 30,000 dengue cases this year, due to the El Nino phenomenon and the change in the type of dengue virus circulating among the population.

The previous record was 22,170 dengue cases in 2013.

Speaking to the media after the briefing, NEA director-general for Environmental Public Health Derek Ho said: “(Male mosquitoes) do not pose any additional pressure in the public sphere. People should not be experiencing more mosquito bites as a result of this study.”

There are two types of Aedes mosquitoes in Singapore: Aedes aegypti and Aedes albopictus. The former species thrives in the urban environment and is a more efficient vector for diseases such as dengue, chikungunya and the Zika virus. Native to Singapore, the latter prefers the greenery but are not as efficient as a vector for dengue.

Wolbachia is a naturally-occurring bacteria found in over 60 per cent of insect species, but not in the dengue-carrying Aedes aegypti mosquito.

The NEA’s Environmental Health Institute managed to rear local Aedes aegypti mosquitoes carrying the Wolbachia in their laboratories, and will be releasing only the male Wolbachia-carrying mosquitoes to mate with female Aedes mosquitoes. Eggs produced from their mating will not hatch because they are biologically incompatible.

Over time, this will reduce the Aedes aegypti mosquito population and the potential spread of diseases such as dengue.

While Singapore is looking at using the Wolbachia technology to suppress the Aedes mosquito population, there are also other countries using the same technology but to replace the Aedes population to block the transmission of diseases.

The latter approach involves releasing both male and female Wolbachia-carrying Aedes aegypti mosquitoes. As Wolbachia is passed on from the female Wolbachia-carrying Aedes aegypti mosquitpes to their offspring, this method is used to rear subsequent generations of Wolbachia-carrying Aedes aegypti.

The Wolbachia bacteria would protect the mosquitoes from viral infection and reduce the risk of dengue transmission. This is being tested in countries such as Australia, Vietnam and Indonesia even though its impact on dengue has not been proven yet.

An NEA spokesperson said Singapore went with the former approach as it was consistent with what the agency has been doing – to promote source reduction of mosquito population by removing breeding sites for instance.

Releasing both male and female Wolbachia-carrying Aedes aegypti mosquitoes might not be an acceptable approach to the public here, since more people may may get bitten by the female mosquitoes, the spokesperson added.

Other countries adopting this approach to control the mosquito population includes the United States, Thailand and Guangzhou, China. In the latter’s field study, it managed to suppress more than 90 per cent of its Aedes albopictus population.

The field study comes after the authorities spent the last four years carrying out various risk assessment and laboratory research studies to ensure that the Wolbachia technology is safe. During this time, they also consulted stakeholders such as academic experts, medical and healthcare professions and non-governmental organisations.

A Dengue Expert Advisory Panel was also appointed by NEA in June 2014 to continue to review study results.

Speaking to reporters on Saturday (Aug 27) after announcing the small-scale field study sites to Tampines residents, Minister for the Environment and Water Resources Masagos Zulkifli said it was important to engage the public for the study.

The Government could have "just released it (the Wolbachia-carrying Aedes aegypti mosquitoes) quietly" and shared the results thereafter, he said. But it is important for the public to know what the small-scale field study is about such as the methods behind it.

"For this stage, we are calibrating, we are not actually releasing (them) in millions or billions," said Mr Masagos.

While the NEA and public having been working very hard over the years to bring down and suppress the mosquito population and spread of dengue, Mr Masagos noted that Singapore is situated in a region which is endemic with dengue.

He added that as climate change will also affect Singapore's weather pattern,there is a need to have a "new line of defence" to deal with the situation. In this case, it is the release of the Wolbachia-carrying Aedes aegypti mosquitoes, said Mr Masagos.

Mdm Winnie Lim who has been living in Tampines Ave 4 for the past 15 years said the method used is good in the long-term. The 49-year-old housewife noted that fumigation may not completely eradicate the mosquitoes, but with this method, it would prevent the eggs from hatching. Additional reporting by Amanda Lee