Land use gradients drive spatial variation in Lassa fever host communities in the Eastern Province of Sierra Leone.

The natal multimammate mouse (Mastomys natalensis) is the primary reservoir host of Mammarenavirus lassaense (LASV), a zoonotic pathogen causing Lassa fever and endemic to West Africa. The occurrence and abundance of this species is regulated by the human environment and biotic interactions with other small-mammal species, but these ecological drivers remain poorly understood in the regions where Lassa fever outbreaks are observed. We developed a Bayesian multi-species occupancy model incorporating incomplete detection to assess habitat use from data obtained as part of a multi-year small-mammal trapping study (43,226 trap nights across four village sites in Sierra Leone, 2020-2023). We investigated the effects of land use gradients and small-mammal community dynamics on the spatial distribution of M. natalensis. Mastomys natalensis occupancy increased along a gradient from forest to agriculture to village habitats but was reduced in the peri-urban site compared to rural settings. Invasive rodent species influenced this pattern, with Mus musculus presence associated with reduced M. natalensis occupancy in the peri-urban site. We did not observe a similar effect when considering the co-occurrence of invasive Rattus rattus with M. natalensis in rural settings. These findings suggest that land use and species interactions drive spatial heterogeneity in M. natalensis populations, potentially explaining reduced Lassa fever incidence in urban areas. The results highlight the importance of considering community dynamics when predicting the risk of outbreaks of endemic zoonoses and the need to widen the context of studies of LASV transmission beyond the primary reservoir host species. To better assess public health risk and improve allocation of limited resources, we recommend more precise characterisation of small-mammal communities in LASV endemic regions, particularly in areas undergoing rapid land use change which may alter community-level small-mammal biodiversity.