Determining Ancestry between Rodent- and Human-Derived Virus Sequences in Endemic Foci: Towards a More Integral Molecular Epidemiology of Lassa Fever within West Africa.

Ayodeji Olayemi,Thomas Strecker,Adetunji Samuel Adesina,Elisabeth Fichet-Calvet,N’Faly Magassouba

doi:10.3390/biology9020026

Abstract

Lassa fever is a viral hemorrhagic illness responsible for thousands of human deaths in West Africa yearly. Rodents are known as natural reservoirs of the causative Lassa mammarenavirus (LASV) while humans are regarded as incidental, spill-over hosts. Analysis of genetic sequences continues to add to our understanding of the evolutionary history, emergence patterns, and the epidemiology of LASV. Hitherto, the source of data in such investigations has mainly comprised human clinical samples. Presently, a rise in the quantity of virus strains accessed through ecological studies over the last 15 years now allows us to explore how LASV sequences obtained from rodents might affect phylogenetic patterns. In this study, we phylogenetically compared LASV sequences obtained from both rodents and humans across West Africa, including those from two localities highly endemic for the disease: Ekpoma in Nigeria and Kenema in Sierra Leone. We performed a time-calibrated phylogeny, using a Bayesian analysis on 198 taxa, including 102 sequences from rodents and 96 from humans. Contrary to expectation, our results show that LASV strains detected in humans within these localities, even those sampled recently, are consistently ancient to those circulating in rodents in the same area. We discuss the possibilities connected to this preliminary outcome. We also propose modalities to guide more comprehensive comparisons of human and rodent data in LASV molecular epidemiological studies.

Highlights

Lassa fever causes a deadly zoonotic disease that kills 5000 or more people every year in WestAfrica [1]
The six viral populations isolated from M. natalensis and M. erythroleucus show very different emergence periods according to the rodent populations from which they originated in West Africa
The Lassa mammarenavirus (LASV) clade obtained from rodents in Mali is the oldest and dates back to 96 years, whereas that obtained in Denguedou near the Sierra Leonean border is very recent and seems to have emerged 20 years ago

Summary

Introduction

Lassa fever causes a deadly zoonotic disease that kills 5000 or more people every year in West. The Lassa mammarenavirus (LASV), which causes this illness, is borne by rodents. The Natal multimammate mouse Mastomys natalensis is recognized as the main LASV reservoir while the Guinea multimammate mouse M. erythroleucus and the African wood mouse Hylomyscus pamfi were recently identified as additional hosts [2,3]. Primary (rodent-to-human) transmission of the virus occurs when humans come in contact with excreta or body fluids of infected rodents. Further knowledge on the epidemiology of Lassa fever, at the rodent–human interface, is needed to control this disease

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