Dengue Fever Activity in Sri Lanka Remains Elevated Amid Persistent Rainfall and Flood Conditions

• Dengue fever activity is likely to remain elevated through the southwest monsoon period. 

• The Western Province is most affected due to high population density, mobility, and persistent urban breeding environments.  

• Healthcare facilities in high-burden areas are likely to remain under sustained operational pressure.  

• Any shift in circulating dengue fever serotypes could increase population susceptibility to secondary infections.  

• Current vector control efforts are unlikely to fully contain the outbreak without improved waste management, drainage maintenance, and community-level source reduction.  

The Western Province is the most affected, accounting for 29,093 cases, or 52.5 percent of the national total. Within the province, the Colombo District has reported 11,392 cases, while the Gampaha District has reported 11,006 cases. Together, these two densely populated urban centers and regional transport hubs account for nearly 40 percent of all reported infections and continue to drive national transmission. Other affected provinces include Southern (8,822 cases), Sabaragamuwa (4,843 cases), Central (4,234 cases), Eastern (2,783 cases), North Western (2,458  cases), Uva (1,382 cases), Northern (896 cases), and North Central (895 cases).

Transmission has persisted since January but accelerated sharply following heavy rainfall and flooding in mid-May. Health authorities have linked this increase in part to unplanned urbanization and environmental conditions, such as debris from Cyclone Ditwah, that have expanded mosquito breeding habitats across most of the country. 

Local authorities have intensified dengue fever control measures in response to the recent surge in cases. Nationwide clean-up campaigns are targeting sites where standing water may accumulate, with additional prevention and awareness programs concentrated in Colombo and other high-risk districts. 

Dengue fever is endemic in Sri Lanka, with cases reported throughout the country. Transmission follows a well-defined seasonal pattern, with two annual peaks typically occurring during and after the southwest monsoon (May-September) and the northeast monsoon (October-January). Increased rainfall during these periods creates favorable breeding conditions for Aedes mosquitoes, particularly in areas with inadequate drainage and water management. The Western Province, particularly the Colombo, Gampaha, and Kalutara districts, has consistently reported the highest dengue fever burden.

Since the early 2000s, Sri Lanka has experienced several major dengue fever epidemics, including large outbreaks in 2002, 2004, 2009, 2017, and 2019. The 2017 outbreak remains the country's largest on record, with more than 186,000 cases reported.  

Several factors continue to complicate outbreak control efforts. Urban areas such as Colombo and Gampaha function as major economic and transportation hubs, facilitating the movement of large numbers of people daily. Rapid urbanization, population growth, and unplanned development have further expanded mosquito breeding habitats.

Entomological surveys indicate that a substantial proportion of breeding sites are located in semi-public and public spaces, including schools, government facilities, workplaces, religious institutions, and construction sites. These settings often lack clearly assigned responsibility for routine vector control, allowing breeding habitats to persist even where household-level awareness and prevention measures are relatively strong. 

Environmental conditions remain highly conducive to transmission. Inadequate waste collection allows water-holding containers to accumulate in public spaces, while poorly maintained drainage systems and flood-damaged infrastructure produce the standing water Aedes mosquitoes require. Additionally, the post-cyclone debris environment and ongoing monsoon rainfall create conditions that are difficult to remediate rapidly through vector control alone, particularly across densely populated urban and peri-urban areas. 

Surveillance, community outreach, and risk communication activities also face limitations. Although public awareness of dengue fever is generally high, this knowledge does not consistently translate into sustained preventive behaviors. Household source reduction remains inconsistent, and monitoring is not always sufficient to detect emerging transmission hotspots at an early stage. 

Operational constraints present additional challenges. Response efforts continue to be affected by limited human resources, healthcare capacity pressures, possible insecticide resistance among mosquito populations, and logistical constraints. Health authorities have warned that a further rise in patient numbers could put public hospitals under severe strain. Furthermore, dengue fever's clinical similarity to other febrile and arboviral illnesses can complicate early diagnosis and treatment. 

Dengue fever activity is likely to remain elevated throughout the remainder of the monsoon season, particularly in densely populated urban areas. The highest risk is expected to remain concentrated in Colombo and Gampaha districts, where population density, extensive transport networks, ongoing construction activity, and environmental conditions continue to favor transmission. Healthcare facilities in these areas are likely to face sustained pressure if case numbers continue to rise.

There is also concern among health officials that changes in the dominant circulating dengue serotype could increase the proportion of secondary infections, raising the risk of severe disease. While the national case fatality rate remains low by historical standards, delayed healthcare-seeking behavior remains a concern because early symptoms often resemble influenza and other common viral illnesses. Patients who present late may be at greater risk of severe dengue and dengue hemorrhagic fever, further straining hospital and laboratory capacity.  

The following basic precautions may help protect travelers from disease transmission: 

• Cover as much skin as possible; wear long sleeves, long trousers, socks, shoes/boots, and a hat. Avoid sandals, and tuck shirts into pants and pant legs into socks. 

• Sleep in protected areas with screens or air conditioning, or use a permethrin-impregnated sleeping bed net. Check and eliminate any mosquitoes from inside rooms and sleeping areas before retiring.  

• Insecticide sprays may be used to clear rooms of mosquitoes. Coils should only be used outdoors; avoid using coils that contain DDT. Be aware that many foreign-produced repellents and insecticides are extremely toxic. 

• Use an insect repellent that contains DEET (around 35 percent) on exposed areas of skin and clothing. DEET formulations of as high as 50 percent are recommended for both adults and children older than two months of age. All insect repellents should be applied after sunscreen and reapplied after swimming or bathing. Avoid eyes and inhalation of the spray. 

• Repellents containing permethrin can be sprayed on clothing, bed nets, and camping gear. This is an effective repellent for mosquitoes, ticks, and other insects.

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