Lassa fever
Lassa mammarenavirus · Arenaviridae
Strains
- Lineage I (Nigeria)
- Lineage II (Nigeria)
- Lineage III (Nigeria)
- Lineage IV (Sierra Leone, Guinea, Liberia)
- Lineage V (Mali, Ivory Coast)
- Lineage VI (Togo)
- Lineage VII (Benin)
Transmission
- Contact with urine, faeces, or saliva of infected multimammate rats (Mastomys natalensis)
- Handling or consuming infected multimammate rats
- Direct contact with blood or body fluids of an infected person
- Healthcare-associated exposure without adequate PPE
- Sexual transmission during convalescence — virus persists in semen up to ~3 months
Incubation
6-21 days
Mortality rate
~1% overall (most cases mild/asymptomatic); 15-25% in hospitalised patients; foetal mortality approaches 80% in third-trimester pregnancy
Vaccine
No
Treatment
Ribavirin is effective when started early — ideally within 6 days of fever onset — together with supportive care (IV fluids, electrolyte correction, treatment of secondary infections). Sensorineural hearing loss affects 25-30% of patients and may be permanent.
Vaccine status
No WHO-approved Lassa fever vaccine as of 2026. Multiple candidates (e.g. INO-4500, MV-LASV, rVSV- and measles-vectored vaccines) are in clinical development, several under CEPI funding.
Endemic regions
- Nigeria — highest reported burden, thousands of cases annually (NCDC), peak transmission January-April
- Sierra Leone — highly endemic
- Liberia — highly endemic
- Guinea — highly endemic
- Mali, Ivory Coast, Benin, Togo — sporadic cases
- Europe and North America — occasional imported cases among returning travellers
Frequently asked questions
transmission
How is Lassa fever transmitted?
geography
Which countries report the most Lassa fever cases?
severity
What is the mortality rate of Lassa fever?
symptoms
What are the symptoms of Lassa fever?
treatment
Is there a treatment for Lassa fever?
comparison
How is Lassa fever different from Ebola?
Sources
Last update Jun 9, 2026 · ⚠ Not medical advice.