Activity and bionomic aspects of primary and secondary vectors for dengue transmission in Gampaha district, Sri Lanka

Abstract

Sri Lanka had experienced periodic dengue epidemics every two to three years for the last two decades. In this scenario, understanding bionomics aspects, and the prevalence of dengue vector species in varied settings will help to develop more accurate and focused vector control approaches. Thus, the current study is intended to elucidate the bionomics of Aedes aegypti and Ae. albopictus, in urban (Negombo, Wattala, Kelaniya), suburban (Attanagalla, Gampaha, Minuwangoda) and rural (Dompe, Meerigama, Divulapitiya) areas of the Gampaha District, Sri Lanka from April 2017 to December 2019. The effectiveness of the insecticide space spraying depends on susceptibility status and the behaviour of the targeted species, hence a part of this study included testing susceptible levels of Ae. aegypti and Ae. albopictus against malathion and deltamethrin insecticides. The study also sought to establish area-specific threshold values for determining the risk of dengue transmission based on larval indices, and a forecasting model for predicting impending dengue outbreaks within the district. Total 19,835 possible breeding habitats were investigated at 13,563 premises where Aedes larvae were identified in 11.03% of premises and 1856 habitats were positive for larvae (Ae. aegypti; 9.8%; Ae. albopictus; 90.2 %). Results showed that Ae. aegypti prefers urban locations while Ae. albopictus is the predominant vector in all spatial settings. Ae. albopictus (54.5%; n= 999) dominated the adult collection, followed by Ae. aegypti (45.5%; n= 835), with a 1:4 male to female ratio. Ae. aegypti mosquitoes demonstrated endophilic resting behaviour, whereas Ae. albopictus were exophilic (Chi-square analysis between the two species; P < 0.001). Resting places of Aedes aegypti recorded as on cloth hangings (36.9%, n=308) and under furniture (40.4%) predominantly in bedrooms (40.4%) and living rooms (24.8%) while Ae. albopictus preferred to rest on outdoor vegetation Xxix 46%(n=460). The majority (43.7 %; n=801) of Aedes mosquitoes rest 1-2 m above ground, and 34.4 % (n=399) rested 1 m or less. The host-seeking cycle of Ae. albopictus was bimodal, with morning peak occurring between 05:00 and 11:00 and afternoon peak between 14:00 and 19:00. While that of Aedes aegypti was with a minor peak between 05:00 and 09:00 and a major peak between 13:00 and 19:00. Physicochemical parameters of mosquito breeding water changed significantly across breeding categories (Kruskal—Willi’s statistics, p < 0.001) showing the change of mosquito susceptibility status. There was a strong positive association between rainfall, larval vector density, and the likelihood of high dengue incidence. When Breteau Index for Ae. aegypti (BIA) exceeds 3.00 and relative humidity exceeds 80%, an early epidemic alert is triggered while BIA > 6.0, case incidence reached an epidemic level in urban areas. In suburban areas, when BI for Ae. albopictus (BIB) > 14.0 an early epidemic alert is triggered. At the BIB > 20, case incidence reached epidemic levels even in absence of Ae. aegypti. Case incidence exceeds the epidermic level in rural areas when the BI for Ae. albopictus is 10 with a one-month latency. In conclusion, area-based entomological thresholds and rational use of insecticides can be proposed for future control of rising dengue epidemics in the Gampaha district based on vector biology/bionomic related evidence through regular monitoring.