Open Outbreak

Lassa fever

Also known as: Lassa, Lassa virus, arenavirus, Lassa hemorrhagic fever

Lassa fever is a viral illness found mainly in West Africa. The virus is carried by Mastomys rats, also called multimammate rats. People usually get infected through food, dust, or household items contaminated with rat urine or faeces. Human-to-human spread can happen through direct contact with blood or other body fluids, but WHO says this rate is low and it is mainly a concern in health-care settings without good infection prevention. Most infections are mild or have no symptoms. A smaller share become severe, with bleeding, shock, or failure of several organs. WHO says early supportive care improves survival. Ribavirin has been used, but WHO states its efficacy is unproven. There is no approved vaccine. Lassa fever is endemic in several West African countries and causes repeated outbreaks, especially in Nigeria. Imported cases have also been reported in Europe and North America after travel. Because symptoms can look like many other infections, lab testing and rapid reporting matter.

Key facts

TransmissionUsually from contact with food, household items, dust, or surfaces contaminated with urine or faeces from infected Mastomys rats; less often by direct contact with blood or other body fluids from an infected person, especially in health-care settings.
Incubationunknown
Case fatalityOverall about 1% according to WHO outbreak assessments; usually 1% to 15% among patients hospitalized with severe disease; in Nigeria's 2023 outbreak, 152 deaths among 877 confirmed cases (17%).
Reproduction numberunknown
VaccineNo approved vaccine reported by WHO.
TreatmentEarly supportive care with rehydration and symptom treatment improves survival. Ribavirin has been used, but WHO says its efficacy is unproven.
ReservoirMastomys rats, especially the multimammate rat.
Endemic regionsWest Africa, including reported endemicity or outbreaks in Nigeria, Benin, Burkina Faso, Côte d’Ivoire, Guinea, Ghana, Liberia, Mali, Sierra Leone and Togo.

Transmission

Lassa fever is mainly a rodent-borne disease. WHO says people usually become infected through direct contact with infected Mastomys rodents, or through food and household items contaminated with the rodents’ urine or faeces. The virus can also be inhaled in dust particles. This means risk is often linked to homes, stored food, and places where rats live close to people. Human-to-human spread can happen, but WHO describes the rate as low. When it does happen, it is usually through direct contact with blood or other body fluids from an infected person. Hospitals and clinics are a particular concern if a case is not recognized early and infection prevention measures are weak. Several WHO outbreak notices also mention infections in health workers, which fits this pattern. For most people outside affected settings, the bigger risk is contact with contaminated rodent waste, not casual contact with another person.

Symptoms and severity

Lassa fever can be hard to spot because many infections do not look dramatic. WHO says about 80% of infections are asymptomatic or mild. When symptoms do happen, they can include fever, weakness, headache, vomiting, and muscle pain. In some patients the illness becomes severe, with bleeding, shock, or failure of several organs. WHO notes that the disease can range from no symptoms at all to multiple organ dysfunction and death, so laboratory testing is often needed to confirm the diagnosis. Severity is not the same in every group. WHO highlights late pregnancy as especially dangerous, with maternal death and or fetal loss in the third trimester in over 80% of cases. WHO outbreak assessments state that the overall case fatality rate is about 1%, but among hospitalized patients with severe disease it is usually between 1% and 15%. Nigeria’s 2023 outbreak reported a 17% case fatality ratio among confirmed cases.

Treatment and vaccines

There is no approved vaccine for Lassa fever in the sources reviewed here. That means control still depends mostly on preventing exposure, finding cases quickly, and giving good supportive care. WHO says early supportive care with rehydration and treatment of symptoms improves survival. In practice, that means careful fluids, monitoring, and treating complications while the body fights the infection. Ribavirin has been used for Lassa fever for many years, and WHO outbreak reports say it is given in response efforts. But WHO also states that its efficacy is unproven, so it should not be described as a clearly proven cure from the evidence available here. WHO notices from Togo and Nigeria mention shortages of antiviral treatment and supportive therapy as response challenges, which matters because even a useful treatment is less helpful if clinics cannot get it quickly. For now, the best protection is reducing rodent exposure and improving infection prevention and control in health-care settings.

Where it occurs

Lassa fever is mainly a disease of West Africa. WHO says it is endemic there and lists confirmed outbreaks or sporadic cases in Benin, Burkina Faso, Côte d’Ivoire, Guinea, Ghana, Liberia, Mali, Sierra Leone, Togo and Nigeria. Nigeria appears often in the WHO archive and had a large outbreak in 2023. WHO also notes that transmission occurs throughout the year in Nigeria, with larger seasonal outbreaks during the dry season, typically from December to April. Outside West Africa, cases are usually imported after travel from an affected country. WHO has published notices on imported cases in the United Kingdom, the Netherlands, Germany, Sweden and the United States. Those events matter because they show the virus can travel with people, but they do not mean the disease is established in those countries. The main reservoir remains Mastomys rats in endemic areas, so the pattern of risk is closely tied to where those rodents are common and where they live near people.

Recent history

On this site’s archive, Lassa fever has 29 WHO notices since 1996. That tells a clear story: this is not a one-off event but a disease that returns repeatedly, especially in West Africa, with occasional imported cases elsewhere. Recent WHO notices show continued activity in Nigeria, including a large outbreak reported on 2023-05-01 with 4,702 suspected cases, 877 confirmed cases, and 152 deaths by epidemiological week 15 of 2023. In 2022, WHO published notices for Guinea, Togo, the United Kingdom, and Nigeria. Earlier archive entries include Benin, Liberia, Sierra Leone, Ghana and imported cases in Germany, the Netherlands, Sweden, the United Kingdom, and the United States. The archive pattern suggests two linked realities: regular endemic transmission in West Africa, and periodic exportation through travel. WHO’s risk language in recent notices generally keeps regional and global risk low, because the main transmission route is still contact with contaminated rodent waste rather than efficient person-to-person spread.

What to watch

Several signals would make Lassa fever more concerning. First, more cases in new countries or more sustained outbreaks outside the usual West African belt would suggest geographic expansion. The 2023 WHO assessment and recent reviews already point to concern about possible further spread of the reservoir or spillover risk. Second, any sign that human-to-human transmission is becoming more frequent would matter a lot, especially larger hospital outbreaks or spread without direct body-fluid exposure. Third, pressure on health systems is important: WHO notices mention shortages of isolation rooms, antiviral stock, supportive therapy, and delays in testing. Those problems can turn a controllable outbreak into a larger one. Fourth, treatment and prevention gaps remain important because there is still no approved vaccine and ribavirin’s benefit is described by WHO as unproven. Finally, infections in health workers are a warning sign that infection prevention measures may be failing. Better surveillance, testing, rodent control, and reliable clinical supplies are key markers to follow.

Updated