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

Author

Affiliation

David Simons

The Royal Veterinary College

Abstract

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.

Authors

David Simons ^1,2,3, Ravi Goyal ⁴, Umaru Bangura ^5,6, Rory Gibb ^2,9 Ben Rushton ⁸, Dianah Sondufu ⁶, Joyce Lamin ⁶, James Koninga ⁷, Momoh Jimmy ⁷, Mike Dawson ⁶, Joseph Lahai ⁶, Rashid Ansumana ⁶, Elisabeth Fichet-Calvet ⁵, Richard Kock ¹, Deborah Watson-Jones ^3,10, Kate E. Jones ^2,9

¹ Centre for Emerging, Endemic and Exotic Diseases, The Royal Veterinary College, London, United Kingdom

² Centre for Biodiversity and Environment Research, Department of Genetics, Evolution and Environment, University College London, London, United Kingdom

³ Department of Clinical Research, London School of Hygiene and Tropical Medicine, London, United Kingdom

⁴ Department of Medicine, University of California, San Diego, USA

⁵ Implementation Research, Zoonoses control, Bernard-Nocht Institute for Tropical Medicine, Hamburg, Germany

⁶ Njala University, Bo, Sierra Leone

⁷ Kenema Government Hospital, Kenema, Sierra Leone

⁸ Panadea Diagnostics GmbH, Hamburg, Germany

⁹ People & Nature Lab, UCL East, Department of Genetics, Evolution and Environment, University College London, London, United Kingdom

¹⁰ Mwanza Intervention Trials Unit, National Institute for Medical Research, Mwanza, Tanzania

Motivation

This study was designed to explore the contacts among individual rodents trapped in our ongoing rodent trapping study in a Lassa fever endecmic region of Sierra Leone. While there is some understanding of where different individuals of rodent species are located across landuse gradients in the endemic region there has been little research on the probability of contact among individuals of these species and what this may mean for viral transmission in these settings. Direct and indirect contact between infected and susceptible individual rodents will modify how viral pathogens are transmitted in these settings and there may be different rates of contacts between individuals of different species or in different landuse types that could be important for understanding viral transmission.

We used data from our repeated, systematic, rodent trapping study in the Eastern province of Sierra Leone to investigate the following questions. First, what are the potential contact networks of individuals of different species. Second, do these contact rates vary by landuse type. Third, do we observe differential inter- and intra-specific contact rates among the primary rodent reservoir of LASV (Mastomys natalensis). To achieve this we reconstructed contact networks of rodent communities using geolocated trap data for each of our study visits. We further report the prevalence of antibodies to LASV among rodents in our study but do not currently combine this data with our contact networks due to few positive samples.

Preprint

This manuscript has been submitted to the American Journal of Tropical Medicine and Hygiene. The pre-peer reviewed version is available from bioRxiv.

Data availability

Data are available in the project’s GitHub repository. Individual level data on detected rodents and their serological status for LASV antibodies are available on PHAROS.

ASTMH 2023 presentation

This work has been presented at the American Society for Tropical Medicine and Hygiene annual conference 2023 in Chicago.