The curious case of dengue in Singapore
Why have clusters surged in 2020?by Gwyneth Cheng
With COVID-19 constantly on our minds, we’re lowering our guard around a more persistent disease in our community: dengue fever. While dengue is easy to prevent, Singapore has now hit a record high number of cases, and the situation will likely worsen if we do not take collective action quickly.
An old enemy that has plagued us for centuries has recently resurfaced with a vengeance.
In our battle against COVID-19, measures that were taken to protect us have ironically spurred the spike of yet another disease within our community: dengue fever.
Transmitted most efficiently via females of the Aedes aegypti mosquito species, dengue fever is rampant in places with hot and rainy tropical climates, large populations, high human flow, and developing urban landscapes.
Southeast Asia fits these criteria. Within the region, high numbers of dengue cases every year have prompted governments to act to keep the disease under control. Singapore has been claimed as one of the best in the region at dealing with the disease.
Yet, despite our expertise in disease prevention, a worrying trend has brought dengue to the forefront of local epidemiological conversations. In 2020, a year when most Southeast Asian countries have seen a considerable dip in the number of dengue cases compared to the same period in the previous year, Singapore is one of the few that has seen cases soar.
While Singapore’s numbers seem low compared to the likes of Malaysia or the Philippines, our National Environment Agency (NEA) has predicted that our total annual cases will top that of 2013, breaking the record for the largest dengue outbreak in Singapore’s history. This year, nineteen people have died of dengue—twice the death count in 2013 and slightly short of our current COVID-19 death toll.
As our authorities start to panic and people begin to worry, we can’t help but wonder—why is this happening to us?
The risk of dengue always rises during the wet season, which typically occurs from the middle to end of the year within the region. More frequent rainfall results in the constant formation of puddles, increasing the number of potential breeding grounds for A. aegypti mosquitoes and consequently the risk of dengue infection.
Within Southeast Asia, high rainfall patterns are the norm. While Singapore records relatively lower amounts of rain from May to December compared to our neighbours, our precipitation levels remain high throughout the year. This likely means that we require consistent efforts throughout the year to remove breeding spots nationwide, more so than the other nations.
Unfortunately, with the onset of COVID-19, society has mostly been forced indoors, leaving commercial buildings, public spaces, and construction sites idle. Most lockdowns in Southeast Asia started in March or April, exactly when most Southeast Asian nations experienced lower rainfall and hence a lower risk of mosquito breeding—except Singapore.
The slowdown of consistent efforts to eradicate breeding spots during this period makes idle spaces a haven for mosquitoes. Given that Singapore is the only country in the region which is 100 percent urbanised, these areas tend to be situated close to residential areas as well, which could be a contributing factor to the spike in local dengue cases.
Interestingly, A. aegypti mosquitoes have been discovered to be particularly adept at navigating domestic spaces and are especially bloodthirsty during the day. In the months of May to July, despite the easing of lockdown measures across the region, Singapore’s population has largely remained at home compared to people in other countries. Instead of the closed, air-conditioned spaces of our offices or open spaces of the outdoors, staying at home when mosquitoes roam can increase the risk of getting bitten. Clearly, this factor alone is not enough to contribute much to the surge in cases, but it plays an undeniable role in increasing the overall risk of dengue infection.
When it rains, it pours. As if high rainfall and COVID-19 weren’t enough, Singapore is now facing an unpredicted shift in the dominant virus strain of the disease. Dengue fever is caused by a type of virus known as the Flavivirus, which has 4 different types: DenV-1, DenV-2, DenV-3, and DenV-4.
For more than 30 years, DenV-1 and DenV-2 have been dominant in Singapore, allowing our community to develop some immunity against them and consequently provide some protection for those who aren’t immune. This year, DenV-3 became the dominant strain, which means a much larger portion of the community isn’t immune, allowing the virus to thrive and infect more people than usual.
With the lack of awareness about the new dominant strain, coupled with our attention on COVID-19, dengue has taken us by surprise. In Singapore, the National Environment Agency announced a worrying fivefold increase in the number of occurrences of Aedes mosquito larvae found in households and corridors during the circuit breaker.
NEA has predicted that Singapore has yet to witness its peak in dengue cases, despite our gradual ease back to work. Weather conditions will persist, and so will the dominant strain. As there is currently no common vaccine for dengue, the main method of control is still to prevent the surge in population numbers of the A. aegypti mosquito.
A rise in dengue cases spells trouble for our COVID-19 situation too. As the symptoms of dengue and COVID-19 are similar, the increased risk of dengue in the community could delay the detection of or create false results for actual COVID-19 patients. Moreover, this is an easily preventable disease that our community has been able to keep at bay for years—letting our guard down at a crucial time like this will only result in unnecessary deaths.
This year, NEA has been stepping up its measures in dengue control. Using a combination of enforcement, vector control efforts, public communications, as well as community engagement and mobilisation, the agency hopes to put a stop to this trend of rising dengue cases.
From 15 July 2020, households with repeated offences in mosquito breeding face heavier penalties and, in more extreme cases, the possibility of being sent to court. NEA is also working with local Town Councils to enhance checking of mosquito breeding spots such as drains, rooftops, and water tanks. The agency has improved surveillance efforts at construction sites during the Circuit Breaker period, and this will likely continue after.
But the most essential role in dengue prevention still belongs to us:
“The majority of mosquito breeding is found in residential homes. Vector control remains key to eliminating potential mosquito breeding habitats and breaking the dengue transmission. We need urgent collective community effort to prevent this situation from worsening.”
—Mr. Masagos Zulkifli, Minister for the Environment and Water Resources
So check for stagnant water, overturn those pails, and be more alert about potential breeding spots at home. The year 2020 has been unpredictable in the craziest ways, which is why it’s even more important that we keep predictable—and preventable—events under control.