SINGAPORE: Project Wolbachia is still in the research study phase and is not ready to be deployed in dengue clusters, said the National Environment Agency (NEA) on Wednesday (Jul 24).
In a joint statement with the Ministry of Health (MOH), NEA said Project Wolbachia, which took home a distinguished award at the Minister for National Development’s R&D Awards on Tuesday (Jul 23), has shown great promise.
AdvertisementAdvertisementThe project involves infecting male Aedes mosquitoes with Wolbachia bacteria, so that when they mate with females, the latter’s eggs do not hatch. Wolbachia-infected males also do not bite.
The NEA first began small-scale field studies in Braddell Heights, Nee Soon East and Tampines West in 2016, which led to a 50 per cent suppression of the Aedes mosquito population.
Since then, the study areas forhas expanded steadily by about 3.7 times since the study begun in 2016.
NEA confirmed that Project Wolbachia has shown up to 90 per cent suppression of the Aedes mosquito population at the Yishun and Tampines study sites.
AdvertisementAdvertisement“Such a focused approach is needed to derive good comparative results, and a consistent and comprehensive data set over time, to ensure the robustness of the study before scaling-up to more areas beyond Yishun and Tampines,” said NEA.
[h=3]READ: Wolbachia mosquitoes to be released at expanded Nee Soon, Tampines sites in next phase of study[/h]Project Wolbachia involves the use of male Wolbachia-Aedes mosquitoes to further suppress the Aedes aegypti mosquitoes in the community, in turn lowering the transmission of diseases like dengue and Zika.
According to experts, Project Wolbachia “shows great promise” in its pilot studies in Singapore and other countries.
“It is a possible solution together with existing control measures and continued public education and awareness rising,” added Professor Tikki Pang, advisor to the Asia Dengue Voice & Action Group and a visiting professor at the Lee Kuan Yew School of Public Policy.
Assistant Professor Vincent Pang, Director of the Centre for Infectious Disease Epidemiology & Research at the NUS Saw Swee Hock School of Public Health, said: “However, different underlying population structure and environments may have different impacts on the intervention, and findings from other countries cannot be fully generalisable to Singapore's context.”
NEA said comprehensive mosquito surveillance, source eradication of breeding habitats and spraying of insecticide where necessary continue to be Singapore’s key strategies for dengue prevention and control.
As of Thursday (Jul 18), there were 188 active dengue clusters in Singapore, of which more than 45 are listed as high-risk. There have also been 7,808 dengue cases so far this year, about five times more than the same period last year.
The largest dengue cluster in Woodlands, where there were 216 reported cases, has closed and is now under surveillance, NEA said in an advisory on Friday (Jul 19).
Five people have died from dengue this year.
NEA explained that the hot weather and low herd immunity are the main reasons for the spike in cases this year, with almost triple the Aedes aegypti mosquito population since the last major dengue outbreak in 2013.
WARMER TEMPERATURES, MORE MOSQUITOES
According to Prof Vincent Pang, warmer temperatures can enhance the growth of mosquito larvae and cause them to grow into adult mosquitoes faster.
“Some studies showed that warmer temperatures may also increase the number of eggs laid per mosquito,” he said.
Prof Tikki Pang noted that warmer temperatures also cause the virus to multiply more rapidly in infected mosquitoes.
Experts confirmed that the population of the Aedes mosquito is likely to increase further in the coming months. Based on Singapore’s historical climate, the estimated mean temperature range between Aug and Nov is 27 to 28 deg C, which is also the most optimal temperature for Aedes mosquito growth, said Prof Vincent Pang.
However, dengue transmission may decrease if the temperature goes beyond 32 deg C, not uncommon in Singapore. He added that some studies show this may reduce the mosquitoes’ lifespan, blood-feeding behaviour and flight competence.
[h=3]READ: More mosquitoes or mutating virus? Experts have different views on dengue spike[/h]The warmer months may not be solely to blame. Prof Vincent Pang noted that Singapore has warmed since the mid-1970s due to rapid urbanisation.
Total rainfall in Singapore has also increased since 1980, and there have been “upward trends” in average annual rainfall and the frequency of heavy rainfall over the past few decades.
“It is likely that with the increase in rainfall days, there is a higher chance of new mosquito breeding sites. However, more research is still required.”
LOW HERD IMMUNITY ALSO TO BLAME
Experts said with low herd immunity, a dengue-infected mosquito is more likely to bite a susceptible individual, increasing the chances of dengue transmission.
Prof Paul Tambyah, infectious diseases expert and president of the Asia Pacific Society of clinical microbiology and infection said Singapore has historically had a low herd immunity to dengue due to highly effective vector control programs that have made dengue “very rare” in children.
Prof Tambyah added: “In contrast, in rural parts of Southeast Asia, most children would have been infected with all four strains of dengue by the time they are teenagers, so they are immune to all four strains. In these areas, dengue is a disease of children.
“That is not the case in Singapore where young adults are generally not immune to dengue and they are thus at risk for symptomatic dengue fever.”
Prof Leo Yee Sin, Executive Director of the National Centre for Infectious Diseases said the low herd immunity can be partly attributed to the successful implementation of the Aedes control programme in the 1970s and 1980s.
Prof Leo added that most of the current dengue patients are having dengue for the first time.
According to NEA, the proportion of adults who have had dengue before has reduced from 59 per cent in 2004 to 41 per cent in 2017.
A cleaner environment compared to countries like Indonesia and Malaysia could also contribute to a low herd immunity could also contribute to a lower herd immunity, said Prof Tikki Pang.
[h=3]READ: Bigger Aedes mosquito population identified as key reason for surge in dengue cases[/h]In the joint statement, NEA and MOH confirmed that all four dengue virus serotypes are circulating in Singapore, and Dengue-2 has been the predominant serotype since 2016. Dengue-1 was the predominant serotype in the 2004-5 and 2013-14 outbreaks.
All four dengue virus serotypes are circulating in Singapore, and DEN-2 has been the predominant serotype since 2016. DEN-1 was the predominant serotype in the 2004-5 and 2013-14 outbreaks.
“The progressive build up of new residents over the years, with limited or no prior exposure with our predominant serotypes Dengue-1 and Dengue-2 would be susceptible to dengue infection,” said Prof Vincent Pang.
Almost all the experts CNA spoke to believe the number of dengue cases is set to continue increasing.
Infectious diseases specialist Dr Leong Hoe Nam believes the number could exceed 10,000 cases.
Prof Tambyah said: “However, given that the dominant strain this year is still apparently Dengue-2, it is likely that after a period of time, the level of herd immunity will build up in the local population to prevent sustained transmission of the disease.
“Of course, if the serotype switches to Dengue-3 or Dengue-1, then the high numbers may unfortunately continue.”
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