• Dominant virus strain now accounts for about 75 per cent of infections 
• Cases exceed 52,000 and deaths rise to 31

The re-emergence of Dengue Virus Serotype 2 (DENV-2), which now accounts for around 75 per cent of confirmed dengue infections, together with the lingering effects of last year's cyclone and continued rainfall, has played a major role in the sharp increase in dengue cases this year, according to the National Dengue Control Unit (NDCU).

Speaking to The Daily Morning, NDCU Director Dr  Kapila Kannangara explained that DENV-2 should not be viewed as a newly emerging virus, but rather as the return of a strain that had previously been brought under control, although never completely eliminated.

"There is a misconception that DENV-2 disappeared after the major outbreak in 2017. We managed to control it, but we did not eliminate it. The virus is still present and so is the mosquito vector. Unless either the virus or the mosquito is eliminated, communicable diseases such as dengue will continue to recur," he said.

He explained that unlike bacterial infections, viruses cannot simply be eradicated with medicines. "There is no treatment that eliminates dengue viruses from the body. It is the body's immune system that clears the virus. Although DENV-2 was controlled after 2017, it continued to circulate between humans and mosquitoes."

He noted that the dominant dengue serotype changes over time. DENV-2 was the main circulating strain during the 2017 outbreak, before DENV-3 became dominant throughout 2023, 2024 and 2025. However, DENV-2 has once again become the predominant serotype this year.

"The viruses compete with one another while circulating between humans and Aedes aegypti and Aedes albopictus mosquitoes. At present, DENV-2 has overtaken DENV-3. Around 75 per cent of dengue patients currently have DENV-2 in their blood, while only about 25 per cent are infected with DENV-3."

According to Dr  Kannangara, the change in the dominant serotype has also increased the number of people vulnerable to infection. "For the past two to three years, many people developed immunity against DENV-3 because it was the predominant strain. Now that DENV-2 has become dominant again, more people are susceptible because their immune systems are less prepared for this serotype."

He also attributed the unusually high number of dengue cases this year to environmental factors. "We started 2026 with a much higher incidence than in previous years because of the cyclone experienced in November 2025. The floods and landslides created numerous mosquito breeding sites, while the debris left behind could not be cleared immediately by local authorities. As a result, dengue incidence in January itself was much higher than in 2024 and 2025."

The continued intermittent rainfall following the cyclone has further worsened the situation. "Normally, Sri Lanka records two dengue peaks associated with the South-West and North-East monsoons. This year, however, intermittent rainfall has continued almost without interruption since the cyclone, allowing mosquito breeding to continue throughout the period."

Dr  Kannangara also warned that poor waste management and public complacency continue to undermine dengue control efforts despite repeated awareness campaigns. "We continue to find discarded cups, cans, bottles and other containers collecting rainwater. Even if your own premises are free of breeding sites, mosquitoes can fly about 150 to 200 metres from neighbouring properties and transmit the disease. Unless communities remove breeding sites and local authorities improve waste collection, this situation will continue."

According to data published by the National Dengue Control Unit's NaDSys surveillance system, the number of reported dengue cases had exceeded 52,000 by midnight on 27 June, while the death toll had risen to 31. The NDCU has also identified 124 high-risk Medical Officer of Health (MOH) areas by the 25th epidemiological week of the year.