Characterizing the escalation of pyrethroid resistance and its impact on bed nets efficacy alongside molecular basis in Anopheles funestus from Cameroon.

Background: Malaria prevention still relies greatly on vector control interventions. However, increasing levels of resistance to pyrethroids across Africa have significantly reduced the efficacy of pyrethroid-based interventions leading to an increase of malaria burden. Consequently, it is imperative to characterize the extent and molecular basis of this resistance.

Methods: This study was conducted from 2020 to 2021 in a South-North transect across Cameroon. WHO tube assay was used to assess the susceptibility profile of Anopheles funestus to the four main classes of insecticides. The efficacy of bed nets was evaluated using cone assay. Known genetic resistance markers and gene expression were determined using PCR and quantitative PCR techniques, respectively. Taqman assay and nested polymerase chain reaction (PCR) were used to determine Plasmodium sporozoite infection.

Results: High pyrethroid resistance intensity was noticed in all sites with mortalities ranging from 80-93.9%, 84.9-96.7% and 82% for permethrin, deltamethrin and alphacypermethrin at 10 × concentration respectively. This high level of resistance led to dramatic inefficacy of pyrethroid-only nets with 0-17% mortality recorded 24-h post exposure while PBO-based nets displayed optimal efficacy. Sporozoite infection rates ranged from 0-16.5% across the study sites. However, there was no clear relationship between the infection rate and the intensity of pyrethroid resistance. The L119F-GSTe2 allele was higher in the South (56-68%) compared to the North (20-37%) meanwhile the P450-linked 4.3 kb structural variant was fixed contrasting with the absence of the CYP6P9a/b-R, 6.5 kb insertion and N485I-Ace1 alleles. Furthermore, the L119F-GSTe2 allele confers significant ability to mosquito to survive permethrin. In addition, the CYP325A, CYP6P5, CYP6P9a/b, and the Carb2514 were the most overexpressed genes in pyrethroid resistant mosquitoes. However, no further association was noticed between these alleles/genes and increasing doses of pyrethroids.

Conclusion: This study confirms the escalation of pyrethroid resistance across Cameroon and the inefficacy of pyrethroid-only nets and highlights genes potentially implicated in the aggravation of insecticide resistance with implications on vector control strategies.