Background: Malaria remains a leading cause of mortality in Kenya, despite concerted efforts in malaria vector control. Reducing outdoor transmission is a key factor in addressing residual malaria. Outdoor latrines are characterized as semi-sheltered structures with humid environments, which may provide an ideal resting site for Anopheles species to feed on human blood and subsequently rest. This study aimed to quantify the abundance of adult mosquitoes in houses and outdoor latrines, as well as explore the environmental factors associated with mosquito abundance. Methods: Monthly mosquito sampling was conducted in 50 houses and their corresponding outdoor latrines using Prokopack aspirators from July 2023 to April 2024. Household interviews were conducted concurrently to collect data on the number of individuals sleeping in the houses and the quantity of bednets used within the households. In addition, blood meal sources were identified through polymerase chain reaction (PCR) analysis of blood-fed mosquitoes collected from December 2023 to April 2024. Results: Among anopheline species, the An. funestus group was the most common, followed by An. gambiae s.l. in both houses and latrines. In anophelines, the human blood index was 50.0% (n = 15) in houses and 33.3% (n = 4) in latrines, while bovine blood was 60.0% (n = 18) and 66.7% (n = 8), respectively. Ventilated improved pit (VIP) latrines were associated with a 61% decrease in An. gambiae s.l. abundance (adjusted incidence rate ratio [aIRR] = 0.39, 95% confidence interval [CI] = 0.16–0.96) and a 62% decrease in Culex quinquefasciatus abundance (aIRR = 0.38, 95% CI = 0.24–0.60), compared with pit latrines. The presence of a bathing space in latrines was associated with a 23% increase in Cx. quinquefasciatus abundance (aIRR = 1.88, 95% CI = 1.23–2.89) compared with latrines without a bathing space. There was an inverse association between the number of individuals using the latrines and Cx. quinquefasciatus abundance (aIRR = 0.93, 95% CI = 0.89–0.97). Conclusions: VIP latrines were associated with a lower abundance of An. gambiae s.l. and Cx. quinquefasciatus compared with pit latrines, whereas the presence of a bathing space in latrines was associated with a higher abundance of Cx. quinquefasciatus compared with latrines without bathing spaces. Integrative public health programs that promote the construction of VIP latrines as a low-cost intervention may provide co-benefits for both sanitation and vector control.
