Lassa fever

Infectious Disease

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Lassa Fever is an infectious disease caused by the Lassa Virus. Although it can occasionally spread from person to person, most people become infected through contact with the urine or faeces (excrement) of an infected Mastomys rat.

These rodents carry and spread the virus amongst themselves without becoming ill, and shed the virus in their excrement.

Virus-carrying rodents often live in and around households and communities in parts of West Africa where food or surfaces can become contaminated. Unlike the rodents, humans can become unwell when infected with the Lassa virus, developing Lassa fever.

Lassa fever has been reported in Benin, Ghana, Guinea, Liberia, Mali, Nigeria and Sierra Leone, and it is likely present in other West African countries too. Due to international travel, cases have also sometimes been diagnosed outside of West Africa, including in the United Kingdom and the United States.

For many people, Lassa virus infection causes either mild symptoms or none at all. However, about 1 in 5 people may develop more serious illness, potentially affecting several organs and, in some cases, leading to bleeding from the mouth, nose or other parts of the body.

Approximately 1 in every 100 people infected with the virus will die from the disease. The risk of death is higher amongst those who need hospital treatment because of the disease, or those who catch the virus during pregnancy. 

Because early symptoms can be mild and similar to those of other illnesses that are common in Africa, such as malaria, it can be difficult to recognise Lassa fever straight away. However, quick diagnosis is critical to support recovery, and stop the disease spreading further.

 

Symptoms of Lassa fever start between 2-21 days after exposure to the virus. Initially people may suffer from the gradual onset of fever, malaise (extreme tiredness), and general weakness.

After a few days a headache, muscle pain, sore throat, chest pain, nausea, vomiting, diarrhoea, cough and abdominal pain may develop. If the disease becomes more severe, facial swelling, fluid in the lungs, bleeding from the mouth, nose, vagina and bowels, as well as low blood pressure may develop. In late severe disease, shock, seizures, and coma may be seen.

Among hospitalised patients, approximately 15% die from Lassa fever, with death typically occurring within 14 days of the onset of symptoms in fatal cases.

Infection in pregnancy, especially in the later stages, is usually associated with more severe disease. When contracted in the third trimester, the baby dies in around 80% of cases, and the mother in approximately 30%.

About 25% of people who survive the infection experience hearing loss. It can start during the illness or after a person begins to recover. In some cases, hearing may improve a little in the first few months, but it is often permanent.

Lassa fever can also affect the nervous system (the brain, spinal cord and nerves), which may cause trouble with walking or moving normally. Other effects can include changes in vision, joint pain, hair loss, and sometimes mental health problems.

 

Humans most commonly contract Lassa fever by contact with food or other household items contaminated with the excrement of infected Mastomys rats, or through direct contact with them.  Lassa virus has been detected in other types of rodents and some other mammals in West Africa too.

It can also be spread from human to human through direct contact with an infected person’s blood, urine, faeces or other bodily fluids. This can occur in the community or in healthcare settings, so infection control measures such as wearing plastic gloves or aprons are key to preventing the spread of infection when managing patients with Lassa fever.

Lassa virus has also been detected for a few months in the semen of those who are recovering from Lassa fever and patients are usually advised to abstain from sexual intercourse for at least 3 months.

There is no evidence of airborne spread of Lassa fever virus between humans.

 

There are no licensed vaccines available yet for the treatment or prevention of Lassa fever, but there are several currently in development.

There are currently no approved medications for treating Lassa fever. Ribavirin, an antiviral drug, is sometimes used, but there isn’t strong evidence of its effectiveness. Patients who develop Lassa fever should receive careful medical care, focusing on good fluid management and addressing any specific symptoms or complications as they arise.

Strict infection control measures are important when caring for patients with Lassa fever, to prevent contact with blood, bodily fluids or contaminated surfaces and materials. This includes good hand hygiene, using protective equipment such as gloves, gowns and masks, as well as following safe burial practices. Laboratory workers should also take proper precautions when managing samples from patients potentially infected with Lassa fever. 

Given the uncertainty around the limited treatment options currently available, preventing Lassa fever is very important.

Prevention requires individual households and communities to make practical changes to discourage rodents from co-existing in their homes, to reduce contact with them. This involves keeping living spaces clean, storing food securely in rodent-proof containers, using proper waste disposal systems located at a safe distance from homes and ensuring all food consumed is thoroughly cooked.

As Mastomys, and other rodents that could carry the disease, are common across West Africa, it isn’t possible to stop contact with them completely or get rid of them from the environment.

 

Lassa fever was first formally identified in 1969 in the Nigerian town of Lassa, from which the disease got its name. However, doctors had begun noticing patients with symptoms similar to what we now recognise as Lassa fever as early as the 1950s.

Outbreaks

Lassa fever outbreaks happen most often during the dry season, although infections can occur at any time of the year. Although outbreaks continue to appear, the World Health Organisation (WHO) has recorded the following historical outbreaks in the following countries and years:

Nigeria – 2023, 2022, 2020, 2019, 2018, 2017, 2016, 2012

Guinea – 2022

Togo – 2022, 2017, 2016

Liberia – 2018, 2016

Benin – 2017, 2016

Burkina Faso – 2017

Sierra Leone – 2004, 2003, 2000, 1997, 1996

Ghana – 1997

Netherlands (following travel to Sierra Leone) – 2019

 

The UK Health Security Agency (UKHSA) reports there have been 16 confirmed cases of Lassa fever in the UK to date. All of them were connected to travel.

Diagnosis

Samples collected from suspected Lassa fever cases should be handled under maximum biological containment conditions.

Current diagnostic methods available for Lassa fever are

  1. RT-PCR - Reverse transcriptase polymerase chain reaction assay
  2. ELISA - Enzyme-linked immunosorbent assay
  3. Antigen detection tests
  4. Virus isolation by cell culture

 

Page last updated, Monday 1 December 2025