First report of widespread kdr-L995F pyrethroid-resistant An. arabiensis and Temporal trends of pyrethroid resistance in urban Ilorin, Kwara State, Nigeria

Effectiveness of currently deployed insecticide-based malaria vector control measures depends on continuous insecticide resistance monitoring and management. Urban malaria is becoming a serious health problem in Africa partly due to vector resistance in urban areas. To provide insecticide resistance trends information for effective resistance management, this study assessed insecticide susceptibility status and occurrence of knock down resistance mutation in Anopheles mosquitoes from urban Ilorin, Nigeria in two different years. Malaria vectors in urban Ilorin were assayed for susceptibility to WHO-recommended insecticides in 2017 and 2020 using WHO test kits. Sub-sets of exposed An. gambiae s.l. mosquitoes were PCR-assayed for sibling species and knock down resistance (kdr) mutation detection. Full mosquito susceptibility (98–100% mortality; Niger and Baruba) and suspected resistance (95–97% mortality; Shao garage and Edun) to alphacypermethrin were recorded in 2017. Significantly (p ≤ 0.010) reduced mosquito mortalities indicating increased alphacypermethrin resistance were recorded in 2020. Permethrin, lambda-cyhalothrin and DDT resistance increased significantly (p ≤ 0.03) from 2017 (42–89%) to 2020 (5.8–58%) but not deltamethrin (p ≥ 0.086). Mosquitoes tested were mostly susceptible to bendiocarb and pirimiphos-methyl. The kdr-L995F detected increased from 15 to 20% for An. arabiensis, 24–36% for An. gambiae and 0–18% for An. coluzzii from 2017 to 2020. For both 2017 and 2020, most of the pyrethroid resistant (≥ 80%) and susceptible (≥ 75%) mosquitoes from each site were identified as An. arabiensis. This study presents a first report on widespread occurrence of kdr-L995F mutation in An. arabiensis mosquito populations in urban Ilorin, Kwara State, Nigeria. The report of pyrethroid-resistant An. arabiensis prevalence in urban Ilorin triggered the switch from pyrethroid only to dual active ingredient ITN distribution in the city since 2020. Extension of dry season periods due to climate change may drive more preponderance of An. arabiensis in the study area. Therefore, the implementation of supplementary vector control measures targeted at An. arabiensis is required in this locality.