The Sri Lankan military has been deployed to help fight a spiralling dengue outbreak, after more than 1,000 people were infected in a single day.

The South Asian country has seen a sharp uptick in cases of the mosquito-borne disease in the last month, with some 19,000 cases so far reported in June – more than double the 8,590 recorded in May, according to official data.

There are mounting concerns that the escalating toll threatens to overwhelm the health system. Researchers in the capital Colombo, which is at the heart of the outbreak, told the Telegraph that dengue patients are already sharing beds or sleeping on the floor, as specialist units and generalised wards are flooded with people.

“Hospitals are absolutely full, in public hospitals you have two patients per bed and in some hospitals there are even patients on the floor,” said Prof Neelika Malavige, a dengue specialist at the University of Sri Jayewardenepura and the Sri Jayewardenepura General Hospital – one of the country’s largest healthcare institutions.

“If you remember the situation during Covid times, when we had cases increasing like a flagpole – we have a similar situation here as far as dengue is concerned,” she said.

In an effort to curb transmission, the President’s office said a special unit of army, navy and air force officers will be deployed to identify and destroy mosquito breeding sites, as part of a larger national clean-up “mission” launching on Wednesday.

“Laws will also be strictly enforced against those allowing mosquito breeding on their premises, in addition to setting up the special military unit,” the government added after a dengue situation briefing this week.

The law requires that householders and landowners take care not to allow pools of stagnant water to gather on their land.

The global threat posed by dengue has been rising across the globe for years, as climate change, urbanisation and international travel expand the territory where the Aedes Albopictus (or Asian tiger) mosquito can live and thrive.

But researchers are also racing to understand why the virus has erupted so severely in Sri Lanka now, including whether the virus has mutated or outcompeted other diseases, as the current outbreak is the country’s largest in years.

It has raised uneasy memories of 2017, when Sri Lanka saw 186,000 patients and 440 deaths in a massive epidemic – still far higher than the 47,000 cases so far detected this year.

Dengue, sometimes called ‘breakbone fever’, causes symptoms including a headache, high fever, muscle pain and, in severe cases, extreme bleeding. Although it can be deadly, the case fatality rate is low, at around 0.6 per cent. Yet the disease is debilitating.

The virus can take weeks to recover from and sometimes leaves lasting health issues, extracting a significant toll on productivity and the economy. It can also clog up health facilities, which has knock-on impacts on a range of other conditions. While a vaccine is available, it has not yet been approved in Sri Lanka.

“Hospitals are already under pressure,” Dr Kapila Kannangara, head of the National Dengue Control Unit, warned this week. “We don’t want to have a situation like the one we faced in 2017.”

Prof Malavige said the current transmission may be linked to a major cyclone in December, followed by extremely hot weather in March and April, which created the ideal breeding ground for mosquitoes.

Cases are also highest in areas of Colombo and neighbouring Gampaha which have seen rapid urbanisation – the Asian tiger mosquito is a city-dweller.

But her research team is also exploring other hypotheses about why the virus has found such fertile ground in Sri Lanka this year.

First, that the virus has rebounded after chikungunya – another arbovirus that’s spread by the same mosquito – “outcompeted” it last year. Sri Lanka saw its first major outbreak of the virus in 16 years in 2025, as the disease surged across the globe, and this may have dampened dengue’s ability to transmit.

Another unconfirmed theory is that dengue has evolved to better infect humans. Viruses are prone to mutations, and many of these changes have little real world impact. But Prof Malavige is investigating whether the serotype currently responsible for most of the current outbreak – dengue 2 – has any significant changes. She expects answers “within weeks”.

“It’s our dengue season and we usually have outbreaks [at this time] every year, but not this many cases,” she said. “Understanding why is the most important question… and this is not just something related to Sri Lanka.

“Dengue has caused outbreaks all over the world because of climate change and rapid, unplanned urbanisation. But when there is higher transmission everywhere, it also means the virtual evolution speeds up,” Prof Malavige said. “So surveillance is important.”

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