Reservoir displacement by an invasive rodent reduces Lassa virus zoonotic spillover risk

Abstract

The black rat (Rattus rattus) is a globally invasive species that has been widely introduced across Africa. Within its invasive range in West Africa, R. rattus may compete with the native rodent Mastomys natalensis, the primary reservoir host of Lassa virus, a zoonotic pathogen that kills thousands annually. Here, we use rodent trapping data from Sierra Leone and Guinea to show that R. rattus presence reduces M. natalensis density within the human dwellings where Lassa virus exposure is most likely to occur. Further, we integrate infection data from M. natalensis to demonstrate that Lassa virus zoonotic spillover risk is lower at sites with R. rattus. While non-native species can have numerous negative effects on ecosystems, our results suggest that R. rattus invasion has the indirect benefit of decreasing zoonotic spillover of an endemic pathogen, with important implications for invasive species control across West Africa.

Fig. 1. Patterns of Mastomys natalensis and Rattus rattus catch per trap across 28 study sites in Sierra Leone and Guinea.

Map of catch per trap for M. natalensis (a) and R. rattus (b), and a scatterplot of the same data (c). Catch per trap was calculated using only house traps from a given site (n = 9588 trap-nights). d shows the implied values of M. natalensis catch per trap for sites without and with R. rattus present, as derived from a visit-level Bayesian statistical model (n = 20,000 posterior samples; see main text for details). Colors indicate sampling season, points indicate posterior means, thick lines represent 90% HPDIs, and thin lines represent 99% HPDIs. Source data are provided as a Source Data file.

Fig. 2. House-level analyses suggest Rattus rattus negatively affects Mastomys natalensis.

Jitter plot showing M. natalensis catch per trap across 572 houses in Sierra Leone from sites where R. rattus is either apparently absent or known to occur (a). b shows posterior estimates for house-level M. natalensis occupancy probability for sites without and with R. rattus present (analysis based on 560 houses with repeated sampling suitable for occupancy modeling; n = 100,000 posterior samples). Colors indicate sampling season, points indicate posterior means, thick lines represent 90% HPDIs, and thin lines represent 99% HPDIs. Source data are provided as a Source Data file.

Fig. 3. Lassa-positive Mastomys natalensis catch per trap as an index of Lassa virus zoonotic spillover risk.

Map of zoonotic spillover risk index values across 28 study sites in Sierra Leone and Guinea (a). These values were calculated using only M. natalensis captures and Lassa virus testing results from within human dwellings. b shows the implied values of the zoonotic spillover risk index for sites without and with R. rattus present, as derived from a visit-level Bayesian statistical model (n = 20,000 posterior samples; see main text for details). Colors indicate sampling season, points indicate posterior means, thick lines represent 90% HPDIs, and thin lines represent 99% HPDIs. Source data are provided as a Source Data file.