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CNA Explains: Why is Singapore at risk of a surge in dengue cases?

SINGAPORE: The National Environment Agency (NEA) on Wednesday (Sep 6) warned that Singapore is at risk of a surge in dengue cases.

Members of the public were urged to take immediate action to prevent mosquito breeding.

According to NEA, more than 6,300 dengue cases have been reported in Singapore this year as of Thursday. There are currently 53 active clusters around the country, up from the 48 that were announced on Wednesday. Twelve of them are high-risk clusters with more than 10 cases.

CNA takes a look at why the situation has worsened this year despite measures to keep the virus under control, with infectious diseases experts weighing in.

Why is there a risk of a surge in dengue cases? 

NEA attributed the risk of a rise in dengue cases to the Dengue virus serotype 1 (DENV-1) gaining dominance against a backdrop of high weekly dengue cases, several large and persistent dengue clusters, as well as high Aedes mosquito populations in many places.

DENV-1 cases have been on the rise over the past two months and the serotype has replaced the previously dominant dengue virus serotype 3 (DENV-3), the agency said.

In July, the monthly proportion of DENV-1 cases was approximately 55 per cent, which was almost triple the proportion of DENV-3 cases at 17 per cent. 

The rise in proportion of a previously less prevalent dengue virus serotype is “of concern”, as this has historically been associated with a surge in dengue cases months later, said NEA. 

However, one expert told CNA in a news programme that the DENV-1 serotype is not new. 

“When you have a switch in the type of dengue, then the worry of course is that the population immunity to this DENV-1 isn’t as high as say, against DENV-3, which has been circulating in Singapore at least for the past three years,” said Professor Ooi Eng Eong from the Emerging Infectious Diseases programme at Duke-NUS Medical School.

“But having said that, DENV-1 is not uncommon in Singapore. It actually has been around at least in the past 20 years. It’s caused a couple of outbreaks, I think, one in 2005 and another one in 2013,” he said, adding that the country’s immunity to DENV-1 is “not that low”. 

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Which areas are most affected by dengue?

In its news release on Wednesday, NEA highlighted “large and persistent” dengue clusters in several areas. 

This includes Toa Payoh, where persistent dengue transmission has been noted such as a 321-case cluster at Lorong 1 and Lorong 2 Toa Payoh, as well as a 178-case cluster at Lorong 1A Toa Payoh.

In response to queries from CNA on Wednesday, NEA said that about 280 mosquito breeding habitats have been detected at these large Toa Payoh clusters. More officers have also been deployed to inspect other parts of Toa Payoh.

Other large dengue clusters include a 66-case cluster at Angklong Lane in the Sin Ming area, and a 46-case cluster at Eng Kong Road in Bukit Timah.

A 44-case dengue cluster at Science Park Drive and 28-case cluster at Lentor Loop were also highlighted, which have a fast rate of dengue transmission, according to NEA. 

Why are certain areas more susceptible to dengue?

This is not the first time Toa Payoh has been affected by dengue. In June, the area also had several dengue clusters.

Dr Borame Sue Lee Dickens from the Saw Swee Hock School of Public Health at National University of Singapore (NUS) told CNA that there are most likely two factors why some areas are more affected than others: Ongoing increased risk of mosquito breeding due to environmental features, and the failure to continually remove breeding sites when they appear.

“These factors are universally important for HDBs, condominiums and landed property areas. Mitigating this risk requires building management to minimise mosquito breeding and doing regular checks to ensure that there are no mosquito breeding habitats,” she said. 

On why Toa Payoh is more susceptible to dengue cases, Dr Dickens highlighted that no matter the location, once an outbreak begins, it can be “difficult” to stamp out quickly and it requires time for control measures to have effect.

“If an outbreak isn’t quickly controlled, more hotspots can emerge as mosquitoes move and become established in more areas nearby,” she said. 

Building features that facilitate mosquito breeding and lack of breeding habitat surveillance could be part of the reason for any location having sustained transmission, noted Dr Dickens.

“There is always a risk wherever people live here. Increasing risk of dengue clusters come from highly populated areas with many micro-environments that facilitate mosquito breeding,” she said, referring to bins, drains, pots and building cracks.

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Why is dengue a continuing issue?

Asked whether the dengue situation in Singapore is getting worse, Dr Dickens noted that the serotype DENV-3 is being replaced by DENV-1, and that many outbreaks have occurred alongside these shifts in serotype.

“Singapore is part of a larger global community that suffers from dengue and it should be noted that other countries in our region have recorded higher outbreaks in July 2023 compared to July 2022, including Cambodia, Laos and Malaysia,” the assistant professor said.

Dr Dickens added that the reasons behind why the outbreaks occur are “complex”. It is a mixture of environmental factors such as weather, serotyping and the management of our urban spaces, the assistant professor noted.

Population immunity has also been falling because of reduced dengue transmission, Dr Dickens said. “Over shorter time frames in terms of months, however, we will always have the risk of outbreaks as long as we have mosquito breeding ongoing,” she added. 

Similarly, NEA also said on its website that the warmer months from May to October usually see higher dengue transmission.

This is due to an accelerated development of the Aedes mosquito vector and a faster multiplication of the dengue virus in mosquitoes.Dr Chia Po Ying, a consultant at the National Centre for Infectious Diseases (NCID), said that the dengue situation in Singapore is “not getting worse” due to the “continuous efforts and education of the public” by NEA and the medical community, including NCID. 

Effective control of the dengue situation requires a multifaceted approach, Dr Chia said. This involves controlling the Aedes aegypti mosquito population; early diagnosis, which requires patients who have symptoms of dengue – high fever, headache, joint and muscle pain – to seek medical attention early; prompt recognition by healthcare professionals; and early appropriate management. 

She also noted that the dengue cases identified so far this year are comparatively fewer than the 32,225 cases in 2022. “However, what is worrying has been the serotype switch from serotype 3 to serotype 1 since June 2023,” she added. 

Dengue has four serotypes, and infection by one of the four serotypes leads to lifelong type-specific immunity, but only a short-term cross-protection to the other serotypes, said Dr Chia. 

“This means a person can be infected by dengue a total of four times, and generally the second dengue infection can have more complications and a poorer outcome,” she said. 

Reiterating NEA’s point that historically, there is a surge in dengue cases when a dengue serotype switch occurs, Dr Chia added the surge may also be in part due to the overall decreasing host immunity against dengue.

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CNA Explains: Singapore ramps up Wolbachia mosquito project. How does it work in the fight against dengue?

What are the current measures against dengue and are they working?

In 2016, NEA started the first field release of Project Wolbachia-Singapore. The idea behind the project is to release male Aedes aegypti mosquitoes that carry the Wolbachia bacteria so that they mate with the female Aedes aegypti in an urban environment. Their resultant eggs do not hatch and no offspring are produced. 

Continued releases lead to a decline in the urban Aedes aegypti mosquito population, and therefore less dengue transmission, according to NEA. 

In May this year, Senior Parliamentary Secretary for Sustainability and the Environment Baey Yam Keng said that more than one million people have benefitted from the project, covering about 30 per cent of all HDB blocks and 9 per cent of landed residential areas in Singapore.

At Tampines, Yishun, Choa Chu Kang and Bukit Batok, the Aedes aegypti mosquito population was suppressed by more than 90 per cent, Mr Baey added. 

NEA’s Environmental Health Institute (EHI) also developed the Gravitrap, which is designed to attract and trap female Aedes mosquitoes that are looking for sites to lay their eggs.

Female mosquitoes attempting to lay their eggs in these containers will be captured and prevented from subsequently biting other people. The Gravitrap also traps and prevents the emergence of any mosquitoes from eggs that are laid in the trap.

NEA said it has deployed close to 70,000 Gravitraps at public and landed housing estates around Singapore.

Gravitraps themselves do not add to the mosquito population already in the area, but conversely help to reduce the number of mosquitoes emerging from other breeding sites, said NEA.

Additionally, knowing which Gravitraps are capturing these mosquitoes helps NEA to “prioritise its resources” by directing officers to search for and destroy the breeding habitats at locations with higher Aedes mosquito populations, the agency added.

To avoid a surge in dengue cases, NEA said members of the public can remove stagnant water and carry out the “S-A-W” steps: Spray insecticide in dark corners around the house, apply insect repellent regularly and wear long-sleeve tops and long pants.

Those diagnosed with or suspected to be infected with dengue are also advised to avoid further mosquito bites, by using insect repellent and carrying out the other S-A-W actions, to prevent passing on the dengue virus to mosquitoes and other people in their neighbourhoods.

All residents must also take immediate action to suppress the Aedes mosquito population and break disease transmission, said NEA. They can do so by regularly practising the “B-L-O-C-K” steps: Break up hardened soil, lift and empty flower pot plates, overturn pails and wipe their rims, change water in vases and keep roof gutters clear and place BTI insecticide inside.

On the effectiveness of these methods, Dr Dickens of NUS Saw Swee Hock School of Public Health noted that it is a “group effort”. 

“If everyone is unified in their S-A-W and B-L-O-C-K responses, mosquitoes will struggle to bite and breed, and the risk of dengue outbreaks will go down,” she said, adding that it only takes one breeding habitat for hundreds of mosquitoes to appear.

“It is particularly important for B-L-O-C-K to happen all the time, not just when an outbreak starts. Among individuals diagnosed or suspected to have dengue, S-A-W can critically prevent the passing on of the virus to the other mosquitoes and the people in the wider neighbourhood,” Dr Dickens added. 

The infectious diseases expert also said there is “no one-stop solution” for dengue, highlighting measures such as Wolbachia technology, breeding habitat removal as well as personal protection. 

“The key is not to be vigilant only when an outbreak is already here but to be preventative all the time,” Dr Dickens added. 

In addition, NCID’s Dr Chia said continuous vigilance to eliminate mosquito breeding, as well as ongoing education of the medical community, is needed to curb Singapore’s dengue problem.

“Dengue outbreaks hinge on three factors: The dengue virus, the host (i.e. us humans), as well as the vector (i.e. the dengue mosquito),” she said, adding that only two of the factors are more easily modified – vector and host.

On anti-viral treatment, Dr Chia noted that there is currently no such treatment that is dengue-specific. 

“Management of dengue has been largely supportive, with emphasis on ensuring adequate fluid intake as well as appropriate monitoring for early detection of complications,” she added. 

As for host immunity, while there is currently a commercially available dengue vaccine, Dengvaxia, it is only licensed for use locally for individuals aged 12 to 45 years who have had previous dengue infection.

“There are other dengue vaccines that are being studied with hopes to increase protection as well as circumvent the need for prior dengue infection,” said Dr Chia.

“The medical community on the whole is also looking towards research to develop safe and effective anti-viral medications and medications to modulate host immune response.” 

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