Singapore may face a major dengue threat this year, with the recent rise in infections stemming from a strain of the virus that has not been prominent for years.

The National Environment Agency (NEA) said yesterday that there has not been an outbreak of DENV-3 virus here in the past three decades. This means there is low immunity in the population to this serotype.

There are four dengue serotypes, with the outbreaks here caused by DENV-1 and DENV-2. DENV-2 has been the dominant strain since 2016. DENV-3 generally accounts for a minority of cases.

The NEA said in a statement: "We have seen an increase in dengue virus serotype 3 cases and clusters over the past month.

"As Singapore has not seen a DENV-3 outbreak in the last three decades, the population immunity for DENV-3 is low and, therefore, (people are) more susceptible to transmission of the virus. It is thus critical that all residents and stakeholders work closely together with NEA to break the dengue transmission in these clusters, and curtail the spread of the virus."

It added that DENV-3 infections have been detected in three of the six largest clusters - in Jalan Bangau, Cashew Road and Jalan Paras.

Dengue infections have been on the rise for three weeks. Last week, 345 people were diagnosed with dengue - a 50 per cent rise over the number of infections seen in mid-December. This is the highest number of infections seen at this time of the year, since 2016.

Said the NEA: "The high Aedes aegypti mosquito population in the community, the current high number of dengue cases, and the increase in circulation of DENV-3 could lead to weekly dengue cases rising above current levels in 2020."

Professor Leo Yee Sin, executive director of the National Centre for Infectious Diseases, said: "The recent increase in the proportion of DENV-3 cases could raise the likelihood of having more clinical cases because of the probable lower herd immunity."

Related Story NEA boosts dengue fight with new mozzie breeding facility

Related Story NEA to test releasing mosquitoes via drone as Project Wolbachia moves to phase 4 of fight against dengue

Professor Teo Yik Ying, dean of the School of Public Health, said NEA was right to sound a note of caution over the rise of this strain.

NEA also warned that the mosquito population, which is already high, may go up some more with the advent of warmer months.

It added that in some of the large dengue clusters, "we have detected mosquito breeding multiple times in the same homes".

At Bukit Mugliston, for example, mosquito breeding was discovered more than once in 10 homes. In all, 85 people have been infected in the cluster which has been active since October, including eight over the past fortnight. Generally, one in five people diagnosed with dengue is so ill that he needs hospital care.

The current rise in infections come after 15,998 reported cases last year - the third highest annual number seen here. Dengue claimed 20 lives last year.