Intensity of Insecticide Resistance in the Major Malaria Vector Anopheles Funestus From Chikwawa, Rural Southern Malawi

Background: Malaria vector control using insecticide-based approaches, has proven to be an effective strategy. However, widespread insecticide resistance among malaria vector populations across sub-Saharan Africa threatens to derail control efforts. This study was conducted in Chikwawa district, an area in rural southern Malawi characterised by persistent malaria transmission and reports of insecticide resistance in the local mosquito population. The study aimed to characterise the intensity of insecticide resistance within a population of Anopheles funestus, a major vector of malaria in this district . Methods: Live adult females belonging to the An. funestus group were collected from households by indoor aspiration. A modified CDC bottle assay was used for phenotypic quantification of resistance to deltamethrin, permethrin and alpha-cypermethrin at 1X, 2.5X, 5X and 10X the recommended diagnostic doses. WHO tube assays were used to determine susceptibility to bendiocarb, dichlorodiphenyltrichloroethane (DDT) and pirimiphos-methyl insecticides at diagnostic concentrations. Results: Anopheles funestus exposed to 10x concentrations was highly resistant to alpha-cypermethrin (mortality 95.4%) whereas there was 100% mortality to both deltamethrin and permethrin at the same dose. Despite susceptibility at 10X, mortality to deltamethrin and permethrin at 5X was 96.7% and 97.1% respectively, indicating moderate resistance to the two insecticides. WHO susceptibility assays indicated strong resistance against bendiocarb (mortality 33.8%, exposed n= 93) whereas there was full susceptibility to DDT (mortality 98.9%, n= 103) and pirimiphos-methyl (mortality 100%, n= 103). Conclusions: Strategies for managing resistance, particularly against pyrethroids, must be urgently implemented to maintain the effectiveness of bed nets, which remain the primary vector control intervention in the area. This includes the wide scale introduction of third generation synergist ITNs and next-generation dual active ingredient ITNs. The use of effective non-pyrethroids such as pirimiphos-methyl, clothianidin and potentially DDT, could provide a window of opportunity for IRS across the district. This would support the current Malawi Insecticide Resistance Management Plan which aims at rotating insecticides to minimise selection pressure and slow down the evolution of resistance to approved insecticides. This will help to prevent malaria vector control failure and improve progress towards malaria elimination.