It sounds like a contradiction in terms. Singapore is trying to combat its worst-ever year for dengue fever by releasing millions of mosquitoes into the most affected areas of the City State.
But the male mosquitoes, which scientists are breeding and releasing into the wild, are a kind of Trojan horse carrying a bacterium called Wolbachia inside of them. When the males mate with female Aedes aegypti mosquitoes, which transmit the virus, the bacterium stops the eggs from hatching, thereby decimating the mosquito population.
That’s the theory. Singapore certainly needs to find something that works, as does the rest of Asia. Dengue is now endemic to the region: initially carried by slaves from Africa in the 19th century and accelerated by troop movements during World War II.
The World Health Organisation estimates that there are about 100 million to 400 million infections every year, with half the world’s population at risk. Exact numbers are hard to calculate, however, because only 25% of those affected show symptoms.
But one thing no one doubts is how deadly it is. The virus has four strains (serotypes) and getting one doesn’t offer protection against the others, quite the opposite.
That’s one of the problems afflicting Singapore. Four years ago, DENV1 and DENV2 were most prevalent. This year it’s been DENV3.
At the end of September, the Lion City passed a depressing threshold: 30,000 cases and 20 deaths since the beginning of the year. That’s nearly double 2019’s caseload.
Thailand is also suffering its worst outbreak in 20 years with 61,662 cases as of early-October and 41 deaths. Across the rest of Asia, the most recently reported figures show that Indonesia stands at over 70,000 cases and 500 deaths (mid-July), Malaysia 75,804 cases and 124 deaths (late-September) and the Philippines 60,819 cases and 236 deaths (late-August).
A second reason for the spike is Covid-19. When people stayed at home, commercial and retail spaces stood idle. Construction sites, in particular, are notorious breeding grounds for mosquitoes because they abound with stagnant water pools.
This year, the water hasn’t been cleared away, or sprayed as quickly as it normally would. And unfortunately, it only takes a patch of water the size of a small coin to provide a comfortable home for the Aedes aegypti.
They also like residential accommodation, especially plant pots and toilet brush holders. And there’s been plenty of bare skin to come out and feed on this year given what working-from-home means sartorially-speaking: ditching those long sleeve shirts and trousers for shorts and T-shirts.
Asian government have tried-and-tested methods to combat dengue, including spraying infected areas with insecticide and skin with repellent. They also encompass inspections, fines, water clearance and ongoing public awareness campaigns. None of the above has been able to stop infections from escalating, although the measures have stopped the situation from being even worse than it would otherwise have been.
One of the biggest issues is the lack of specific treatments for those who are unlucky enough to get infected. Doctors prescribe paracetamol for pain relief, while cautioning against non-steroidal anti-inflammatory drugs (NSAIDs), which increase haemorrhage risk.
The biggest risk is when patients pass from the disease’s fever phase to dengue shock syndrome, which causes internal bleeding and potentially organ failure. However, doctors have got better at spotting the signs (abdominal pain, rapid breathing, restlessness) and combating it by protecting patients from dehydration, which would put them at risk of severe disease.
In the meantime, scientists continue rolling out Wolbachia-related projects across the region. One of the most well known is Australia’s Monash University World Mosquito Program (WMP).
Its scientists are individually infecting Aedes aegypti eggs with Wolbachia, which then competes with dengue, making it harder to reproduce inside the insects’ bodies. When they hatch, the females don’t carry the virus and die more quickly as well.
Scientists believe Wolbachia could be the magic bullet that will finally stop dengue in its tracks. But it will take time to work its way through the region’s insect population.
In the meantime, perhaps the region can take some comfort from research by America’s Duke University. It has just completed a study, which concludes that recent dengue infection might confer some immunity against Covid-19.