Dengue: What is the mosquito-borne disease that Sri Lanka is using drones to fight?

Battling its worst dengue outbreak in nearly a decade, Sri Lanka has deployed military drones to scan for stagnant pools of water on rooftops and other mosquito-breeding sites.

The virus has killed 56 people and infected more than 77,000 others in the South Asian nation this year, with more than a quarter of them reported in July alone.

Dengue fever, which is spread to humans through mosquitoes, is common in tropical climates.

But as temperatures rise worldwide, these disease-carrying mosquitos are now moving into new areas, including Europe and the Mediterranean. According to the WHO, the number of global reported cases surged from around 500,000 in the year 2000 to 14.4 million in 2024.

Dengue is a viral fever caused by the dengue virus, which is transmitted to humans through the bite of infected Aedes mosquitoes.

These mosquitoes breed in stagnant water like pails, vases and flowerpot plates. They are most active during daylight hours and usually bite just after sunrise and around sunset.

Many people who get dengue do not develop symptoms. For those who do, the most common symptoms include high fever, headache, muscle and joint pains, nausea and rashes. These usually resolve within a week or two.

In severe cases, however, dengue can lead to internal bleeding or even death.

Those who get infected a second time are at a greater risk of severe dengue.

There are four strains of the dengue virus and healing from one would only protect you from that specific strain, says Kapila Kannangara, director of Sri Lanka's Dengue Control Unit.

"That immunity does not extend to other strains of the virus. That means someone infected by one strain of the virus could still get dengue again if infected by a mosquito carrying another strain."

"This could mean that they get an even more serious infection," he tells BBC Sinhala.

As there is no universal vaccine for dengue, treatment often involves managing pain with medication such as paracetamol. Patients who develop severe symptoms would often require careful monitoring in hospital, as complications can be lethal.

Sri Lanka's health authorities attribute the surge in dengue infections to a cyclone that tore through the island last December, scattering debris that have turned into new breeding grounds for mosquitoes.

The government has tasked the military to use drones to scan the rooftops of homes, schools and construction sites, and deployed police officers to go to people's homes for physical inspections,

Nearly 11,000 mosquito breeding sites have been cleared so far this month, while more than 4,000 premises fined for harbouring those sites.

Although infections have been reported across the country, they tend to concentrate in urban areas, such as in the capital city Colombo.

Hospitals have added beds and extended work hours to cope with the outbreak, and public health workers are "stretched to their limit", Kannangara from the National Dengue Control Unit tells Reuters news agency.

"The number of patients is high, and the severity of the cases is high," he says, noting that about 75% of the cases were linked to the more virulent dengue strain.

Capacity constraints have prompted doctors to advise patients with mild symptoms to rest and recover at home.

Authorities are also considering a dengue vaccine, but health experts have suggested that it would be hard to develop such a vaccine given the four distinct strains of the virus.

It is not the first time Sri Lanka has experienced a dengue epidemic.

In 2017, the disease left 450 people dead, in a year driven by heavy monsoon rains.

Dengue is most commonly found in tropical and sub-tropical climates worldwide, where warm temperatures and high humidity allow the Aedes mosquitoes to thrive.

High concentrations of dengue hotspots are found in South and South East Asia, Latin America and the Caribbean.

But rising temperatures worldwide has helped these disease-carrying mosquitos push into new regions like Europe and the Mediterranean.

Wetter and longer monsoons are giving them more time to reproduce in the tropics.

The incidence of dengue has increased 30-fold over the last 50 years, according to the World Health Organisation, which categorises the disease among the top ten global health threats.

Nearly half of the world's population are now at risk.

Removing stagnant water, such as by emptying flowerpot plates, overturning unused pails and breaking up hardened soil, can help break the dengue transmission cycle.

Some governments also use Wolbachia, a naturally occurring bacterium found in about half of all insect species, to curb the spread of mosquito-borne diseases like dengue.

The bacteria makes it harder for the insects to spread the virus, rather than kill them off.