SINGAPORE: A confluence of warm and rainy weather, a shift in the dominant dengue strain and the "circuit breaker" could have resulted in a surge in dengue cases in recent months, an expert has said.
As of 3pm on Tuesday (Jun 9), the number of dengue cases in Singapore has reached 10,234, with 870 cases in the first week of June.
The increase since the start of May has been particularly steep. While the year started off with about 300 to 400 cases per week from January to April - already a higher number than usual; in May, that number ranged between 500 and 800.
The National Environment Agency (NEA) said on Jun 3 that the number of dengue infections in the first five months of 2020 was the highest since 2013, when Singapore saw its largest outbreak in recent history.
The number of dengue cases this year is expected to exceed the 15,998 cases reported in 2019, and may even surpass the 22,170 cases reported in 2013. There have also been 12 deaths from the disease this year, with victims aged between 56 and 80 years old.
There are several reasons for this year's increase in cases, said Associate Professor of Infection and Immunity Luo Dahai from Nanyang Technological University’s Lee Kong Chian School of Medicine.
Recent weather conditions have encouraged mosquitoes breeding, while more people stayed at home during the circuit breaker period, providing fodder for the mosquitoes.
The Aedes aegypti mosquito, which spreads dengue fever, lives mainly indoors and generally bites during the day, according to NEA.
"Warmer weather and rainy days since the beginning of this year favour mosquito breeding across the country," Assoc Prof Luo said.
"Circuit breaker due to COVID-19 is another likely factor, making this year’s situation worse. When more people stay at home all day, there could be more residential mosquito breeding and more opportunities for 'blood meals'."
The mean temperature in the last three months is about 1 degree Celsius higher than the preceding months, said NEA.
The agency added that there has been a five-fold increase in the incidence of Aedes mosquito larvae detected in homes and common corridors in residential areas during the two-month circuit breaker, compared to the two months before. The incidences of larvae being observed at construction sites has also doubled.
As of Tuesday, 192 active dengue clusters were identified, including large clusters reported in Woodleigh Close, Tampines Avenue 7, Potong Pasir Avenue 1, Aljunied Road and Elizabeth Drive, among others.
READ: Construction firms rely on on-site workers, pest control companies to prevent mosquito breeding during circuit breaker
The increase in the number of cases this year is compounded by the fact that a less common dengue virus serotype 3 (DENV-3) has been making the rounds.
On why a lesser known serotype may lead to more infections, Assoc Prof Luo explained that there are four dengue serotypes, and the body’s immune system develops customised protective responses to each one.
"Protective immune response against one dengue type may not be effective in protecting against the challenge from a second dengue serotype," he said. "More concerning is how secondary dengue infection may lead to more severe dengue disease."
NEA said that DENV-3 was last dominant in Singapore about three decades ago, which means there is now low immunity in the population.
"This likely explains the unusually high number of weekly cases, hovering between 300 (and) 400, since the start of the year," the agency said.
Cases are also peaking as Singapore enters the traditional dengue peak season, which could last until October, said NEA.
"Weekly dengue case numbers are likely to continue to rise, and more dengue clusters will form in the coming weeks to months," said Mr Chew Ming Fai, director-general of NEA’s Environmental Public Health Division.
Urging the public to take preventive measures, NEA said that it has continued with dengue inspections despite circuit breaker measures.
Assoc Prof Luo said: "Based on past experience, increase in mosquito population is likely the major factor. So the advice is to eliminate mosquito breeding and avoid being bitten by the mosquito."