Mozzies pose unexpected challenges to Singapore’s Wolbachia project

SIAU MING EN Today Online 9 Dec 17;

SINGAPORE – A larger trial of Wolbachia-carrying Aedes aegypti mosquitoes will be delayed, after a recent field study at three sites threw up unexpected challenges in the form of mosquito movements.

Aedes aegypti mosquitoes from surrounding areas moved easily into the three sites – Braddell Heights, Tampines West and Nee Soon East – where their laboratory-modified male counterparts had been released. This hampered the latter’s ability to suppress the Aedes aegypti population at the release sites, said the National Environment Agency (NEA) on Friday (Dec 8).

A second challenge was the insufficient numbers of male Wolbachia-infected mosquitoes that reached higher floors of some housing blocks. This hampered suppression at high-rise blocks that had more Aedes aegypti mosquitoes at the higher floors.

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.

When male Aedes aegypti mosquitoes that have been infected with Wolbachia are released and mate with females, the eggs do not hatch.

In addition to longstanding measures such as the destruction of breeding sites, this is a potential way to keep the mosquito population at a level where dengue transmission cannot be sustained.

The field study involving Braddell Heights, Tampines West and Nee Soon East began in October and November last year and lasted for about six months.

A larger suppression trial was to have started this year, but no date has now been set, after an expert panel advised the NEA to conduct further field studies.

A second field study will start in the second quarter of next year, with details on its scope and locations to be announced later, said the NEA.

Chairman of the Dengue Expert Advisory Panel, Professor Duane Gubler of Duke-NUS Medical School, said NEA would need to understand if there are potential barriers – such as roads, expressways and parks – that can stop the Aedes mosquitoes from entering the test sites.

“NEA’s Phase One field study has garnered valuable data, but it is important that further field studies be conducted to address the unique challenges that were surfaced during the study so that future application of this exciting technology can proceed more effectively,” he said.

The field study found that half the Aedes mosquito eggs collected from the release sites did not hatch. This means the Wolbachia-carrying male mosquitoes successfully mated with some Aedes aegypti females. But a larger reduction of hatched eggs and the adult population will be needed to suppress the Aedes population, said NEA.

The authorities also found that small releases of female Wolbachia-Aedes – which inadvertently slipped through during the sorting process - could see them taking over and becoming the dominant mosquito strain here. This would hamper the effort to use male Wolbachia-Aedes to suppress the population.

Improvements to existing sorting methods need to be explored, said NEA.

Careful and thorough studies have to be conducted over “several years” to ensure the technology is applied in the most effective way in Singapore’s unique urban landscape, NEA added.


Wolbachia-carrying mosquito study reports 50% suppression rate, phase 2 to start next year: NEA
Channel NewsAsia 8 Dec 17;

SINGAPORE: The first phase of a study on mosquitoes carrying the Wolbachia bacteria has been completed, with the successful suppression of 50 per cent of the targeted Aedes aegypti population, the National Environment Agency (NEA) said on Friday (Dec 8).

Since October 2016, male Wolbachia-carrying Aedes aegypti mosquitoes have been released on a regular basis at three selected sites at Braddell Heights, Nee Soon East and Tampines West. This is to understand their behaviour and ecology, and see if they can suppress the population of urban Aedes aegypti mosquitoes.

Since only female Aedes mosquitoes spread dengue by biting humans, if a male carrier of the Wolbachia bacterium mates with an uninfected female mosquito, the resulting eggs will not hatch.

NEA hopes that by releasing sufficient numbers of Wolbachia-carrying Aedes aegypti males, they can compete successfully against wild males and eventually drive down mosquito numbers as the population fails to reproduce.

Over time, this could also reduce the potential spread of dengue. NEA expects that the method could also help prevent the transmission of other mosquito-transmitted diseases such as Chikungunya and Zika.

Phase 1 of the study has met its objectives and a second recommended phase will commence in the second quarter of next year, NEA said.

The study reported that much fewer Aedes aegypti adult mosquitoes were found at sites where the male Wolbachia-Aedes mosquitoes were released, implying that the method is effective.

At the same time, half of the collected Aedes mosquito eggs did not hatch at the released sites, which provided strong indication that the released Wolbachia-Aedes males had successfully competed with the urban Aedes males and mated with some of the urban Aedes aegypti females.

HIGH-RISE URBAN LANDSCAPE POSES CHALLENGES

The field study also revealed two ecological challenges that are unique to Singapore’s high-density and high-rise urban landscape, and should be addressed to increase the impact of the suppression, the NEA said.

Firstly, the report noted how Aedes aegypti mosquitoes were able to move easily from surrounding areas into the release sites, thus reducing the suppression effect of Wolbachia-Aedes.

Secondly, it was noted that there is a high density of Aedes aegypti mosquitoes on higher floors, where not enough Wolbachia-Aedes mosquitoes can reach.

Data collected on how high and far the male Wolbachia-carrying Aedes aegypti (Wolbachia-Aedes) mosquitoes can fly, how long they live and their mating competitiveness in actual field conditions, will contribute to future field studies.

The Wolbachia technology, if proven effective, will further strengthen our capabilities to tackle dengue and other mosquito-borne diseases, the agency added.

"This is especially crucial as higher global temperatures resulting from climate change can have an impact on the spread of mosquito-borne diseases and public health," NEA said.
Source: CNA/kc