Commentary: Uncovering the Factors Fueling Singapore’s Record Dengue Cases

SINGAPORE: 2020 will be remembered above all for the COVID-19 pandemic, which has killed more than a million people and infected many more around the world.

But lurking in the shadows of the pandemic is another pestilence, a long-standing public health threat in tropical and subtropical regions of the world.

We are referring, of course, to dengue, which has infected more than 30,000 people in Singapore this year, with more than 1,000 cases per week for most of June to August.

This is an all-time high, beating the highest past record of 22,170 dengue cases in 2013 and the highest weekly peak of 891 cases seen in 2014.

EARLY SIGNS

Earlier this year, we saw signs of a large imminent dengue outbreak, both in the high number of dengue cases and in the increased incidence of the rare serotype DENV-3.

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Therefore, the National Dengue Prevention Campaign was launched in March by the National Environment Agency (NEA), ahead of the usual May launch that coincides with the start of the “dengue season”, to mobilize the nation. to be conscientious in your internal management. , get rid of standing water and deprive Aedes mosquitoes of breeding habitats.

However, despite preventive measures and extensive public communications, weekly reported infection numbers soon exceeded trends from previous outbreaks, suggesting that other factors may have exacerbated the increase.

Dengue transmission intensifies when there is more contact between Aedes mosquitoes and humans, either from increased exposure of people to Aedes mosquito bites or from an increase in the mosquito population. Could they have been a by-product of our response to the COVID-19 pandemic?

CIRCUIT BREAKERS CONTRIBUTE TO DENGUE OUTBREAK AS MORE STAY AT HOME

The answer is yes. With more people spending more hours of the day in naturally ventilated homes, the possibility of mosquitoes acquiring the infection from people infected with dengue and then transmitting it when they bite healthy people has increased.

This is due to the biology of the Aedes aegypti mosquito, the main vector of dengue fever. The mosquito is known to bite during the day, peaking at dawn and dusk, rather than at night, and inhabits human habitats.

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In August, the NEA and the Saw Swee Hock School of Public Health conducted an epidemiological study to examine the independent effect of the breaker period on the number of reported dengue infections.

We found almost 50% more infections during the switch period than it should have been according to our model. This is even after taking into account the typical increase in dengue cases in the middle of the year and the initial high level of infection in the first two months of the year.

Most of the excess infections occurred among working-age adults (ages 20 to 64), people who spent much of their day at home and who would normally have been in offices, factories and shops.

The younger population (5 to 19 years old) only had 12% more cases, possibly because schools were closed for a period shorter than the total duration of the switch.

Also, unlike working adults, children going to school would have spent much of the Aedes evening rush hour at home, even before the circuit breaker.

Mosquito exposures are less likely to change for the elderly and preschool-age children, as they would spend more time at home even before the switch period and thus represent a suitable reference population for analysis.

It is clear that the closer proximity of humans and mosquitoes has driven a rapid increase in the number of new dengue infections.

CIRCUIT BREAKER MEASURES LARGER MOSQUITO BREEDING

Despite repeated public communications about the risk of dengue, Aedes aegypti Mosquito breeding in Singapore steadily increased during the circuit break period.

The average percentage of inspected construction sites that were found to breed mosquitoes from January to March was six percent. This figure tripled to 18 percent from April to June, although it has since fallen to around 10 percent in August.

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This is likely because the suspension of construction activities since the start of the breaker period on April 7, 2020, had left construction sites largely unattended and exposed to the tropical climate with regular rainfall.

Additionally, the closure of many of our essential workers to contain the transmission of COVID-19 in the bedrooms had the effect of reducing the frequency of cleaning of public spaces.

The number of mosquitoes breeding in homes also remained high during the circuit breaker, challenging some expectations that people who spend more time at home have the ability to remove standing water.

NEA reported a five-fold increase in the incidence of Aedes Mosquito larvae detected in common homes and corridors in residential areas during the two-month circuit break period compared to the previous two months.

Could the unexpected rise in mosquito breeding in homes be due to the challenges faced by residents juggling working from home, the increased burden of housework with more people at home, or housewives having to pay more attention to preparing homemade meals and looking after children than otherwise being in school?

In recent weeks, although dengue cases have declined for a successive seven-week period, it remains high at 595 last week. Given that a substantial proportion of our workforce continues to work from home and faces an increased risk of contracting dengue, the decline in dengue cases could be long and slow.

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So far this year, more than 20 lives have been lost to dengue, and there may be more unless we collectively take immediate and determined action.

INDIVIDUAL RESPONSES NEEDED TO PROTECT FAMILIES

In the fight against dengue we all have a role, because each one of us can become a node of transmission of dengue, and the Aedes aegypti mosquito breeds in our homes.

One way to combat dengue is to avoid being bitten. This is especially important for people who have dengue, as they are a potential source of infection for other people.

If you live in a high-risk area for dengue, you and your family should wear long pants and long-sleeved clothing, and use mosquito repellent to protect against mosquito bites, especially during the Aedes aegypti peak mosquito bite periods early in the morning and late afternoon.

You can protect your home by spraying insecticide on mosquito hiding places. These places can be under sofas, tables or shelves, behind curtains, or in dark parts of the kitchen.

A light spray of insecticide in these places, not vaguely in the air, can help kill mosquitoes in the home. NEA has a video showing how to do this.

Another long-term solution is to install mosquito nets on the windows.

To help you remember to check and remove standing water, consider putting a reminder on your phone at a convenient time (like after dinner) with a two-day repeat.

SOCIAL RESPONSES NEEDED: PROTECT COMMUNITIES

During a large outbreak like this, we must take a community approach to preventing dengue in our neighborhoods. Prosocial encouragement in the neighborhood can help overcome one of the main barriers to dengue prevention, namely the inertia that arises from the habit of not acting.

There are inspiring initiatives in the community. For example, concerned about the increase in dengue cases that had infected many people in their neighborhood, members of an East Coast Residential Council took proactive efforts to monitor residents on their property and make sure they knew how to protect themselves. themselves and their families from being infected.

What makes the COVID-19 pandemic so different from past pandemics is that we have launched an unprecedented response: closing schools, borders, mosques, churches and temples, shops, cinemas, workplaces, wearing masks, social distancing, restriction. about travel and contact tracing.

As a society, we have done more to stop the spread of COVID-19 than for any other disease in recent times.

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The answer, both how we have accepted them and how the pandemic has been controlled thus far, shows what can be achieved when we work together to contain a threat to public health.

Associate Professor Alex Cook is the Vice Dean for Research at the Saw Swee Hock School of Public Health at the National University of Singapore.

Associate Professor Ng Lee Ching is the group director of the Institute of Environmental Health of the National Environment Agency. He has provided expert advice to the World Health Organization.