Lassa fever is a viral hemorrhagic fever caused by the Lassa virus, spread primarily through contact with infected rodents in West Africa. About 80% of people who catch the virus have no symptoms at all, but 1 in 5 infections leads to severe illness affecting the liver, spleen, and kidneys. The overall case fatality rate is around 1%, but among hospitalized patients with severe disease, roughly 15% die.
How the Virus Spreads
The Lassa virus lives in a common West African rodent called the Natal multimammate mouse. These mice thrive in and around homes, and their droppings are frequently found in kitchens, on bedroom floors, and even in bedding. Humans get infected through contaminated food, by breathing in tiny particles from rodent urine or droppings, or through direct contact with the mice during hunting or food preparation.
Person-to-person transmission is possible through contact with an infected person’s blood, urine, or other bodily fluids. This is a particular concern in healthcare settings where protective equipment may be limited, though casual contact does not spread the virus.
Symptoms and Timeline
After exposure, the incubation period ranges from 2 to 21 days. When symptoms do appear, they start gradually with fever, general weakness, and a feeling of being unwell. In most people, the illness stays mild and may be mistaken for many other tropical infections.
In the roughly 20% of cases that become severe, the virus begins damaging multiple organs. Signs of serious disease include facial swelling, fluid buildup in the lungs, bleeding from the mouth, nose, or gastrointestinal tract, and dangerously low blood pressure. In the late stages, patients may experience seizures, tremor, disorientation, and coma. In fatal cases, death typically occurs within 14 days of symptom onset.
Hearing Loss in Survivors
The most significant long-term complication of Lassa fever is sudden hearing loss. Approximately one-third of people who recover from a symptomatic infection develop some degree of permanent hearing impairment. This happens because the virus and the body’s immune response together damage the delicate hair cells and nerve structures of the inner ear. Research in animal models shows that even after the virus clears, ongoing inflammation in and around the ear continues to cause damage. This hearing loss is the single largest source of lasting disability among Lassa fever survivors.
Diagnosis
Lassa fever is difficult to diagnose based on symptoms alone because early signs overlap with malaria, typhoid, and dozens of other infections common in the same regions. Confirming it requires laboratory testing. The gold standard is growing the virus from a patient’s blood sample in cell culture, but this takes time and requires the highest level of biosafety containment. In practice, clinicians rely on PCR-based tests that detect the virus’s genetic material and antibody tests that identify the immune system’s response to the infection. Even these methods face challenges because the Lassa virus varies significantly across different regions.
Treatment Options
There is no approved cure for Lassa fever. Supportive care, including maintaining fluid balance, managing blood pressure, and treating specific organ complications as they arise, forms the backbone of treatment. The antiviral drug ribavirin has been used off-label for decades, typically given intravenously over a 10-day course. Recent re-analysis of clinical data suggests ribavirin may help patients with severe disease but could actually be harmful in mild cases. The WHO currently recommends using ribavirin only within structured clinical protocols designed to further evaluate its safety and effectiveness.
Early medical attention makes a significant difference. Patients who receive supportive care before organ damage becomes extensive have much better outcomes than those who present late.
Where Lassa Fever Occurs
Lassa fever is endemic to West Africa, with the highest burden in Nigeria, Sierra Leone, Liberia, and Guinea. The multimammate mouse’s range extends across the region, meaning cases also occur in neighboring countries. Estimates suggest hundreds of thousands of infections occur each year, though the true number is hard to pin down because most cases are mild or asymptomatic and never reach a clinic. Occasional cases appear outside Africa in travelers returning from endemic areas.
Vaccines in Development
No vaccine is currently approved to prevent Lassa fever, but one candidate is further along than any before it. A single-dose vaccine developed by the nonprofit research organization IAVI is now in Phase 2a clinical trials in Nigeria, Liberia, and Ghana, funded by the Coalition for Epidemic Preparedness Innovations (CEPI). The trial is evaluating safety and immune response across adults (including people living with HIV), adolescents, and children as young as two. It is the most advanced Lassa vaccine candidate to date, but approval is still likely years away.