Spatial exploration of the impact of cattle on risk of malarial infection using data from a cluster randomised trial

In the context of stalled progress in reducing malaria burden and ongoing efforts to develop novel vector control methods, there is ongoing debate about the impact of livestock on malaria transmission. Zooprophylaxis refers to the use of livestock as an alternate source of blood that diverts mosquito vectors away from humans, potentially reducing malaria transmission. However, conflicting evidence highlights the concept of zoopotentiation, where livestock may instead increase vector populations and enhance malaria transmission. Prior studies that explore these concepts have examined livestock as a household asset, which does not account for the geospatial effect of animals owned by neighbours around the home.

We implemented a novel spatial approach to explore the impact of cattle on the risk of malarial infection in coastal Kenya. Using data with high granularity from a recent randomised controlled trial, we measured the presence of animals at increasing concentric areas around the household. We fit binomial regression models with generalised estimating equations to explore how cattle density and cattle-to-human-ratio measures were related to malaria prevalence in children 5-15 years old, accounting for relevant environmental and socio-economic variables.

Our findings show that cattle density measured at 400, 500, and 600 m around the household (ORs: 1.63, 95% CI 1.20 to 2.21; 1.84, 95% CI 1.21 to 2.80 and 2.04, 95% CI 1.13 to 3.67, respectively) and cattle-to-human ratio at 500 and 600 m (ORs: 2.73, 95% CI 1.45 to 5.41 and 2.32, 95% CI 1.11 to 4.86, respectively) are significantly related to higher odds of testing positive for malaria. We did not find a significant association at closer distances (<400 m) or when cattle were evaluated only as a household asset.

To better understand the impact of livestock on malaria transmission and potentially develop vector control strategies that account for these impacts, future research should consider animals as an environmental exposure and not only as a household asset.