Contact networks of small mammals highlight potential transmission foci of Lassa mammarenavirus

Lassa fever, caused by Lassa mammarenavirus (LASV), is an endemic zoonosis in several West African countries. Human infections primarily result from transmission from rodent hosts, with the primary reservoir species being Mastomys natalensis , a synanthropic rodent. In Sierra Leone, small-mammal communities are structured along land use gradients, and this structure is hypothesized to influence the risk of Lassa fever outbreaks in human populations. However, whether anthropogenic habitats facilitate significant LASV transmission among small mammals is not well understood.

To address this, we conducted rodent sampling over 43,266 trap nights, detecting 684 rodents and shrews in the Lassa fever-endemic Eastern Province of Sierra Leone. We reconstructed small-mammal contact networks to investigate whether contact rates and network structures differed by land use to identify settings more conducive to viral transmission among hosts. We found that small-mammal communities were larger in villages and agricultural settings than in forests, but contact rates were similar across habitats. Network structures differed by land use, with villages exhibiting more disconnected networks compared to agricultural settings. Notably, intra-specific contact among M. natalensis was more likely in agricultural environments than in villages, suggesting that land use can influence LASV transmission dynamics.

Overall, LASV seroprevalence was 5.7% across the small-mammal communities, with LASV antibodies detected in nine rodent and shrew species. These findings underscore the importance of expanding rodent surveillance across various habitats to understand habitat-specific pathogen transmission dynamics. A more systematic approach to LASV surveillance in villages and agricultural settings will help identify key host species driving pathogen dynamics.