Lassa fever is an acute viral haemorrhagic illness caused by Lassa virus, a member of the Arenaviridae family, and is classified as a zoonotic disease with rodents serving as the natural reservoir [1][2]. The primary animal host is the multimammate rat (Mastomys), which sheds virus without becoming ill [1][2].
Disease overview
Evidence-backed information EN 5/7 applicable sections · ZH 5/7 applicable sections Updated Oct 23, 2026
Lassa fever is an acute viral illness caused by an arenavirus first identified in Nigeria in 1969, endemic in parts of West Africa with frequent spillover from rodents to humans [1][2][3]. Most infections are mild or asymptomatic, but severe disease affects about one in five infected persons and can involve multiple organs [1][2]. The condition is the most common imported viral haemorrhagic fever in non-endemic countries, with higher case fatality observed among returned travellers than in endemic settings [4].
Read the full clinical and epidemiological profile 6
Approximately 80% of infected individuals experience no or mild symptoms, while severe disease occurs in about 20% and may affect the liver, spleen and kidneys [1][2]. Symptomatic onset is typically gradual with fever, weakness and malaise, followed by headache, sore throat, muscle pain, chest pain, nausea, vomiting, diarrhoea, cough and abdominal pain [2]. Severe presentations can include facial swelling, fluid in the lung cavity, bleeding, low blood pressure, proteinuria, shock, seizures, disorientation, coma and deafness in some recovered patients [2].
The disease is endemic in the rodent population across parts of West Africa, notably Benin, Ghana, Guinea, Liberia, Mali, Sierra Leone, Togo and Nigeria, with likely presence in other regional countries [2]. Annual outbreaks occur in endemic regions with high mortality among symptomatic patients [5][3]. Imported cases to non-endemic countries outside West Africa have been documented repeatedly over five decades, with a case fatality of about 35% in that setting [4].
Humans are primarily infected through contact with food or household items contaminated by urine or faeces of infected Mastomys rats, or by direct contact with the rodents themselves [1][2]. Person-to-person spread and laboratory transmission can occur, particularly in healthcare settings lacking adequate infection prevention and control [2]. Other rodent species have also yielded Lassa virus, suggesting additional potential reservoir sources [1].
Reducing risk depends on limiting human contact with rodent populations and promoting community hygiene to discourage Mastomys from entering homes, though complete elimination of these abundant rats is not feasible in endemic areas [1][4]. Early detection with prompt isolation and care, strong infection prevention and control in healthcare, and community rodent-prevention efforts help curb further spread [1][2]. WHO supports endemic countries through surveillance, laboratory services, infection control and community engagement to reduce mortality and limit facility-based transmission [1].
- 1 World Health Organization. Lassa fever [Internet]. cited 2 Sept 2026. Available from: https://www.who.int/news-room/fact-sheets/detail/lassa-fever
- 2 World Health Organization. Lassa fever [Internet]. cited 2 Sept 2026. Available from: https://www.who.int/health-topics/lassa-fever
- 3 Garry 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/
- 4 Wolf T et al. Fifty years of imported Lassa fever: a systematic review of primary and secondary cases. J Travel Med. 2020 Jul 14. PMID: 32219400. doi: 10.1093/jtm/taaa035. PubMed: https://pubmed.ncbi.nlm.nih.gov/32219400/
- 5 Eberhardt KA et al. Ribavirin for the treatment of Lassa fever: A systematic review and meta-analysis. Int J Infect Dis. 2019 Oct. PMID: 31357056. doi: 10.1016/j.ijid.2019.07.015. PubMed: https://pubmed.ncbi.nlm.nih.gov/31357056/
- A96.2
- 1D61
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