Vector prevalence and insecticide resistance status of Aedes sp. in dengue high and low risk areas in the Colombo District

Abstract

BACKGROUND: In areas where dengue transmission is high, space spraying against adult Aedes sp. is practiced to reduce the density of infected mosquitoes and thereby reduce transmission within a very short time. Repeated use of insecticides in this manner, in dengue high risk areas, lead to the development of insecticide resistance in vector populations. The aim of the present study was to determine the prevalence and status of insecticide resistance of Aedes vectors in dengue high risk and low risk areas in the Colombo District. METHODS: The study was carried out in the area of Regional Director of Health Services, Colombo (RDHS area Colombo) and the Colombo Municipal Council area (CMC area), from December 2014 to June 2015. One each of comparatively high risk and low risk MOH area were selected from each study site based on the reported number of dengue cases during last five years. MOH areas Kolonnawa and D3 (Borella) were selected as high risk areas while Padukka and D5(Wellawatta) were selected as low risk areas respectively from the RDHS area Colombo and the CMC area. Ovitrap survey method was used to determine the vector prevalence and to collect Aedes eggs for bio assay tests. Larval and adult bio assay tests were conducted according to WHO guidelines. RESULTS: Both Ae. aegypti and Ae. albopictus were recorded from all MOH areas, except Padukka where there was only Ae. albopictus present. The vector prevalence in terms of Ovitrap Index and Egg Density Index was higher in Padukka (p=0.001 and p=0.029 respectively) than those in Kolonnawa in the RDHS area Colombo. In the CMC area, there was no such significant difference in the low risk and the high risk areas. Vector prevalence and the number of reported dengue cases showed no significant correlation in any of the MOH area. Aedes larvae in all four MOH areas have acquired resistance to diagnostic dosage of temephos (0.012 mg/l). Ae. aegypti in Kolonnawa, Ae. albopictus in Padukka and Ae. aegypti in D3 (Borella) showed possible resistance to 0.025% Deltamethrin while, Ae. aegypti in D5 (Wellawatta) has already acquired resistance. Ae. albopictus in D5 (Wellawatta) was still susceptible to the same insecticide. CONCLUSION: The vector prevalence does not significantly correlate with disease incidence, but further correlations with patients in localities within MOH and the vector densities will reveal reliable information. Despite the comparative status of high and low risk, all MOH areas have acquired resistance to insecticides, specifically to temephos and deltamethrin. Study reveals the emerging trends in resistance development of Aedes sp. in the Colombo District which has to be taken into account when designing further studies on resistance monitoring with a view to design effective control strategies.