Mosquito Exposure and Malaria Morbidity: A Microlevel Analysis of Household Mosquito Populations and Malaria in a Population-Based Longitudinal Cohort in Western Kenya

Abstract

Background: Malaria morbidity is highly overdispersed in the population. Fine-scale differences in mosquito exposure may partially explain this heterogeneity in individual malaria outcomes.

Methods: In 38 households we explored the effect of household-level mosquito exposure and individual insecticide-treated net (ITN) use on relative risk (RR) of confirmed malaria. We conducted monthly active surveillance (n = 254; 2624 person-months) and weekly mosquito collection (2092 household-days of collection), and used molecular techniques to confirm human blood feeding and exposure to infectious mosquitoes.

Results: Of 1494 female Anopheles (89.8% Anopheles gambiae sensu lato), 88.3% were fed, 51.9% had a human blood meal, and 9.2% were sporozoite infected. In total, 168 laboratory-confirmed malaria episodes were reported (incidence rate 0.064 episodes per person-month at risk; 95% confidence interval [CI], .055-.074). Malaria risk was directly associated with exposure to sporozoite-infected mosquitoes (RR, 1.24; 95% CI, 1.11-1.38). No direct effect was measured between ITN use and malaria morbidity; however, ITN use did moderate the effect of mosquito exposure on morbidity.

Conclusions: Malaria risk increases linearly with vector density and feeding success for persons with low ITN use. In contrast, malaria risk among high ITN users is consistently low and insensitive to variation in mosquito exposure.

Keywords: Anopheles; cohort; insecticide-treated net; malaria.

Figure 1.

Weekly entomological exposures (A) and monthly confirmed malaria cases (B) from June 2017 to July 2018.

Figure 2.

Predicted probability of malaria episode by mosquito exposure for high (dots) versus low (squares) net use: total female Anopheles (A), Anopheles fed by any source (B), human-blood–fed Anopheles (C), and sporozoite-infected Anopheles (D).

Figure 3.

Estimated marginal R² for models of malaria incidence where covariates are added sequentially (base model) to measure the effect on model fit. Separate models are fit with base model plus 1 exposure variable or 1 exposure variable with interaction on net use. Abbreviations: HH, household; ITN, insecticide-treated net.