Nigeria reported 1,068 confirmed Lassa fever cases and 196 deaths in 2024, an 18 percent case fatality rate among diagnosed patients. The disease is endemic across Sierra Leone, Liberia, Guinea, Benin, Togo, Ghana, and Nigeria, with the Nigeria Centre for Disease Control declaring outbreak status nearly every dry season. Most travelers never hear about Lassa until a returning aid worker triggers a hospital biosafety scramble.
WHO estimates between 300,000 and 500,000 Lassa infections happen each year across West Africa, killing roughly 5,000 people annually. Yet most cases stay invisible because mild infections look like malaria. This post sits inside the disease severity scoring framework and complements coverage of Ebola outbreaks and Marburg virus.
Key Takeaways
- Lassa virus is carried by the multimammate rat (Mastomys natalensis), which lives inside rural West African homes.
- Roughly 80 percent of infections are mild or asymptomatic. The 20 percent that progress can include hemorrhage, hearing loss, and multi-organ failure.
- Case fatality among hospitalized patients is 15 to 25 percent. Untreated severe pregnancy cases approach 80 percent maternal mortality.
- Ribavirin reduces mortality if started within 6 days of symptom onset.
- One in three Lassa survivors develops permanent or transient sensorineural hearing loss.
- No licensed vaccine exists in 2026. CEPI is funding multiple candidates in Phase 2 trials.
What is Lassa fever?
Lassa fever is an acute viral hemorrhagic illness caused by Lassa virus, an arenavirus first identified in 1969 in the town of Lassa, Nigeria. The virus circulates in a rodent reservoir and spills over to humans through contaminated food, dust, or direct contact. Severe cases involve bleeding, shock, and organ failure similar in clinical picture to Ebola, though with lower overall lethality.
The virus belongs to the Old World arenavirus group. It is enveloped, single-stranded RNA, and segmented. Four genetic lineages dominate, with Lineage IV in Sierra Leone and Liberia and Lineages I-III in Nigeria.
Lassa earns its place on WHO's R&D Blueprint priority pathogen list because of high case counts, expanding range, and the absence of a licensed vaccine.
How does Lassa virus spread to humans?
Most human infections trace to contact with rodent excreta from Mastomys natalensis. The rat enters homes, contaminates food and water, and sheds virus in urine for life. Inhaling dust from contaminated floors, eating contaminated rice or maize, or handling the rats themselves all transmit the virus.
Human-to-human spread happens through direct contact with blood, urine, vomit, or semen from a symptomatic patient. Healthcare workers face the highest secondary risk, particularly when patients arrive misdiagnosed as malaria. Nosocomial outbreaks have killed entire surgical teams.
Lassa is not airborne in casual contact. Routine household contact with an asymptomatic survivor carries near-zero risk. Sexual transmission can occur for up to 90 days after recovery.
What are the symptoms and timeline?
Symptoms appear 6 to 21 days after exposure. The first week looks like flu or malaria: fever, headache, sore throat, malaise, muscle pain. About a fifth of cases progress in the second week to facial swelling, bleeding gums, conjunctival hemorrhage, vomiting, encephalopathy, and shock. Death typically comes from multi-organ failure between day 10 and day 14.
| Phase | Days | Clinical features |
|---|---|---|
| Incubation | 1 to 21 | No symptoms |
| Early | 1 to 7 | Fever, malaise, headache, sore throat |
| Severe | 7 to 14 | Facial edema, bleeding, encephalopathy, hypotension |
| Recovery or death | 14+ | Recovery with possible hearing loss, or fatal organ failure |
Pregnancy outcomes are particularly grim. Maternal mortality runs around 30 percent overall and approaches 80 percent in the third trimester. Fetal loss is over 85 percent. Evacuating the fetus often improves maternal survival.
How is Lassa fever diagnosed and treated?
Diagnosis requires PCR or antigen testing on blood, typically at a reference laboratory with BSL-4 capability for virus isolation. Clinical suspicion in a returning traveler with fever from West Africa should trigger isolation and rapid testing. Read more about how disease tests differ.
Ribavirin, a nucleoside analog, is the only treatment with mortality benefit. Intravenous ribavirin started within 6 days of symptom onset reduces mortality from around 55 percent to under 5 percent in severe cases according to McCormick and colleagues' landmark 1986 trial. After day 6, the benefit shrinks substantially.
Supportive care matters too: fluid resuscitation, electrolyte correction, transfusion for hemorrhage, and dialysis for renal failure. Most patients also need empirical malaria treatment until co-infection is ruled out, since the two diseases co-circulate.
Who is most at risk?
Rural West Africans living in mud-walled homes with grain stored indoors face the highest baseline exposure. Healthcare workers in endemic regions are second. Travelers face very low absolute risk but extremely high consequences if exposed.
US CDC has documented fewer than 10 imported cases since 1969. Each triggers contact tracing, hospital isolation, and ribavirin offering for high-risk contacts. The 2015 New Jersey case (a returning traveler from Liberia who died) cost the receiving hospital over $1 million in remediation.
If you are traveling to a Lassa-endemic area for work, the outbreak-aware travel guide is your starting point. Pre-departure briefings should include rodent-proofing accommodation and the symptom watch protocol on return.
Why is Lassa hard to control?
The rodent reservoir is impossible to eliminate. Mastomys natalensis is one of the most abundant rodents in sub-Saharan Africa, breeding fast and adapting to human dwellings. Trap-and-poison programs fail at scale because vacated niches refill within weeks.
Surveillance is fragmented. Most rural deaths from undifferentiated febrile illness are never tested. Underdiagnosis lets the virus circulate quietly, then surge when the dry season concentrates rodent-human contact.
No vaccine has finished Phase 3 trials as of mid-2026. CEPI has invested over $100 million across multiple candidate platforms (MV-LASV, INO-4500, rVSV vectored). A first licensure would change West African public health dramatically.
FAQ
Can you catch Lassa fever from a tourist destination?
Tourist risk is very low. Lassa transmission requires sustained exposure to rodents in rural living conditions or to a symptomatic patient. Standard travel itineraries through Abuja, Accra, Freetown, or Monrovia airports carry near-zero risk. Risk rises sharply for aid workers, missionaries, journalists, and researchers staying in village housing.
Is there a Lassa vaccine in 2026?
No licensed vaccine exists. Several candidates are in Phase 2 trials including a recombinant vesicular stomatitis virus vector (rVSV-LASV) and an attenuated measles vector. CEPI projects a first licensure in 2027 to 2028 if Phase 3 trials succeed.
How is Lassa different from Ebola?
Both are viral hemorrhagic fevers with bleeding and shock, but Lassa is milder on average (15 to 25 percent case fatality versus 25 to 90 percent for Ebola), has a rodent rather than primate reservoir, and responds to ribavirin while Ebola does not. Lassa also causes hearing loss in around a third of survivors, which is rare with Ebola.
Why does Lassa cause hearing loss?
The mechanism is incompletely understood. Researchers believe the virus damages the cochlea or vestibulocochlear nerve directly, possibly through inflammatory mediators that cross the blood-labyrinth barrier. Hearing loss can appear during recovery or weeks after apparent clinical resolution. About half of cases improve, half are permanent.
Should returning travelers self-monitor?
Yes, for 21 days after leaving an endemic area. Take your temperature twice daily and call ahead to a hospital if you develop fever above 38 degrees C, especially with sore throat, vomiting, or unusual bleeding. Mention West Africa travel before entering any facility so isolation precautions start before you reach the waiting room.