Elevated Plasmodium sporozoite infection and multiple insecticide resistance in the principal malaria vectors Anopheles funestus and Anopheles gambiae in a forested locality close to the Yaoundé airport, Cameroon

Abstract

<b>Background:</b> Reducing the burden of malaria requires better understanding of vector populations, particularly in forested regions where the incidence remains elevated. Here, we characterized malaria vectors in a locality near the Yaoundé international airport, Cameroon, including species composition, abundance, <i>Plasmodium</i> infection rate, insecticide resistance profiles and underlying resistance mechanisms. <b>Methods:</b> Blood-fed adult mosquitoes resting indoors were aspirated from houses in April 2019 at Elende, a locality situated 2 km from the Yaoundé-Nsimalen airport. Female mosquitoes were forced to lay eggs to generate F ₁ adults. Bioassays were performed to assess resistance profile to the four insecticides classes. The threshold of insecticide susceptibility was defined above 98% mortality rate and mortality rates below 90% were indicative of confirmed insecticide resistance. Furthermore, the molecular basis of resistance and <i>Plasmodium</i> infection rates were investigated. <b>Results:</b> <i>Anopheles funestus</i> s.s. was the most abundant species in Elende (85%) followed by <i>Anopheles gambiae</i> s.s. (15%) with both having similar sporozoite rate. Both species exhibited high levels of resistance to the pyrethroids, permethrin and deltamethrin (<40% mortality). <i>An. gambiae</i> s.s. was resistant to DDT (9.9% mortality) and bendiocarb (54% mortality) while susceptible to organophosphate. <i>An. funestus</i> s.s. was resistant to dieldrin (1% mortality), DDT (86% mortality) but susceptible to carbamates and organophosphates. The L119F-GSTe2 resistance allele (8%) and G119S <i>ace</i>-1 resistance allele (15%) were detected in <i>An. funestus</i> s.s. and <i>An. gambiae</i> s.s., respectively <i>.</i> Furthermore, the high pyrethroid/DDT resistances in <i>An. gambiae</i> corresponded with an increase frequency of 1014F <i>kdr</i> allele (95%). Transcriptional profiling of candidate cytochrome P450 genes reveals the over-expression of <i>CYP6P5</i>, <i>CYP6P9a</i> and <i>CYP6P9b.</i> <b>Conclusion:</b> The resistance to multiple insecticide classes observed in these vector populations alongside the significant <i>Plasmodium</i> sporozoite rate highlights the challenges that vector control programs encounter in sustaining the regular benefits of contemporary insecticide-based control interventions in forested areas.