Lassa fever
This page is an overview of Lassa fever, an acute viral haemorrhagic fever caused by the Lassa mammarenavirus and endemic to parts of West Africa. Its natural reservoir is the Natal multimammate mouse (Mastomys natalensis). For clinical guidance refer to current WHO, US CDC and Nigeria CDC (NCDC) resources.
Lassa virus lineages
Lineage II
- Main areas
- Nigeria (south & east)
- Notes
- Dominant in Nigerian outbreaks, including the 2026 season
Lineage III
- Main areas
- Nigeria (north-central)
- Notes
- The second major Nigerian lineage
Lineage IV
- Main areas
- Sierra Leone, Guinea, Liberia
- Notes
- The Mano River basin, where Lassa was first described
Lineage I
- Main areas
- Nigeria
- Notes
- Historically rare; the earliest characterised isolates
Lineage V
- Main areas
- Mali, Côte d'Ivoire
- Notes
- The westernmost focus
Lineage VI
- Main areas
- Togo, Benin
- Notes
- More recently characterised
| Lineage | Main areas | Notes |
|---|---|---|
| Lineage II | Nigeria (south & east) | Dominant in Nigerian outbreaks, including the 2026 season |
| Lineage III | Nigeria (north-central) | The second major Nigerian lineage |
| Lineage IV | Sierra Leone, Guinea, Liberia | The Mano River basin, where Lassa was first described |
| Lineage I | Nigeria | Historically rare; the earliest characterised isolates |
| Lineage V | Mali, Côte d'Ivoire | The westernmost focus |
| Lineage VI | Togo, Benin | More recently characterised |
Frequently asked questions
How is Lassa fever transmitted?
Most often through contact with food or household items contaminated by the urine or faeces of infected Mastomys rodents. Person-to-person spread occurs through direct contact with the blood, urine or other body fluids of an infected person, and is a particular risk in healthcare settings without adequate infection control.
Is there a vaccine or treatment?
There is no licensed Lassa fever vaccine as of 2026, though several candidates are in clinical development. The antiviral ribavirin may improve outcomes when started early in the course of illness, alongside supportive care.
How dangerous is Lassa fever?
About 80% of infections are mild or without symptoms. Among people ill enough to be hospitalised the case fatality rate is roughly 15–20%, and higher in some outbreaks; sensorineural hearing loss is a common complication in survivors.