Lassa fever is an acute viral haemorrhagic illness that comes on quickly after infection. The Lassa virus, which causes the fever, belongs to the Arenaviridae virus family. The disease was first identified in 1969 in Nigeria, but has also been found in Benin, Ghana, Guinea, Liberia, Mali and Sierra Leone.
Lassa fever is a zoonotic disease, meaning that it spreads from animals to humans. The main animal reservoir for the virus is the multimammate rat (Mastomys natalensis), which is common throughout sub-Saharan Africa. These rats do not get sick from the virus but shed it in their urine and faeces.
About 80 per cent of those who are infected display no symptoms but for those who do become ill the onset of Lassa fever is usually gradual, with symptoms appearing one to three weeks after infection.
Mild symptoms may include:
Lassa fever can be hard to diagnose as the symptoms are varied and similar to diseases such as Ebola, typhoid and malaria.
Some people develop severe disease, with the virus affecting organs such as the liver, spleen and kidneys. Lassa fever is fatal in about one per cent of cases – among those who are hospitalised around 15 per cent will die. Other complications include hearing loss, with around a quarter of recovered patients with some level of deafness, which can be permanent. The disease is particularly dangerous in pregnancy with maternal death and or foetal loss in around 80 per cent of cases.
In more severe cases, additional symptoms can include:
The main way to contract Lassa fever is through contact with food or household items contaminated by the urine or faeces of infected rats, that is by eating contaminated food, touching contaminated objects, or breathing in air contaminated with the virus. The virus can also enter through open cuts or sores. Infection can also occur through direct contact with infected rats.
Person-to-person transmission can occur through contact with bodily fluids, such as blood, urine, faeces or semen of an infected person. This is more common in healthcare settings, where the virus can spread via medical equipment.
The antiviral drug ribavirin has been used to treat Lassa fever, although the evidence for its effectiveness is still under debate. Other antiviral treatments are in development. Supportive care is crucial for improving survival rates, this includes: rehydration and management of symptoms such as vomiting, diarrhoea, fever and pain. Blood transfusions, dialysis, and ventilation may also be necessary in severe cases.
Preventing Lassa fever involves controlling rodent populations and avoiding contact with rodents and their excreta. In endemic areas, it’s important to:
People should also avoid contact with the blood and body fluids of infected individuals, as well as contaminated items like bedding.
The non-profit vaccine developer IAVI launched the first ever phase II trial of a vaccine against the disease across three sites in Ghana, Liberia and Nigeria in 2024.
Lassa fever was first documented in 1969, with the first cases occurring in Lassa, Nigeria. About 300,000 people fall ill with the disease across West Africa every year although the true number of cases is thought to be much higher. Between 1971 and 2024 there have been 16 cases in the UK – all linked to travel to the region.
Climate change and population growth have forced humans and the rodents that carry the disease into closer contact. The WHO has classified Lassa fever as an epidemic threat that requires urgent action in terms of developing vaccines and treatments.
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