Detection of Plasmodium falciparum-infected Anopheles sergentii with reduced insecticide net susceptibility in a dryland ecosystem of the Kenyan Rift Valley: implications for malaria surveillance and vector control

Abstract

Abstract Background In Kenya, parts of the dryland ecosystems have disproportionately witnessed a surge in malaria burden amidst poor understanding of the entomologic risk factors. This study investigated whether this trend is linked to shifts in the vector landscape, including the cryptic role of potential new vectors. Methods Adult female anopheline mosquitoes were collected in a village in semi-arid to arid Marigat subcounty, Baringo County of the Kenyan Rift Valley. This was achieved using paired CDC light traps placed indoors and outdoors in randomly selected households during sampling periods, in August 2023, August 2024, and July 2025. A subset of the mosquitoes morphologically identified as Anopheles funestus (s.l.) were analysed by PCR and sequencing for species identification, malaria parasite infection and host meal sources. In addition, using cone bioassays, the efficacy of single and dual-ingredient insecticide-treated nets was evaluated on the mosquito population. Results Analysis of the mosquitoes revealed presence of established Anopheles sergentii populations infected with Plasmodium falciparum (overall infection rate 8.9%, 27/302) in 2023, 2024, and 2025. An. sergentii exhibited low anthropophagy (~ 8%) but high plant-feeding rates (~ 85%). Despite its predominant outdoor activity evidenced from high representation among infected mosquitoes outdoors (25/290) than indoors (0/12), mortality remained low (8–53%) after exposure to piperonyl butoxide- and pyriproxyfen-treated bed nets. Five An. longipalpis C specimens were also found infected with P. falciparum (1.7%; 5/302). Conclusions We provide evidence of An. sergentii , beyond its historical record in Kenya, as a potential malaria vector, underscoring the need to mainstream molecular analysis into routine malaria surveillance activities. The reduced efficacy of dual-ingredient nets against An. sergentii could present a serious challenge to their future deployment in the region, including outdoor control tools with similar active ingredients. This mosquito, and cryptic malaria vectors, such as An. longipalpis C could pose additional threat to malaria control efforts in Kenya.