Urban evolution of insecticide resistance and susceptibility patterns of Anopheles coluzzii in Southeastern Nigeria: implications for malaria vector control.

Insecticide resistance among malaria vectors remains a critical obstacle to effective vector control and malaria elimination across sub-Saharan Africa. This study assessed the phenotypic and genotypic resistance profiles of Anopheles coluzzii populations in three urban centres of Anambra State, Southeastern Nigeria: Awka, Onitsha and Nnewi. Larvae were collected from Awka, Onitsha and Nnewi and reared to adults for WHO insecticide susceptibility tests. Species identification, molecular form determination (PCR-RFLP using HhaI enzyme) and screening for kdr L1014F and ace-1 G119S mutations were performed. Molecular diagnostics confirmed that all specimens were An. coluzzii. Mortality to deltamethrin was low across all sites (Awka: 63%, Onitsha: 61% and Nnewi: 45%), indicating resistance. DDT also showed markedly reduced susceptibility (Awka: 56%, Onitsha: 25% and Nnewi: 20%), consistent with resistance. In contrast, full susceptibility was observed to bendiocarb (100%) and pirimiphos-methyl (100%) in all locations. The kdr 1014F allele occurred at high frequencies across all locations (Awka: 0.72, Onitsha: 0.77 and Nnewi: 0.81), predominantly represented by RR and RS genotypes. In contrast, the ace-1 G119S allele was present at very low frequencies (Awka: 0.07, Onitsha: 0.10 and Nnewi: 0.12), with most mosquitoes carrying the susceptible SS genotype. Urban An. coluzzii populations in southeastern Nigeria exhibit operationally significant resistance to pyrethroids and DDT, driven largely by high kdr allele frequencies, while remaining susceptible to carbamates and organophosphates. These patterns highlight the need for continued resistance surveillance and tailored insecticide-based interventions.