Impact of seasonal change on virus-rodent dynamics in Nigeria's Edo-Ondo hotspot for Lassa fever

The zoonotic Lassa virus (LASV) is naturally maintained in rodents but commonly virulent in humans, killing thousands across West Africa annually. Human cases of Lassa fever surge during the dry season. In a hotspot for this disease, involving seven localities from Edo and Ondo states within Nigeria, we sought to depict how fluctuation patterns of LASV prevalence in rodents and levels of rodent infestation culminate in particularly heightened points of potential rodent-to-human transmission. We also explored how this variability may be driven by environmental and demographic factors. We determined active LASV infection by PCR and previous infection using an indirect immunofluorescence assay for IgG antibodies. Six out of the seven localities had active LASV infections, ranging between 3.7% (1/27) and 75.5% (37/49). In Ebudin and Ekpoma, where longitudinal sampling was conducted, prevalence differences were not statistically significant across locality, habitat, season, or year. Conversely, abundance peaked significantly indoors for Mastomys natalensis (the major LASV reservoir) and Praomys daltoni either during the dry or rainy season, depending on whether each of these rodent species occupied Ebudin or Ekpoma. Our results suggest that, since LASV occurrence is usually widespread in its natural reservoir within highly endemic areas, a more immediate component of zoonotic risk to prioritize for control would be the targeting of rodent infestation peaks when they occur inside human habitations. Over our two-year survey, these peaks in abundance were consistent in the months they occurred yearly for M. natalensis and P. daltoni per locality.