Lassa

Lassa fever, also known as Lassa haemorrhagic fever, is a viral haemorrhagic fever caused by the Lassa virus ^[1][2]. The Lassa virus is a member of the Arenaviridae family and was first described in 1969 from a case identified in the town of Lassa, in Borno State, Nigeria ^[1].

Many individuals infected with Lassa virus do not develop symptoms, and when illness occurs it typically begins with fever, weakness, headaches, vomiting, and muscle pains ^[1]. Less commonly, patients may experience bleeding from the mouth or gastrointestinal tract, and the risk of death once infected is about one percent, frequently occurring within two weeks of symptom onset ^[1]. Among survivors, approximately one quarter experience hearing loss, which improves within three months in roughly half of these cases ^[1].

Lassa virus is endemic in rodent populations in Sierra Leone, Nigeria, and other countries in West Africa, with spillover to humans occurring frequently and resulting in a viral haemorrhagic fever associated with a high case-fatality rate ^[2]. The 2018 outbreak was the largest Lassa fever outbreak recorded; it spread to 18 of Nigeria's states and resulted in 3,498 infections and 171 deaths, with Nigeria accounting for over half of the total regional cases and fatalities ^[1]. That year, Sierra Leone reported 129 infections and 12 deaths, with the outbreak especially severe in Kenema and Bo Districts, while Guinea experienced 98 cases and 8 fatalities and was among the least affected countries in the region ^[1]. Public-health authorities have highlighted concerns about more frequent outbreaks, the potential for further geographic expansion of Mastomys natalensis and other rodent reservoirs, and the emergence of novel LASV strains in densely populated West African settings ^[2].

Lassa fever is usually spread to humans via contact with the urine or faeces of an infected multimammate mouse (Mastomys natalensis), after which person-to-person transmission can occur through direct contact ^[1].

There is no approved vaccine for Lassa virus, making prevention dependent on isolation of infected individuals and reducing contact with reservoir rodents ^[1][2]. Recommended measures include storing food in sealed containers to limit rodent access and keeping cats to hunt vermin ^[1]. Community awareness initiatives promoting hygiene practices, early intervention measures, and stringent border screenings have been employed during outbreaks to limit further spread ^[1].

1. 1Wikipedia contributors. Lassa fever - Wikipedia [Internet]. Wikipedia. cited 2 Sept 2026.Available from: https://en.wikipedia.org/wiki/Lassa_fever
2. 2Garry RF et al. Lassa fever - the road ahead. Nat Rev Microbiol. 2023 Feb. PMID: 36097163. doi: 10.1038/s41579-022-00789-8.PubMed: https://pubmed.ncbi.nlm.nih.gov/36097163/