Association Between Entomological Indices and Malaria Test Positivity Rates in Western Kenya: Implications for Surveillance

The relationship between malaria entomological indices and epidemiological outcomes remains poorly understood at local scales in western Kenya, limiting evidence-based surveillance strategies. This study evaluated associations between vector densities, entomological inoculation rates (EIR), human biting rates (HBR), and malaria test positivity rates in a high-transmission setting. The study was conducted in five villages within Teso South sub-County, Busia County, Kenya, from March to June 2021. Village-level entomological indices including vector densities, human biting rates (HBR), and entomological inoculation rates (EIR) were calculated for Anopheles funestus and An. gambiae s.l. Concurrent malaria test positivity rates were obtained from outpatient department registers in the health facilities serving the five villages (n=2,377 cases). The relationship between entomological indices and malaria test positivity was analyzed using multilevel logistic regression models, accounting for individual- and village-level factors. During this period 1,957 female anopheline mosquitoes, predominantly An. gambiae s.l (89.17%) and An. funestus (10.48%) were collected. Malaria test positivity rates varied significantly across villages (range: 23.6%-68.7%). Multilevel analysis revealed significant associations between malaria test positivity and both anopheline mean density (OR 1.12, 95% CI 1.08-1.16, P<0.0001) and HBR (OR 1.37, 95% CI 1.19-1.57, P<0.0001). Age of humans showed a slight negative association with malaria positivity (OR 0.999, 95% CI 0.998-0.9997, P=0.013), while EIR showed no significant association (OR 1.04, 95% CI 0.96-1.13, P=0.319). Entomological indices, particularly vector density and human biting rate, showed strong associations with malaria test positivity rates. However, entomological monitoring is actually pretty intensive and covers only a few villages, and only parts of those, while routine surveillance for clinical cases of malaria is becoming increasingly reliable. These findings therefore suggest that one solution to higher malaria cases is target transmission through vector control. This information could be valuable for national malaria control programs in optimizing surveillance strategies and evaluating intervention effectiveness.