Phenotypic and genotypic insecticide resistance profiles of main malaria vectors in Kwale county, coastal Kenya.

BACKGROUND

Long-lasting insecticidal nets are the primary malaria vector control measure in coastal Kenya. In 2018, phenotypic resistance to pyrethroids and low frequency of L1014S kdr mutation were reported in the Anopheles gambiae complex. Since then, additional pyrethroid-treated nets were distributed in 2021. The objectives of this study were to determine the insecticide resistance profiles of An. gambiae and Anopheles funestus from Kwale County and evaluate potential resistance mechanisms.

METHODS

From July 2023 to May 2024, adult and larval collections of An. funestus and An. gambiae mosquitoes were done with the aim of conducting insecticide susceptibility bioassays using WHO protocol for permethrin, deltamethrin, bendiocarb, DDT and pirimiphos-methyl. Species found resistant to pyrethroids were subjected to synergism testing by pre-exposure to piperonyl-butoxide (PBO). This was followed by genotyping of resistance-associated mutations in An. funestus (CYP6P9a, CYP6P9b, GSTe2-L119F and 6.5kb S.V) and An. gambiae (kdr L1014S and L1014F). Sibling species identification was done using PCR. The association between genetic markers and phenotypic resistance was explored using logistic regression.

RESULTS

A total of 1826 An. gambiae and 715 An. funestus were used in insecticide susceptibility bioassays. Both An. gambiae and An. funestus were resistant to permethrin (mortality, 58.7% and 57.1, respectively) and deltamethrin (mortality 51% and 76%, respectively), but susceptible to DDT, bendiocarb and Pirimiphos-methyl. Pre-exposure to PBO increased susceptibility to deltamethrin in both species. Both kdr west and east were detected in Anopheles arabiensis (L1014S freq = 0.083, L1014F freq = 0.063) and Anopheles quadriannulatus (L1014S freq = 0.074, L1014F freq = 0.043) at low frequencies. Anopheles funestus sensu stricto and Anopheles rivolurum had the presence of CYP6Pa, CYP6Pb, 6.5kb S.V and GSTe2-L119F, with low allele frequencies. There were no significant associations between the genotypes and phenotypic profile.

CONCLUSIONS

Malaria vectors in Kwale are resistant to pyrethroids. PBO fully restored susceptibility, indicating this resistance could be caused by metabolic mechanism. The presence of kdr and metabolic resistance alleles suggests a recent selection on Anopheles mosquitoes. Pyrethroid-only nets may not fully ensure community protection against malaria in coastal Kenya due to resistance. Operational failure remains uncertain, requiring further studies. Net distribution programs should consider pyrethroid-PBO nets to enhance malaria control effectiveness.