SINGAPORE - More than 4,000 people in Singapore have been infected with dengue fever this year - double the number of cases reported for the same period in 2019.

The surge in cases, ahead of the traditional peak dengue season between June and October, has been fuelled by several factors - 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.

"This unusually high number of cases outside the typical peak dengue period is a cause for concern for all of us. Entering the warmer months of the year, there could be more instances of transmission," said Environment and Water Resources Minister Masagos Zulkifli at the launch of the National Dengue Prevention Campaign in Ang Mo Kio on Sunday (March 22).

The National Environment Agency's (NEA) annual campaign typically begins in April or May, but was started earlier this year in view of the grimmer dengue outlook.

The island has seen 300 to 400 new cases a week since the start of the year. There are now about 100 active dengue clusters across the island, including 28 "red" clusters which have at least 10 cases of the mosquito-transmitted virus. Parts of Jurong West and Ang Mo Kio Avenue 10 are within these high-risk areas.

Dengue fever is caused by four different virus serotypes, or strains. Outbreaks here tend to be caused by the DENV-1 serotype as well as DENV-2, the main dengue virus serotype in the island since 2016.

But DENV-3 infections have been rising in the past four months. In February, they made up 48 per cent of cases, almost double the 26 per cent for DENV-2 cases.

Mr Masagos said that since Singapore has not had a dengue outbreak driven by DENV-3 in almost three decades, the population's immunity to this particular serotype is lower.

It is "still too early" to tell if there has been a switch of predominant dengue virus type, and the NEA is monitoring the situation closely, he added.

"We have a less common dengue virus serotype emerging and lower immunity against it, and second, we have more Aedes mosquitoes carrying this virus. This has resulted in a higher number of dengue cases, which in turn increases the chances of dengue transmission. So that is the third reason for the surge in dengue cases we are seeing..."

"If the three factors I mentioned persist, this number could rise beyond the historical weekly peak of 891 cases recorded in July 2014, during the peak season."

Dengue fever, which can cause a very high fever, severe headache and joint and muscle pains, claimed 20 lives last year.

The NEA has been taking steps to fight dengue, Mr Masagos said.

From January to February, it conducted 161,000 inspections in public areas and housing estates and discovered about 3,200 mosquito breeding habitats, most of which were in people's homes.

At the start of the year, the environment agency also laid 14,000 Gravitraps in newly completed Housing Board blocks and landed housing estates. More than 64,000 of these devices islandwide trap adult female Aedes mosquitoes that are looking for places to lay their eggs, and also stop young mosquitoes in these traps from emerging.

"With better surveillance and analytics capabilities," Mr Masagos added, "we have observed a 90 per cent increase in the detection rate of Aedes larval habitats found in homes for the past three years."

But everyone must continue to do their part to keep the community safe, he said.

"In some of the large dengue clusters, NEA had detected repeated mosquito breeding in the same homes despite giving multiple alerts to residents. I therefore appeal to residents to do your part to remove potential breeding habitats in your homes, to keep you and our communities safe."

People are encouraged to reduce the spread of dengue - for example by ensuring that stagnant water does not gather in potential mosquito breeding areas such as flower-pot plates, pails and litter such as discarded drink cans. Aedes mosquitoes can breed in bodies of stagnant water as small as a 20-cent coin.

Over the past three years, more than 3,000 mosquito breeding habitats have been linked to containers and receptacles that were mostly discarded as litter in public areas.

Mr Masagos added that the SG Clean campaign launched last month "addresses the immediate risks from Covid-19, (but) also sets out to improve public hygiene and cleanliness for the long term".

The NEA is also putting up posters in HDB lift lobbies to inform residents of upcoming inspections - as well as insecticide spraying - for all homes in dengue clusters.

"I ask for your cooperation with NEA officers. Let them carry out the checks and spraying in your homes. We should also inform our family, neighbours and friends about these efforts so that they are aware and lend support," the minister added.

Grassroots leader Eric Chua, 44, chairman of Yio Chu Kang Zone 3 RC, leads a team of volunteers who go door-to-door to raise awareness of the disease. Volunteers visit homes two times a year - and three or four times if these are in more high-risk areas.

He said: "Ang Mo Kio is a mature estate. There are many seniors, and some are staying alone. Sometimes it can be a bit challenging to spread the message to them as well. Youngsters have a lot of platforms for information - such as social media - but some seniors may not know the seriousness (of the situation)."

He stressed the importance of taking precautions - even as public attention has been occupied by the coronavirus outbreak in Singapore.

IT technician Viknesh Jayasekaran, 34, who lives near a high-risk dengue cluster in Ang Mo Kio, takes the usual precautions - leaving pails upturned and emptying water from the plates below plant pots - even as he said he is more worried about the coronavirus.