Dengue cases record a nation-high of 22,400; more than 1,000 dengue cases per week for the past eight consecutive weeks

A new dominant dengue strain is circulating; work-from-home and warm climates contributed to outbreak

Singapore recorded 22,403 dengue cases as of Wednesday (5 Aug), a new high for any single year in Singapore’s history, according to the NEA. 

Within 7 months, the number of cases for 2020 thus far exceeded the previous high of 22,170 cases recorded in the year of 2013. 

20 people have died from dengue this year as of 2 August, said the Ministry of Health (MOH). The youngest victim was 25 years old, while the oldest was 92, 

MOH elaborated that 18 of the victims had worked or lived in active dengue clusters, but declined to comment more on the victims and the specific clusters.  

The ministry said that about 0.2 per cent of the dengue patients this year developed dengue haemorrhagic fever, a more severe form of dengue that can be fatal. The proportion of dengue haemorrhagic fever cases has ranged between 0.1 and 0.8 per cent in the past 10 years.

It further added that although most dengue patients recover from the infection, elderly patients and those with other medical conditions are at higher risk of developing complications. 

According to the National Environment Agency (NEA), Singapore has reported more than 1,000 dengue cases per week for eight consecutive weeks

Last week between 26 July – 1 August, NEA reported 1,380 dengue cases. It recorded 1,792 cases for the previous week between 19 – 25 July.

 

NEA said the weekly numbers remain “persistently high” at above 1,000. Before 2020, the highest weekly number of dengue cases recorded was 891 in 2014. 

As reported by TOC on 29 July, there are close to 400 clusters that NEA has identified as hot spots, with the north and east of Singapore hosting most of the clusters.

The 391 active dengue clusters in Singapore, as of Tuesday (4 Aug), hold more than 600,000 households within them. 

The largest clusters are around Aljunied Road/Geylang Road and Arthur Road near Katong.

NEA expects to see dengue cases numbers in the high range in the coming months due to low herd immunity, work-from-home arrangements, and climates that are conducive for the Aedes mosquito breeding. 

Singaporeans have low immunity to new dengue strain that is circulating 

There are four related dengue strains, DENV-1 to DENV-4, and infection provides immunity against only one serotype. The majority of the infections this year are of DENV-2 and DENV-3.

DENV-2 has been the dominant strain circulating in Singapore since 2016, said Associate Professor Luo Dahai from Nanyang Technological University’s Lee Kong Chian School of Medicine. 

However since 2019, DENV-3 has started to become the dominant serotype that was circulating in Singapore. 

This serotype was last dominant in Singapore about thirty years ago, but it has been dominant from January to April this year, said NEA.

According to the latest study by the agency published in 2015, the level of exposure to dengue among young adults here is relatively low. 

The study found that only 14 per cent were immune in the 16 to 20 age group, but about 87 per cent of those aged 56 to 60 were immune to DENV-2. 

This partially explains the population’s susceptibility to explosive outbreaks and the high rate of dengue cases among young adults. 

However it was found that immunity against DENV-3 is lower than against DENV-2 in all age groups.

The age group between 16 and 20 are especially susceptible to the DENV-3 strain, with only 2.7 per cent of the participants immune to DENV-3.

“The local outbreaks are usually caused by DENV-1 and DENV-2 in the past. The immunity built among the community residents against these DENVs may not be protective against the emerging DENV-3. More symptomatic dengue infection cases may rise for this reason,” said Associate Prof Luo. 

A single housing block in Senja (Bukit Panjang) has become one of the most affected by the nation’s most severe dengue outbreak. 

33 residents are infected, and though there have been intensified efforts in defogging, residents suspect that the breeding is occurring outside of homes as well.

While NEA’s quarterly surveillance data showed that the proportion of DENV-3 cases had fallen while that of DENV-2 rose from April to June, there are more DENV-3 viruses circulating in Singapore than in the past.

“We would need to monitor the serotypes for a longer period to see a clearer trend, whether DENV-2 is back to dominant or there is going to be a shift to DENV-3,” said Assoc Prof Luo.

The NEA added that “the high base of dengue cases in the first few months of the year means a high ‘viral load’ within the population; this is a situation that is conducive for an outbreak once the other favourable conditions are aligned.” 

This leads to the other favourable conditions for breeding such as working from home and climate changes. 

Circuit breaker provides more human hosts for Aedes mosquitoes to target

The sharp spike in case also coincided with the two-month COVID-19 “circuit breaker” period, said NEA.

“With more people staying at home, there would be more human hosts for the female Aedes aegypti mosquitoes to target, which is a day-biter and harbours within the indoor environment, and this would have partly contributed to the recent increase in cases,” said the agency.

Associate Prof Luo agreed, saying: “When more people stay at home all the days, there could be more residential mosquito breeding and more opportunities for ‘blood meals’.”

Other areas affected concurrently with the lockdown include the decrease in landscaping works owing to manpower shortage, that would have curbed the spread of dengue, and a halt in most construction activities which would have ensured good maintenance of construction sites.

However, the surge in number of cases could be because of higher rates of detection –  as people spend most of their time at home now, and also engage themselves in more housekeeping activities whilst at home. 

It is also possible that more cases of dengue were reported because of “better health-seeking behaviour” by the public who felt unwell or noticed symptoms during the COVID-19 pandemic, said the NEA.

Conducive climates that perpetuate dengue strains 

The warmer weather and rainy days since January this year have favoured mosquito breeding across the country, said Associate Prof Luo. 

This increase in the mosquito population in multiple areas then “carried the momentum of the high dengue case load of the first four months of the year over to the following months”, said NEA.

Between January and July this year, NEA detected about 13,800 mosquito breeding habitats islandwide. This is about 40 per cent higher than in the same period in 2019.

“As the traditional dengue peak season lasts a few months, from May to October, NEA urgently seeks the cooperation of all residents and stakeholders to do their part to complement NEA’s efforts,” it said.

Precautions to adhere to and symptoms to watch out for

Residents living in dengue cluster areas are advised to

  • Spray insecticide in dark corners around the home, such as behind curtains and under beds
  • Apply mosquito repellent regularly
  • Wear long-sleeve tops and long pants to protect themselves from mosquito bites

“We urge persons with symptoms suggestive of dengue to see a medical practitioner for timely diagnosis and management,” said MOH.

The symptoms indicative of dengue include:

  • Sudden onset of fever for two to seven days
  • Severe headache with retro-orbital (behind the eye) pain
  • Joint and muscle pain
  • Skin rashes
  • Nausea and vomiting
  • Bleeding from the nose or gums
  • Easy bruising of the skin
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