SINGAPORE: Thousands of male Aedes aegypti mosquitoes carrying the Wolbachia bacterium will be released regularly into three former dengue clusters at Tampines Avenue 4, Yishun Street 21, as well as Jalan Riang and Jalan Sukachita in Serangoon over six months from October 2016, the National Environment Agency (NEA) announced on Saturday (Aug 27). It is part of a "small-scale" field study and comes after a comprehensive risk assessment found it would be safe to release such mosquitoes, with no risk to human health and insignificant impact on ecology, NEA said.
STOPPING THE CYCLE
Only female Aedes mosquitoes spread dengue by biting humans. Should a male carrier of the Wolbachia bacterium mate 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.
NEA said it carried out a four-year evaluation of the process, involving critical reviews of existing research, consultations with various stakeholders such as academic experts, medical and healthcare professionals, and non-governmental organisations such as nature groups. It found the bacterium – which is naturally found in insects in the wild – to offer suitable biological properties
The field study will observe how far the mosquitoes are able to disperse outside the lab, as well as how high they can fly. It will also gauge their lifespans in the wild, and how well they can compete for mates. An average of one to three mosquitoes per resident will be released regularly in areas such as stairwells, void decks, open spaces between blocks of high-rise homes, and outside landed homes of the three estates. NEA stated that mosquitoes will not be released directly into homes.
The three estates represent a cross-section of typical housing estates in Singapore, and provide a good baseline from which to make comparative studies.
The findings will support the design of another field suppression trial to be held over one to two years in 2017, to test if the technology is effective in bringing down mosquito populations – and by extension, possibly impacting the spread of dengue. If the tests are successful, NEA could roll-out this method to fight dengue in high-risk areas from 2019.
WOLBACHIA STUDIES ELSEWHERE
Similar small-scale releases of Wolbachia-carrying mosquitoes were carried out in other jurisdictions such as the Unites States and Thailand, with tests in China and French Polynesia having met with some success. A 2015 study in Guangzhou reported an over 90 per cent reduction in the Aedes mosquito population.
The trial was previously announced by Environment and Water Resources Minister Masagos Zulkifli in April 2016, during the Committee of Supply debates. It also comes after the NEA previously warned that Singapore could see a record 30,000 dengue cases in 2016. Despite efforts to eradicate mosquito breeding habitats, the NEA said Singapore remains vulnerable due to its location in a dengue endemic region, and low herd immunity in the community.
“A NEW LINE OF DEFENCE”
Speaking at a community event on Saturday to raise awareness of the study, Mr Masagos said it was important to engage the public on the move, adding that “a new line of defence” was needed in the fight against dengue. But he also hopes the community can play its part. For instance, residents can volunteer to use fan traps from NEA to capture mosquitoes in their homes, and all should keep up efforts to remove possible mosquito breeding habitats. Should mosquitoes continue to breed, Mr Masagos said, it would negate the effect of introducing the Wolbachia-carrying male mosquitoes.
“(The technology) does not replace the source eradication system. Therefore, whatever we’ve been doing all these years successfully – bring down the mosquito breeding places in our homes, in our construction (sites), in our public places, must continue,” said Mr Masagos.
EFFECTIVENESS COULD VARY:EXPERT
Despite the international success of the use of Wolbachia in countries like China and Myanmar, one expert said the technology remains in its infancy. Infectious diseases expert at Mount Elizabeth Hospital, Dr Leong Hoe Nam said while he sees no major ill effects on human health, Wolbachia’s effectiveness could vary from environment to environment.
He said: “The biggest drawback is we don't really know what's going to happen. Many experiments have been done in the lab, trying to look at the different forms or possibilities - even looking into the different strains of Wolbachia, and how they will affect (mosquitoes). Going into a small trial in selected areas is the way forward."
Dr Leong said the lack of certain strains of dengue in Singapore could indicate that the mosquitoes spreading the disease could differ slightly from that of the region.
"Singapore has a very unique property in that only Dengue Strains 1 and 2 exist in Singapore - 3 and 4, we hardly see them. Now if you cross the Causeway, we'll see 3 and 4 in Malaysia. And similarly in the Philippines, Indonesia and Thailand. This tells you that there is something unusual about Singapore, where we're extremely effective in getting rid of 3 and 4," he said.
"If the mosquito is slightly different, then would the use of Wolbachia make a change? Would we have the same result? We don't know. The only way to take it forward is with field testing, field experiments and gathering data. This data will not only help Singapore, but help the region and the whole world”.