Project Wolbachia, Singapore's stealth weapon against dengue fever, will be expanded to Choa Chu Kang and Bukit Batok this month to suppress the Aedes aegypti mosquito population as the country battles a surge in dengue cases as well as the Covid-19 outbreak.
As part of the project, male mosquitoes carrying the Wolbachia bacterium are released to mate with female mosquitoes, causing them to lay eggs that do not hatch. The National Environment Agency (NEA) has been studying the programme since 2012.
Male Wolbachia-Aedes mosquitoes will be released at more than 200 housing blocks in Choa Chu Kang, Keat Hong and Hong Kah North this month, the NEA announced yesterday.
These areas were chosen because of their consistently high populations of the Aedes aegypti mosquito, which transmits dengue.
This comes after successful field studies in Yishun and Tampines, which achieved more than 90 per cent suppression of the urban Aedes aegypti mosquito population at test sites last year. The studies in Tampines and Yishun are now in their fourth phase. A preliminary analysis of last year's data by the NEA showed a 65 per cent to 80 per cent drop in dengue cases in sites where the male Wolbachia-Aedes mosquitoes were released.
Singapore has seen a rise in dengue infections this year, with about 300 to 400 new cases reported each week. There have been more than 6,500 cases of dengue fever since January, more than double the number of cases for the same period last year. The number of cases this year is projected to exceed last year's figure of 16,000.
The surge in cases, ahead of the traditional peak dengue season between this month and September, has been driven by factors such as the rise of a less common dengue virus serotype, as well as warmer temperatures and more rain, which have caused the population of the Aedes aegypti mosquito to multiply.
Experts warn the higher number of dengue cases at a time when the country is also facing a coronavirus pandemic could lead to issues such as competition for healthcare resources, and a risk of misdiagnosis, since symptoms of Covid-19 and dengue fever can be similar.
But on the bright side, most of Singapore's public hospitals have good outpatient dengue management clinics, which help reduce the burden on inpatient services, said Dr Paul Tambyah, president of the Asia Pacific Society of Clinical Microbiology and Infection.
Dengue fever, which can cause a very high fever, severe headache and joint and muscle pain, killed 20 people last year. The disease claimed seven lives between January and March this year.
The disease is caused by four different virus serotypes, or strains. Outbreaks in Singapore tend to be caused by DenV-1 and DenV-2. However, DenV-3 infections have been higher this year. In March, they made up 43.1 per cent of cases, compared with 31.5 per cent for DenV-2. The country has not had a dengue outbreak driven by DenV-3 in almost three decades, which means the population's immunity to this serotype is lower.
Number of dengue cases in Singapore since January - more than double the number of cases reported for the same period last year. The number of cases this year is projected to exceed last year's figure of 16,000.
Professor Duane Gubler, the chairman of Singapore's Dengue Expert Advisory Panel (Deap), said it is possible DenV-3 could become dominant here. "Changes in dominant virus serotype are common in dengue endemic areas and that is one of the causes of epidemic transmission."
Others said it is hard to tell if this will come to pass. DenV-3 has not been the dominant strain here for a very long time for reasons "not entirely clear", Dr Tambyah said. He added that there are "many confounding factors" this year, such as having no travellers, who were previously believed to introduce new strains, and a halt to construction, which poses challenges to vector control in building sites.
Associate Professor Vernon Lee, also a Deap member, said Project Wolbachia is just one part of the country's multi-pronged approach to fight dengue. Other strategies range from a dengue vaccine to the National Dengue Prevention Campaign.
"While we are battling Covid-19, we also need to remain focused on other diseases, such as dengue... so they do not surge because we are giving them less attention."