Another dengue outbreak hits Singapore possibly due to warm weather

[ASEAN NOW News]

A worker fogs a housing estate in Singapore. (Photo: AFP/Roslan Rahman)

 

SINGAPORE: It never rains, but it pours. Just when COVID-19 cases in Singapore have fallen back to lower levels, we are experiencing yet another outbreak, this time of a more familiar foe – dengue. 

 

According to CNA, the number of cases reported so far this year has already exceeded the whole of 2021 and is already climbing towards the levels in 2020 when Singapore saw its worst dengue outbreak. 

 

Why is this happening despite efforts by the National Environment Agency (NEA) and the public in mosquito control?

 

To borrow terms, we’ve become familiar with over the last two years of the COVID-19 pandemic, the effective reproductive number – the number of people infected by the mosquitoes infected by one dengue carrier – needs to be kept below 1 to curb transmission. And that isn’t currently happening.

 

We know some circumstantial factors that may be responsible for the current outbreak. The recent warm weather is conducive to mosquito population growth. DENV3, the current dominant serotype behind most cases now, has not circulated at such levels before 2021. 

 

But here are two broader factors we think are responsible for why outbreaks like the current one still persists: Ever lower population immunity and ever better diagnostics. Ironically, both are good problems to have.

 

A seeming paradox of dengue epidemiology in Singapore is that despite thousands of cases in recent years, immunity in the population is dropping. It defies logic until you flip it around: Immunity is falling, so dengue can spread more easily.

 

Low levels of transmission over the last 50 years, thanks to community and government successes in vector control, mean that fewer people have been exposed to dengue. With more people susceptible, a single infected mosquito has more chances to pass on the virus, meaning that controlling the mosquito population must be increasingly effective to get the same results. 

 

Every few years, NEA and the Health Sciences Authority screen residual blood donations for past dengue infection, and each time over the last decade they have done so, the prevalence has fallen. 

 

In 2004, about one in three 30-year-olds had had dengue before and this fell to about one in four by 2017. The fall was starker among those aged 45, with prevalence falling from about 74 per cent to 48 per cent.

 

So how do we reconcile the paradox of the falling prevalence of dengue with the large numbers of cases diagnosed in recent years?

 

The short answer is that we are better now at detecting dengue infections than in the past. The public is also on higher alert for potential infections, thanks to COVID-19. 

 

Dengue and COVID-19 share some early clinical symptoms, like fever, body ache and fatigue, so more may have sought medical attention in the past two years or so. The availability of rapid diagnostics in GP clinics is another factor.

 

In a nutshell, though overall the prevalence of dengue has fallen over the years, we are more susceptible to getting infected if exposed to the virus and more adept at diagnosing infections.

 

A mosquito on a person's skin. (AFP/Luis ROBAYO)
 

So does this mean we just have to accept dengue, as it is for evermore? We don’t think so. 

 

Of course, there are small steps most of us are already familiar with, such as removing stagnant water, using the myENV app to get alerts about clusters, as well as the increased use of sprays, long-sleeved and legged clothing, and residual indoor spraying around the home. 

 

But scientists have always sought out more upstream interventions in control that could create a step change in outcomes. 

 

Fortunately, there are long term technological solutions around the corner, the two most likely over the years ahead being the use of Wolbachia mosquitoes and dengue vaccination.

 

Singapore has been leading the way internationally in using Wolbachia mosquitoes since 2016 in a suppression strategy – where male mosquitoes infected with the Wolbachia bacteria are released to mate but the resultant eggs cannot produce offspring, thus making the wild population crash. 

 

The data from the pilot field sites in Tampines and Yishun over the last few years suggest the approach is very successful in cutting mosquito populations there.

 

[ThaiPauly]

Well at least they look like they are fogging properly.

 

When they fog our streets in Ching Mai it's a quick squirt up in the air and they are gone!!

Or they come and thoroughly  spray a house AFTER the person that lives there has a confirmed  case of dengue..even though they could have caught it anywhere!!