Singapore could see more cases of dengue than usual in 2019 after a lull of a few years, but it is not clear what is causing the current spike, infectious diseases experts told Channel News Asia (CNA).

Dengue cases rose to 666 last week, which the National Environment Agency said was the highest recorded in a week since a previous peak in January 2016, when the number of cases hit 637.

As of  Monday (July 15), there were 7,438 recorded cases of dengue in Singapore, about five times more than the 1,481 cases in the same period last year, NEA said. Five people have died from dengue this year.

Experts said there was insufficient information to determine the cause of this year’s jump in cases, but some pointed to the increase in the number of mosquitoes detected while a few said a different variant or mutation of the virus could be responsible.

The spike in cases this year did not come as a surprise to infectious diseases specialist Dr Leong Hoe Nam, who pointed out that the increase in dengue cases typically comes in “phases.”

“We’ve had two good years in 2017 and 2018. Dengue typically comes in phases of three to four years, so this spike is not surprising,” he said.

Singapore’s latest dengue epidemic was in 2013, with more than 22,000 infections and eight deaths. There were also dengue epidemics in 2005 and 2007, CNA reported.

In contrast, dengue infections in Singapore fell to a 16-year low in 2017, with just 2,772 cases. There were 3,285 cases in 2018.

Dr Leong added that lower incidences of dengue in 2017 and 2018 would have caused the immunity of the general population to fall, making Singaporean residents more susceptible to the virus.

Dr Leong said: “It’s important to note that this is happening not just in Singapore but in the region, including Indonesia, Philippines, Malaysia and Thailand.

“It is too myopic to say this trend is our own doing because it’s happening all over the region.”

Dr Ooi Eng Eong, deputy director at the Emerging Infectious Diseases programme at Duke-NUS Medical School, believes there has not been a drastic change in the Aedes mosquito population.

He also ruled out other possible reasons such as an increase in the number of Singapore residents who are not immune to dengue, or a change in the predominant serotype of the virus, CNA reported.

“If you look at the proportion of the population that has had dengue before and is therefore immune to at least one serotype, that number has not changed in 20 years,” he said.

Instead, he thinks virus evolution might be to blame.

The dengue virus genome is known to mutate, said Dr Ooi, and some of these mutations could have allowed the virus to better spread in Singapore’s urban environment.

“Such mutations have contributed to outbreaks in different parts of the world and may be responsible for the current rise in the number of dengue cases,” he added.

Meanwhile, the Philippines will prepare military hospitals and clinics for a possible surge in dengue patients after a spike in the number of cases this year, the Department of Health said. The DOH declared a national dengue alert for the first time.

A total of 106,630 dengue cases have been reported from Jan 1 to Jun 29 — 85% more than the number of cases in the same period in 2018.

About 450 people have died from dengue this year, Philippine media reported.

Regions that have exceeded the epidemic threshold include Mimaropa, Western Visayas, Central Visayas and Northern Mindanao.