Climate and topography shape malaria transmission in lowland Busia and highland Meru counties, Kenya

Introduction Malaria is a global public health problem especially in sub Saharan Africa. In Kenya, it varies across ecological zones with limited evidence comparing vector ecology, climate, topography and risk of infection in western lowlands and central highlands. Objective To compare levels of malaria infection, vector densities, rainfall, temperature, relative humidity and topography in lowland Busia and highland Meru counties. Methods A cross sectional survey was conducted using larval dipping for aquatic habitats, pyrethrum spray catches for indoor resting mosquitoes, and malaria diagnosis using rapid diagnostic tests and microscopy. Rainfall, temperature, and relative humidity were recorded using automated data loggers. Topography was noted by ground truthing. Data were analysed using chi square tests, analysis of variance, and logistic regression. Results Early instar Anopheles larvae were significantly less likely to be detected in the highland site than in the lowland site (unadjusted Odds Ratio = 0.33; 95% CI: 0.11 - 0.97). Malaria prevalence by rapid diagnostic tests was 0% in the highland site and significantly higher in lowland sites (p < 0.001), with microscopy confirming the absence of infections in the highland area. Highland sites experienced significantly cooler temperatures, including more hours below 16 degrees Celsius (p = 0.006), whereas lowland sites recorded significantly higher minimum, mean, and maximum temperatures (p < 0.001). Rainfall did not differ significantly between the two ecological zones (p = 0.090), average minimum RH in the highland was significantly higher than in the lowland site (p < 0.001). Valleys in Baragu are mainly V-shaped while Maduwa and Budalangi are generally flat areas in the lower courses of rivers prone to flooding. Conclusion Cooler highland climates and topographic features likely limit vector presence, abundance, development and malaria transmission, while warmer lowland environments sustain residual transmission.