Lassa fever
Lassa fever is an infectious disease caused by Lassa virus, a member of the arenavirus family. The disease was first recognised in the 1950s and the virus was later identified in the town of Lassa, Nigeria in 1969. Lassa fever is endemic in parts of West Africa, particularly Guinea, Liberia, Nigeria and Sierra Leone, where the animal reservoir, the Mastomys (multimammate) rat is common. Occasional cases and outbreaks also occur in other parts of the region, mainly in countries next to endemic areas.
People living in risk areas of West Africa are most at risk of Lassa fever. For tourists and most travellers, the risk is very low. Imported cases are unusual and are generally only reported in people who work in endemic areas in high-risk jobs, such as field and agricultural workers. More rarely, cases are sometimes reported in health professionals or aid workers.
Multimammate rats shed the virus in their urine and faeces. Lassa virus usually spreads from these rats to humans by direct contact with rat urine or droppings to broken skin or mucous membranes (usually the mouth or nose) or by contact with infected rodent urine or faeces on floors, home surfaces and in food or water. Transmission is possible when infected rodents are consumed as food and Lassa virus can spread from person to person by direct contact with infected bodily fluids, such as blood, faeces, saliva, urine, semen and vomit. People can also become infected through contact with contaminated objects, like needles. An individual can excrete Lassa virus in urine for between three and nine weeks after the onset of illness and via semen for up to three months, so sexual intercourse must be avoided for three months after recovery.
Initial symptoms may include:
- fever and shivering
- tiredness
- headache
- general aches
- sore throat
Vomiting, diarrhoea or a cough can also occur.
In the later stages of the illness, it can cause bleeding from the mouth, nose, vagina or gastrointestinal tract (stomach and guts).
Most people infected with Lassa virus have few if any symptoms, while approximately 20% of infections cause severe illness. Overall, approximately 1% of infections are fatal. Lassa fever is particularly severe in late pregnancy, with mother and/or foetus (unborn baby) deaths occurring in more than 80% of cases during the third trimester.
Early supportive care with fluid replacement and blood transfusion, and symptomatic treatment can improve survival rates. An antiviral drug called ribavirin may be useful if given early in the illness. However, recently evidence for this has been challenged and more work is needed to find the best treatment regimen.
There is currently no vaccine available for Lassa fever.
Check our Country Information pages for destination specific news and outbreaks.
Prevention
UK travellers planning to visit areas at risk of Lassa fever should consider their plans carefully, in consultation with a travel health specialist.
All travellers to Lassa fever endemic areas should:
- Avoid contact with anyone with symptoms.
- Avoid contact with blood and body fluids and items that might have been contaminated with blood and body fluids, like clothes, bedding or medical equipment.
- Avoid areas with a risk of contamination with rat urine or droppings.
- Avoid eating, cooking or preparing any meat from an unknown source.
- Always wash and peel fruit and vegetables carefully.
- Store food in rodent-proof containers.
- Follow good food, water and personal hygiene advice, including careful, regular hand washing with soap and water (or alcohol gel if soap and water is unavailable).
- Follow safer sex advice - always use a condom.
- At funerals, mourners should avoid all contact with the deceased, their body fluids and their personal property.
Field and agricultural workers, aid workers and health professionals planning to undertake humanitarian work in areas where outbreaks or isolated Lassa fever cases are reported should seek advice, training, and personal protective equipment from their employer/organisation prior to travel. They should also be familiar with information provided on the United Kingdom (UK) Health Security Agency about Lassa fever.
UK travellers experiencing symptoms abroad should seek local medical advice as soon as possible.
Travellers returning to the UK from risk areas with symptoms should phone their GP for advice or call NHS 111. Returning travellers must remember to tell their doctor or health professional they travelled abroad recently and mention every country visited.
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Recent Lassa fever Outbreaks
Lassa fever in Sierra Leone
As of 18 February 2025, a confirmed fatal case of Lassa fever has been reported in Sierra Leone. Please see our Topics in Brief article for further details on Lassa fever.
Lassa fever in Nigeria
Between 12-19 January 2025, 359 suspected and 71 confirmed cases and 17 deaths from Lassa fever have been reported. In 2025, 10 states have reported at least one case, the majority from Bauchi, Edo and Ondo. Please see our Topics in Brief article for further details on Lassa fever.
Lassa fever in Liberia
Between 6 January 2022 and 12 January 2025, 180 confirmed Lassa fever cases and 56 deaths have been reported in Liberia. Most cases were reported from Suakoko and Grand Bassa County. Please see our Topics in Brief article for further details on Lassa fever.
Lassa fever in USA ex West Africa
The US Centers for Disease Control and Prevention (US CDC) and the Iowa Department of Health have reported a fatal case of suspected Lassa fever in an Iowa resident who returned to the US from West Africa in October 2024. Risk to the general public is extremely low. Please see our Topics in Brief article for further details on Lassa fever.
