Non-Mastomys rodents harbour ancient Lassa virus lineages within Benin and Nigeria's Guinea savanna belt.

The zoonotic Lassa virus (LASV), killing thousands of people annually, is maintained majorly by the multimammate mice Mastomys natalensis and M. erythroleucus; but other rodent reservoirs exist. Knowledge regarding LASV ecology remains limited in certain areas where Lassa fever continues to be incident and even recently emergent. Here, we assess community composition and LASV infection-status of small mammals in key localities across the Guinea savanna belt that spans the countries of Benin and Nigeria. Active LASV infection was determined by conventional (gel based)- and quantitative-PCR screening, while previous infection was determined by an immunofluorescent assay on IgG antibodies. Surprisingly, M. natalensis and M. erythroleucus, known to be sympatric across the Guinea savanna, co-occurred in only one locality and were PCR-negative for the virus; but showed IgG antibodies in localities endemic for Lassa fever within Nigeria. Viremia, alternately, was detected in Mus (Nannomys) baoulei and (a newly-discovered reservoir) Lemniscomys striatus, carrying relatively ancient LASV lineages, VIII & IX respectively, within Benin. Our results suggest non-Mastomys rodents assume an increased role in LASV ecology within the Guinea savanna belt of Benin and Nigeria; especially in central Benin, which turns out to be a long-overlooked hotbed of early evolution, host-switching and contemporary emergence.