Resting Behaviour of African Malaria Vectors in an Era of High Indoor Insecticide Use

Abstract

Long Lasting Insecticidal Nets (LLINs) and indoor residual spraying (IRS) represent powerful tools for controlling malaria vectors in sub-Saharan Africa. The success of these interventions relies on their ability to inhibit indoor feeding and resting of malaria mosquitoes. This study sought to understand the interaction of insecticide resistance with indoor and outdoor resting behavioural responses of malaria vectors from Western Kenya. Mark-release-recapture experiments were used to investigate the plasticity of indoor and outdoor resting behaviour while parity rates were used to estimate the physiological ages of Anopheles mosquitoes collected from Kisumu (Kisian) and Bungoma (Kimaeti) counties in Western Kenya. The status of insecticide resistance among indoor and outdoor resting anopheline mosquitoes was investigated in Anopheles mosquitoes collected from study sites. The level and intensity of resistance were measured using WHO-tube and CDC-bottle bioassays, respectively. The mutations at the voltage gated sodium channel (Vgsc) knock down resistance (kdr) gene and Ace 1 gene were characterized using PCR. Microplate assays were used to measure levels of detoxification enzymes, if present. Sporozoite rates were assessed by ELISAs for Plasmodium falciparum circumsporozoite protein. A total of 1094 samples were discriminated within Anopheles gambiae s.l. and 289 within An. funestus s.l. In Kisian (Kisumu County), the dominant species was Anopheles arabiensis 75.2% (391/520) while in Kimaeti (Bungoma county) collections the dominant sibling species was Anopheles gambiae s.s 96.5% (554/574). The An. funestus s.l samples analyzed were all An. funestus s.s from both sites. Pyrethroid resistance of An. gambiae s.l F1 progeny was observed in all sites. Lower mortality was observed against deltamethrin for the progeny of indoor resting mosquitoes compared to outdoor resting mosquitoes (Mortality rate: 37% vs 51%, P=0.044). The intensity assays showed moderate-intensity resistance to deltamethrin in the progeny of mosquitoes collected from indoors and outdoors in both study sites. In Kisian, the frequency of vgsc-L1014S and vgsc-L1014F mutation were 0.14 and 0.19 respectively in indoor resting An. gambiae s.l mosquitoes while those of the outdoor resting An. gambiae s.l mosquitoes were 0.12 and 0.12 respectively. The ace 1 mutation was present in higher frequency in the An. gambiae s.l F1 of mosquitoes resting indoors (0.23) compared to those of mosquitoes resting outdoors (0.12). In Kimaeti, the frequencies of vgsc-L1014S and vgsc-L1014F were 0.75 and 0.05 respectively for the F1 of An. gambiae s.l collected indoors whereas those of outdoor resting ones were 0.67 and 0.03 respectively. The ace 1 G119S mutation was present in progeny of An. gambiae s.l mosquitoes from Kimaeti resting indoors (0.05) whereas it was absent in those resting outdoors. Monooxygenase activity was elevated by 1.83 folds in Kisian and by 1.33 folds in Kimaeti for An. gambiae s.l mosquitoes resting indoors than those resting outdoors respectively. xiv The study recorded high resting behavioural plasticity, physiological (phenotypic, metabolic and genotypic) insecticide resistance and sporozoite rate in indoor resting populations of malaria vectors compared to their outdoor resting counterparts. The indication of moderate resistance intensity and for the indoor resting mosquitoes is alarming as it could have an operational impact on the efficacy of the existing indoor pyrethroid based vector control tools.