Abstract
BACKGROUND: Insecticide-based interventions, including insecticide-treated nets (ITNs) and indoor residual spraying (IRS), are central to malaria vector control in sub-Saharan Africa. In Côte d'Ivoire, increasing insecticide resistance in the key malaria vector, Anopheles gambiae sensu lato (An. gambiae s.l.), has been reported across the country, potentially compromising the current ITN-based control strategy. To assess the feasibility of supplementing control efforts with IRS using clothianidin as an alternative insecticide in areas with high malaria prevalence, we have examined the intensity and molecular mechanisms of insecticide resistance in wild An. gambiae s.l. populations with a focus on pyrethroids and the neonicotinoid clothianidin. METHODS: Using the World Health Organization (WHO) insecticide susceptibility test, we assessed the intensity of insecticide resistance in 2- to 5-day-old female An. gambiae s.l. mosquitoes from Sakassou, Central Côte d'Ivoire, collected locally at the larval stage, against the pyrethroids alpha-cypermethrin, deltamethrin and permethrin, the neonicotinoid clothianidin and the organophosphate pirimiphos-methyl. To characterise the mechanisms underlying insecticide resistance, we conducted synergist assays using piperonyl butoxide (PBO) in combination with pyrethroids. Additionally, we performed diagnostic PCR to determine the An. gambiae s.l. sibling species and to characterise resistance mechanisms targeting the knockdown resistance (kdr) markers L995F and L995S and the insensitive acetylcholinesterase (Ace-1(R)) G280S resistance marker. We also compared expression levels of the key cytochrome P450 monooxygenases (P450s) CYP6M2, CYP6P3, CYP6P4 and CYP6P5 between field-collected mosquitoes and those from a laboratory insecticide-susceptible colony. RESULTS: The diagnostic PCR identified all An. gambiae s.l. specimens that yielded a positive result as Anopheles coluzzii. Anopheles coluzzii individuals showed resistance to all pyrethroids tested, with very low mortality rates for alpha-cypermethrin and deltamethrin (range: 0% to 4%) and permethrin (range: 0% to 1%). In contrast, mortality rates against the organophosphate pirimiphos-methyl ranged from 52% to 97%. However, all mosquitoes remained fully susceptible to the neonicotinoid clothianidin (100% mortality). Pre-exposure to PBO increased the mortality rates following exposure to pyrethroids but did not restore susceptibility completely, with mortality rates of between 4.9% and 30.7% for alpha-cypermethrin, between 5.4% and 68.2% for deltamethrin and between 1.9% and 18.7% for permethrin. Among the four key resistance genes assessed in pyrethroid resistance, only the CYP6M2 gene showed significant overexpression (fold change 2.1, p-value = 0.025) in the field-sampled An. coluzzii compared to the susceptible laboratory colony. Taken together, our results suggest the involvement of a metabolic resistance mechanism. In addition to metabolic resistance, we detected all three target-site resistance alleles (i.e. kdr-L995F, kdr-L995S and Ace.1-G280S), with kdr-L995F predominating, with allelic frequencies ranging from 53% to 71% across years. CONCLUSIONS: Anopheles coluzzii mosquitoes from Sakassou show high levels of resistance to pyrethroids but remain susceptible to clothianidin, indicating that clothianidin-based IRS may serve as an effective complementary strategy for malaria control.