Assessing Dengue Vector Seasonality and Insecticide Susceptibility: Findings from an Ovitrap Survey in Colombo District, Sri Lanka

Introduction: With a high incidence of dengue in the Colombo District of Sri Lanka, recorded as 17,456 cases in 2022 and 18,242 cases in 2023. Maharagama recorded 1,095 dengue cases in 2018, 2,292 cases in 2019, and 1,453 cases in 2022.
Objectives: The study aimed to investigate the seasonal patterns and insecticide susceptibility of Aedes aegypti and Aedes albopictus.
Methodology: An ovitrap-based survey was conducted in Maharagama MOH area from October 2022 to September 2023. Conventional ovitraps were deployed inside and outside 50 randomly selected houses to capture Aedes mosquitoes breeding in both environments, with collections made after five nights. Counted eggs were allowed to hatch and after the identification of larvae were reared under the laboratory conditions. Adults aged 2–3 days were exposed to common insecticides according to the WHO protocol.
Results: Outdoor ovitraps collected a total of 459 Ae. aegypti and 6,610 Ae. albopictus, while indoor ovitraps collected 566 Ae. aegypti and 4,521 Ae. albopictus. The Aedes ovitrap index was highest in December (62%) and April (55%). Ae. aegypti and Ae. albopictus populations fluctuated with the changing seasons. The density of Ae. aegypti peaked in March and May, whereas Ae. albopictus reached its highest levels in January and April. Ae. albopictus showed resistance to 0.8% Malathion, (mortality = 5%), but was susceptible to 4% Malathion, (mortality = 99%). Ae. aegypti was fully susceptible to both 5% Malathion and 0.25% Deltamethrin, with a mortality rate of 100% for each.
Conclusion: The persistent occurrence of the primary dengue vector may have contributed to the high prevalence of dengue in the district. Since Ae. aegypti is more prone to outbreaks and breeds more prevalently indoors (55%) than outdoors, this should be considered when formulating dengue vector control programs