Dengue surge fuelled by more mosquitoes, re-emergence of previously uncommon virus serotype: Experts

SINGAPORE: As dengue cases in Singapore continue to spike, the chance of a major outbreak this year looks to be an increasing possibility.

As of the week ending Apr 9, a total of 3,979 cases have been recorded this year. This is in contrast to a total of 5,258 cases in 2021.

There were 644 dengue cases in the week ending Apr 9 - the highest number recorded at this point in time in the last four years. This includes when a major outbreak in 2020 saw a total of 35,315 cases reported for the year.

Experts attribute this current spike to a number of factors, the most notable being a rise in mosquito numbers and the re-emergence of a previously uncommon virus serotype.

In response to queries from CNA, the National Environmental Agency (NEA) noted that the Aedes aegypti mosquito population in the community was about 48 per cent higher in March this year compared to the same period in 2021.

The agency said this data is obtained from its surveillance network of gravitraps distributed throughout the island. Gravitraps are designed to attract and trap female Aedes mosquitoes looking for sites to lay their eggs. 

Inspections have also found that Aedes mosquito breeding has almost doubled from February to March this year.

"We've seen (an) unusual February where ... it was quite rainy, and also the humidity was higher than usual," NEA deputy chief executive Chew Ming Fai told CNA.

The hot weather could have shortened the incubation cycles of the mosquito, resulting in a faster population growth. 

"The other thing is that within the mosquito itself, the virus also replicates a little bit faster when temperatures are a bit warmer," he added. 

AN "UNUSUAL" SEROTYPE

In addition to higher mosquito numbers, the previously uncommon dengue virus serotype 3 (DENV-3) is now widely circulating. It is now the predominant strain of dengue detected in Singapore, followed by DENV-2 noted, Mr Chew.

Dr Jolene Oon, an infectious diseases expert at the National University Hospital (NUH), told CNA that the "increased circulation" of the DEN-3 serotype would have meant less immunity within the community.

This is because prior to 2020, DENV-3 had not been the dominant virus serotype during any major dengue outbreaks in Singapore over the past three decades, added Mr Chew. 

While recovery from one dengue virus serotype provides lifelong immunity against that particular serotype, it does not guarantee immunity against others.

Instead, it only provides partial protection against subsequent infection by other serotypes, and this will wane over time.

This threat is further exacerbated by more people working from home during the COVID-19 pandemic, said Dr Oon.

With the Aedes aegypti mosquito being predominantly a mosquito that bites during the day, this increases the propensity for individuals to get bitten, added Mr Chew.

WORST YET TO COME?

With the traditional peak dengue season from June to October yet to arrive, experts warn that it is likely the worst is yet to come. 

"As can be seen from the NEA data, dengue peaks tend to occur to the middle of the year, possibly related to small changes in the weather conditions. That is most likely going to happen this year again," said infectious diseases expert Dr Paul Tambyah, who is the president of the Asia Pacific Society of Clinical Microbiology and Infection.

"We usually see higher transmission during the rainy season of months June through to October. As such, we do anticipate a rise in cases during the middle half of the year," agreed Dr Oon, who is also Outpatient Parenteral Antibiotic Therapy (OPAT) clinical director and consultant at NUH's division of infectious diseases.

However, Mr Chew noted that this does not mean that the outbreak of 2020 will be repeated or surpassed. Instead, he said that the NEA is hoping to tackle the issue before it peaks.

"At this point of time, we are still hoping to shave off the peak, but we are very, very concerned because this is early in the season and already the numbers are high," he explained.

What is important is to bridge the gap between awareness and action among the public, he noted.

"In a way, you can say that we are victims of our own success here in Singapore, because in most other countries dengue is a childhood disease ... As we have gotten the dengue numbers lower over time, herd immunity has dropped, which makes us more prone as a population to the virus," Mr Chew said.

"While we can say that awareness is generally high, it really needs to translate into action, and this gap is actually one of those things that over the past years, and whenever we run the (annual) dengue campaign is always to try and narrow the awareness action gap."

One way the NEA is tackling this is by launching the annual National Dengue Prevention Campaign earlier than usual.

It has also taken the additional step of coming up with a new purple dengue alert banner which will be put up in areas with a persistently high population of Aedes aegypti mosquitoes.

"Banners are very useful to give people a visual cue that they are actually within a cluster area ... It works upstream because it actually warns (of) high mosquito numbers," said Mr Chew.

"This is even before you have a cluster in your area, telling you that actually based on our Gravitrap surveillance, network and we see high (amounts of) mosquitoes here, please take the necessary ... measures."

NEA is also ramping up inspections. From January to February, it conducted about 101,000 inspections for mosquito breeding islandwide, including about 1,070 checks at construction sites. About 2,600 mosquito breeding habitats were uncovered, said the agency.

Professor Ooi Eng Eong, an infectious disease expert at the Duke-NUS Medical School, noted that the rise in the number of cases does not mean that Singaporeans are getting complacent.

"When you see the fluctuations in dengue epidemics, it's actually unfortunately normal. It's normal for dengue ... and for that matter, for any infectious disease. So when they spread, they will cause an outbreak but when enough people build up that immunity, the outbreak will disappear, and then the next virus comes in and then it will happen all over again," he said.

"You also have this effect where after infection with one serotype (of dengue), you are actually partially protected against the other serotypes for a brief period. Then after that you become susceptible to those other serotypes. And so all that contributes to this cyclical kind of dengue epidemics.

"So therefore, when when we have an epidemic, it doesn't mean that the population is complacent. And when we don't have any epidemic, it doesn't mean we're doing great," he added. 

While awareness of dengue is high, Prof Ooi noted that the attitude and practices have been that dengue is a public health issue and the Government will step in to protect the public.

"That's always been been the perception. But hopefully, things will change," he said.

Dr Tambyah pointed out that several steps can also be taken to further address the dengue threat.

This includes more information dissemination about clusters, and reminders to seek medical attention if not well, he said.

"We have learned some lessons from the pandemic and perhaps we could have the rapid antigen tests for dengue which have been widely used in Singapore for years by GPs (general practitioners)," Dr Tambyah added.

"The hospitals already have outpatient clinics for dengue management and perhaps these could be rolled out to the PHPCs (public health preparation clinics) as the pandemic winds down. That way people who test positive on self-test Dengue (rapid antigen tests) can get monitored and treated in the outpatient setting without burdening the hospitals."

Dr Oon noted that public education is also important.

"There is definitely some level of awareness but because dengue is endemic, the level of vigilance is somewhat not as heightened as for COVID-19," she said.

"Continued efforts should be made to increase public awareness, something similar to what has been done for COVID-19."