This year is turning out to be a bad one for dengue, with more than 9,700 cases reported so far. This is a five-fold rise, compared to the numbers seen by this time last year, and more than double the total dengue cases for all of 2017. 

This increase is across South-east Asia, with several neighbouring countries also reporting a sharp rise in dengue infections this year.

The outbreak here has claimed nine lives (with a median age of 71 years old) in 2019, a higher case-fatality rate than in the 2013-2014 epidemic outbreak (with 22,170 cases and seven deaths in 2013, and 18,326 cases and five deaths in 2014).

The current dengue surge looks set to continue over the next few months, due to a combination of the different drivers in dengue epidemics.

The first and obvious factor is high mosquito densities with warmer temperatures. So the current dengue surge coinciding with the mid-year monsoon season is consistent with the general pattern for dengue outbreaks.

In addition to effects on vector breeding, a hotter environment also shortens the extrinsic incubation period for dengue virus, allowing mosquitoes to become infectious more quickly after a blood meal.

Climate change is likely to further exacerbate dengue and other mosquito-borne infections.

Dengue outbreaks have an interesting pattern of recurring periodically every six to seven years. This is due to the problem of waning herd immunity and a build-up of susceptible persons.

Singapore’s recent dengue outbreaks occurred in 2005-2006, and in 2013-2014, so a dengue surge in 2019 is not entirely unexpected.

During the inter-outbreak years, when dengue infection rates are low, the number of susceptible persons accumulates as fewer people develop immunity from natural infection. This is akin to a pile of kindling building up, which then catches fire.

How about the higher fatality rate this time round then? This could be an unintended consequence of successful vector control programmes, resulting in people catching dengue later in life when their immune system is weaker.

Population herd immunity is the protective effect when enough individuals possess immunity against dengue due to prior dengue infection. 

However, this protective effect may decrease over time, both in individuals as immunity wanes, and in the population as a whole, as the proportion of individuals with prior infections drops. 

Why might the proportion of immune individuals in the population drop? In addition to the low number of dengue infections between outbreaks, new susceptible persons are constantly being added to the population, due to increasingly mobile global populations. 

These persons can and do acquire dengue infection when exposed in Singapore. Based on the annual Communicable Diseases Reports published by the Ministry of Health, Singapore, non-residents comprised 29 to 40 per cent of all dengue cases from 2012 to 2017. 

These non-residents may also acquire dengue infection due to exposures at work or at their living quarters.

A third factor fueling dengue outbreaks is the switch in predominant dengue serotypes. There are four strains of dengue, known rather unimaginatively as DEN-1, DEN-2, DEN-3, and DEN-4. All four serotypes circulate in Singapore, which means any individual can theoretically get dengue infection four times in their lives.

In 2012, the predominant circulating dengue serotype was DEN-2, which caused 74 per cent of infections, followed by DEN-1, which accounted for 19 per cent of cases. 

Only 7 per cent of cases were due to DEN-3 and DEN-4. Switches back and forth between DEN-2 and DEN-1 have caused moderate blips in dengue incidence in the years 2014 and 2016. 

In the current dengue surge, DEN-2 is the most predominant serotype, but DEN-3 is the second most common serotype circulating, unlike in previous years when it was DEN-1.

Herd immunity has developed against the most common circulating dengue serotypes, but is rendered ineffective when there is a major serotype switch. So, in a population of a specific geographical location that has been exposed to DEN-2, a serotype switch from DEN-1 to DEN-3, can increase the number of severe dengue infections.

What causes dengue virus serotype switching? We don’t know. It is possible that different strains of dengue virus, such as DEN-3 or DEN-4, might be introduced through travel from other dengue-endemic countries, when travellers return with imported dengue infection. 

The Ministry of Health’s reports show that dengue infection acquired overseas accounted for 0.4 to 4.4 per cent of all dengue cases seen in Singapore residents and non-residents from 2012 to 2017.   

What then can we do?

These factors — climate effects, waning herd immunity, and dengue virus switching of strains — may feel overwhelming and not within the control of individuals, communities, or even national environmental and health authorities.

The main tools we have to bend the curve are concerted efforts to control adult mosquitoes by removing mosquito larvae breeding sites. Over the past several years, the National Environment Agency has also been piloting an innovative Wolbachia-Aedes project for mosquito reduction.

Our healthcare systems have made substantial improvements in the care and management of dengue patients with hard-won experience over the past 15 years and two large outbreaks.

Hence, individuals who have high fever, severe muscle/joint pain and rashes should consult their primary care physicians early for prompt diagnosis and management. This also helps the authorities target dengue hotspots for mosquito control interventions.

As with other infectious diseases, the real hope for preventing dengue lies in the development of effective vaccines to achieve a sustainable and effective herd immunity to reduce the risk of transmission.

To overcome this limitation of serotype-specific herd immunity, an effective dengue vaccine would have to confer protection against all four serotypes.

The only commercially licensed dengue vaccine, Dengvaxia, confers approximately 70 per cent efficacy against DEN-3 and DEN-4, but only 50 per cent efficacy against DEN-1 and DEN-2.

If given to individuals without prior dengue infection, there is a higher risk of severe dengue when they acquire their first dengue infection.

Dengue vaccines remain a promising area for pharmaceutical research and development, with several other dengue vaccine candidates in the pipeline.

However, until safe and effective dengue vaccines are developed and licensed, dengue control continues to depend on vector control, through innovative technologies and old-fashioned whole-of-community involvement to reduce breeding sites.

These need to be supplemented by personal protective measures as well as public health interventions, as we weather this current dengue surge together.

ABOUT THE AUTHORS:

Assistant Professor Vincent Pang Junxiong is Director of the Centre for Infectious Disease Epidemiology and Research at the NUS Saw Swee Hock School of Public Health and Associate Professor Lim Poh Lian is Senior Consultant at the National Centre for Infectious Diseases.