Insecticide resistance profiles of Anopheles arabiensis and relationship with microsporidia MB infection in two rice agroecosystems in Kenya

Authors: MUTWIRI, WINFRED - Chuka University; Muturi, Ephantus; NGUNJIRI, JOSEPHINE - University Of Embu; MUTHENGIA, JACKSON - University Of Embu; BUDAMBULA, NANCY - University Of Embu; MULI, JOSHUA - University Of Embu; MURITHI, GENSON - University Of Embu; NYABUGA, FRANKLIN - University Of Embu; KIMANI, NJOGU - University Of Embu; MURAYA, MOSES - University Of Embu; TCHOUASSI, DAVID - International Centre Of Insect Physiology And Ecology

Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/11/2025
Publication Date: 1/22/2026
Citation: Mutwiri, W.K., Muturi, E.J., Ngunjiri, J.W., Muthengia, J.W., Budambula, N.L., Muli, J.K., Murithi, G., Nyabuga, F.N., Kimani, N.M., Muraya, M.M., Tchouassi, D.P. 2026. Insecticide resistance profiles of Anopheles arabiensis and relationship with microsporidia MB infection in two rice agroecosystems in Kenya. Parasites & Vectors. https://doi.org/10.1186/s13071-025-07212-0.
DOI: https://doi.org/10.1186/s13071-025-07212-0

Interpretive Summary: Malaria poses serious health and economic challenges worldwide. Mosquito control is a fundamental component of global malaria control, but malaria vectors have become increasingly resistant to insecticides that are approved for public health use. Knowing the status and extent of insecticide resistance in malaria vectors is crucial for the design and implementation of effective resistance management plans. This study evaluated the susceptibility of a major malaria vector in Africa to six insecticides that are commonly used for mosquito control. The study also investigated the relationship between insecticide resistance and the prevalence and intensity of a microsporidia parasite that blocks malaria transmission and naturally occurs in this mosquito species. Mosquito populations were fully susceptible to bendiocarb, and some populations were also susceptible to DDT and malathion. In contrast, all mosquito populations were resistant to all three pyrethroid insecticides tested, with resistance intensity varying between study sites. The mechanism of pyrethroid resistance was shown to vary spatially with multiple resistance mechanisms identified in one site, and metabolic resistance identified as the sole resistance mechanism in the other site. Microsporidia density but not prevalence was higher in mosquitoes that survived exposure to permethrin doses 2 and 5 times higher than the diagnostic dose compared to those that succumbed to these doses. These findings demonstrate the widespread resistance of this malaria vector to insecticides and the need to devise and implement resistance management plans that are tailored to the local ecology of the vectors. Further studies are needed to determine the link between microsporidia and mosquito resistance to insecticides.

Technical Abstract: Insecticide resistance monitoring in vector populations is a key pillar of the Global Plan for Insecticide Resistance Management in malaria vectors. This study assessed the susceptibility of Anopheles arabiensis populations from Mwea and Ahero, Kenya to six insecticides. The association between insecticide resistance and Microsporidia MB infection, a symbiont known to block malaria transmission in An. arabiensis was also investigated. Mosquitoes were exposed to permethrin, deltamethrin, alphacypermethrin, malathion, bendiocarb, and dichlorodiphenyltrichloroethane (DDT) using the Centers for Disease Control and Prevention (CDC) bottle bioassay. Resistance intensity and synergist bioassays for pyrethroids were conducted to evaluate the strength of resistance and the contribution of cytochrome P450s to pyrethroid resistance. Microsporidia MB infection was detected and quantified using qPCR. A total of 3120 females were tested. Populations from both study sites were susceptible to bendiocarb but resistant to all three pyrethroids. Mortality rates following exposure to alpha-cypermethrin, permethrin, and deltamethrin respectively were 0%, 4.7%, and 25.7% in Ahero, and 25.7%, 6.2%, and 26.6% in Mwea. Mortality increased with increasing permethrin concentration with 1x , 2x , 5x , and 10x values of 4.7%, 17.2%, 70.8%, and 84.4% respectively in Ahero and 6.2%, 29.4%, 85.3%, and 100% in Mwea. The Ahero population was susceptible to malathion but had reduced susceptibility to DDT (92.7%) while the Mwea population was susceptible to DDT and resistant to malathion (69.2%). Pre-exposure to piperonyl butoxide fully restored pyrethroid susceptibility in the Mwea population, indicating metabolic resistance and partially restored permethrin susceptibility (4.7 to 86.7%) in Ahero population, indicating the presence of other resistance mechanisms. Microsporidia MB was detected in Ahero population and mean (± se) infection density was significantly higher in mosquitoes that survived 2x and 5x permethrin doses (1017.6 ± 296.6) compared with those that succumbed to these doses (171.3 ± 78.0). These findings reveal that Anopheles arabiensis populations from the two sites exhibit heterogeneous yet high levels of insecticide resistance, particularly to pyrethroids. The findings highlight the need to incorporate synergist-based interventions into resistance management strategies. This study is the first to document an association between Microsporidia MB density and the intensity of insecticide resistance in An. arabiensis, and further studies are needed to clarify this relationship and its significance to malaria control.