Project Wolbachia still in study phase, will not be deployed in dengue clusters: NEA

Ang Hwee Min Channel NewsAsia 24 Jul 19;

SINGAPORE: Project Wolbachia is still in the research study phase and therefore has not been 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, has shown great results.

The 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.

Now in its third phase, the study areas for the project has expanded by about 3.7 times since it first began in 2016, with a 90 per cent suppression of the Aedes mosquito population in the Nee Soon and Tampines study sites.

“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.

Industry experts note that 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,” said 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, 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.

Five people have died from dengue this year.

WARMER TEMPERATURES, MORE MOSQUITOES

A threefold increase in the Aedes aegypti mosquito population since 2013, hot weather and low herd immunity are the main reasons for the spike in cases this year, experts said.

Warmer temperatures enhance the growth of mosquito larvae and cause them to grow into adult mosquitoes faster, said Asst Prof Vincent Pang.

“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 have forecast 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 August and November is 27 to 28 degress Celsius, which is also the most optimal temperature for Aedes mosquito growth, said Asst Prof Vincent Pang.

However, dengue transmission may decrease if the temperature goes beyond 32 degrees Celsius, not uncommon in Singapore. He added that some studies show this may reduce the mosquitoes’ lifespan, blood-feeding behaviour and flight competence.

Rainfall could also contribute to the problem.

“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,” Asst Prof Vincent Pang said.

LOW HERD IMMUNITY ALSO TO BLAME

Additionally, Singapore's low herd immunity also increases the risk of transmission.

Singapore has historically had a low herd immunity to dengue due to highly effective vector control programmes that have made dengue “very rare” in children, noted Prof Paul Tambyah, infectious diseases expert and president of the Asia Pacific Society of Clinical Microbiology and Infection.

“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," said Prof Tambyah.

“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,” he added.

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 currently, most dengue patients are first time sufferers.

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.

There are four known strains of dengue in Singapore: Dengue-2 has been the predominant serotype since 2016. Dengue-1 was the predominant serotype in the 2004 to 2005 and 2013 to 2014 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 Asst Prof Vincent Pang.

Infectious diseases specialist Dr Leong Hoe Nam believes the number of dengue cases this year could exceed 10,000 cases.

“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," added Prof Tambyah.

“Of course, if the serotype switches to Dengue-3 or Dengue-1, then the high numbers may unfortunately continue,” he said.

Source: CNA/hw(aj)