Characterizing phenotypic and genotypic resistance to insecticide in Anopheles gambiae s.l. from the West Nile region in Uganda

Abstract

In sub-Saharan Africa, Anopheles gambiae sensu lato and Anopheles funestus mosquitoes are the major vectors of Plasmodium that cause malaria; a leading cause of mortalities and morbidity in humans. In order to control Anopheles mosquitoes, Uganda over the past decades has been using indoor residual spraying (IRS) and long-lasting insecticidal nets (LLINs) as recommended by the World Health Organization. However, the effectiveness of these methods has been reduced due to several factors, including reported resistance to insecticides and behavioural change among Anopheles mosquitoes. The management of resistance to insecticides is challenged by limited information on the resistance profile of primary malaria vectors in high transmission regions like the West Nile. This study determined the species diversity, susceptibility and genotypic resistance to insecticides among Anopheles gambiae s.l. collected from October 2023 to January 2024 in Maracha and Terego districts. Larval samples from selected aquatic sites were reared to adults in the insectary at the IDRC laboratory in Butabika, Kampala. Morphologically identified adult females of Anopheles gambiae s.l. were exposed to different diagnostic dosages of deltamethrin, alpha-cypermethrin, pirimiphos-methyl, bendiocarb and pyrethroid (deltamethrin and alphacypermethrin) with piperonyl butoxide (PBO) in the WHO-tube at 25°C and 80% humidity. Percentage mortalities after insecticide exposure were determined after 24 hours. Morphologically identified adults were further differentiated into subspecies using multiplex polymerase chain reaction. Resistance markers Vgsc-L995F/L1014F (kdr-West), Vgsc-L995S/L1014S (kdr-East) and Cyp4j5-L43L were characterised using PCR methods in insecticide exposed mosquitoes. Results showed that the districts of Maracha and Terego consisted of two subspecies: Anopheles gambiae s.s and An. arabiensis. Anopheles gambiae s.l showed high levels of resistance to pyrethroids (3134% mortality) which was significantly associated to Vgsc-L995F/L1014F and VgscL995S/L1014S in Terego (p= 0.03) and Maracha (p= 0.04). The species however had moderate resistance to bendiocarb and full susceptibility to pirimiphos-methyl (100% mortality). Resistance to insecticide was not significantly attributed to Cyp4j5-L43L in samples from both districts. PBO partially improved the susceptibility to pyrethroids but was still below the recommended level by WHO. This indicates presence of metabolic resistance in Anopheles gambiae s.l in West Nile region. Understanding mutations that cause metabolic resistance is therefore important in order to develop more effective vector control method.