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Figuring out the spike in dengue during the circuit breaker period

Some experts point to idle construction sites and more people working from home, though others cite a change in the dominant strain

Even as Singapore adjusts to life in phase one of the post-circuit breaker period amid the Covid-19 pandemic, a threat of a different form has reared its head: dengue fever.

In just the first five months of this year, 9,839 cases of dengue have been reported. The National Environment Agency (NEA) said last week that this was the highest number of cases recorded for the period since 2013, when the largest outbreak in the nation's recent history occurred.

On Tuesday, the number of daily new Covid-19 cases reached the lowest since April 11, with no Singaporeans or permanent residents among the confirmed cases. Meanwhile, the number of dengue cases surpassed the 10,000 mark this week.

Some experts say the dengue spike is an incidental result of the circuit breaker measures. With less activity in commercial areas, estate neglect and idle construction sites pose concerns as potential dengue hot spots, they point out.

Also, more people are staying home, which puts them at greater risk of transmission. This is because the Aedes aegypti mosquito, which transmits dengue, feeds during the day and is well adapted to domestic settings.

With the dengue fever season traditionally peaking between May and October and warmer temperatures set to prevail in the coming months, NEA said numbers might continue to rise. It warned that the total number of dengue cases might exceed the record 22,170 cases in 2013.

CONSTRUCTION SITES POSE RISK

In April, members of the public raised concerns that inactivity at construction sites near their homes could lead to an increase in the number of breeding grounds for the Aedes mosquito.

Since the start of the circuit breaker on April 7, most construction work has been halted, but NEA said a minimal number of people were still allowed on site to carry out housekeeping and pest control work.

Pest control operators, however, told The Straits Times that reduced on-site manpower during the circuit breaker could lead to inadequate housekeeping and mosquito control.

The high rainfall during the current wet season could also undo these efforts. For instance, anti-malarial oil applied to potential breeding grounds could be washed away following a bout of rain, while new puddles that form in crevices after rain could also become breeding habitats.

 
 
 

Inspections by NEA also revealed a twofold increase in the incidence of Aedes mosquito larvae found at worksites since the start of the circuit breaker, compared with the two-month period before. The agency has inspected 89 per cent of all construction sites to date.

In particular, a dengue cluster in Bidadari Park Drive and Woodleigh Link that was notified on May 5 has had 44 cases as of June 5 - all of whom were foreign construction workers residing in dormitories at two construction sites in the area.

This indicates that worksites, if not managed properly for dengue control, could pose a risk not only to those living in residential areas nearby, but also to those who work and reside on site.

STAY AT HOME, BITTEN AT HOME

Apart from idle worksites, the circuit breaker could also have played a part in the surge in dengue cases because more people are working from home during the day.


ST ILLUSTRATION: MIEL

Inspections by NEA also revealed a twofold increase in the incidence of Aedes mosquito larvae found at worksites since the start of the circuit breaker, compared with the two-month period before. The agency has inspected 89 per cent of all construction sites to date.

Figures from NEA also showed a fivefold increase in the incidence of Aedes mosquito larvae detected in homes and common corridors in residential areas during the two-month circuit breaker, compared with the prior two months.

This can lead to more dengue fever transmissions in two ways. First, the Aedes mosquito is well adapted to hiding and thriving in a domestic setting. Second, it is most active in seeking a blood meal during the early morning and late afternoon, said Professor Ooi Eng Eong, deputy director of the emerging infectious diseases programme at Duke-NUS Medical School.

Figures from NEA also showed a fivefold increase in the incidence of Aedes mosquito larvae detected in homes and common corridors in residential areas during the two-month circuit breaker, compared with the prior two months.

Checks by the agency also singled out potential breeding grounds in residential areas such as potted plants and pails, as well as roof gutters and drains within compounds.

OR IS IT DUE TO CHANGE IN STRAIN?

Some experts, however, disagree that the circuit breaker measures, which saw more people staying home, caused the jump in dengue infections. Instead, they point to weakened herd immunity as the cause - one that has resulted from the relatively low number of dengue infections in the past few years.

Assistant Professor Vincent Pang from the National University of Singapore's Saw Swee Hock School of Public Health said herd immunity usually occurs when a large number of individuals in the population develop immunity against a serotype of dengue after being infected by it.

Prof Pang, who is also director of the Centre for Infectious Disease Epidemiology & Research, added that it is likely that herd immunity against the serotypes DenV-1 and DenV-2 - dominant strains in the dengue outbreaks since 2003 - has been established here.

 
 
 

But now that the DenV-2 and DenV-3 strains have become the dominant strains in Singapore, the lack of herd immunity towards DenV-3, coupled with other factors such as the climate, could have contributed to the surge in recent cases, he added.

Prof Ooi agreed that a change in the dominant serotype, or strain, in itself is insufficient to cause outbreaks. Other factors also play a part, including mosquito population density, the fitness of the dominant strain of virus to be transmitted from humans to mosquitoes and back to humans, and the availability of human hosts, he added.

12 DEATHS ALREADY THIS YEAR

Whatever the cause of the spike in numbers, as more people return to workplaces and spend less time at home, and as worker numbers return to normal at construction sites, dengue cases might be expected to taper off. However, given that weather conditions continue to favour transmission, it is unlikely Singapore has already seen the worst for this year's dengue situation.

For the many still working from home, it makes good sense to stay vigilant, especially if you live in a neighbourhood identified as a dengue cluster. After all, most dengue transmissions take place at home.

With the Covid-19 outbreak dominant in people's minds, it is easy to forget how deadly dengue can be. Last year, 20 people died after contracting dengue fever, while there have been 12 deaths so far this year.

About one in five people diagnosed with dengue is sick enough to require hospital care.

 

And at this crucial point, the last thing Singapore needs is for hospitals to be stretched to their limits by two concurrent health outbreaks.

So, wear your mask and wash your hands, but don't forget to overturn your pails as well.

A version of this article appeared in the print edition of The Straits Times on June 11, 2020, with the headline 'Figuring out the spike in dengue during the circuit breaker period'. Print Edition | Subscribe