Lassa fever: origins, reservoirs, transmission and guidelines
Source: gov.uk (2025-07-11)

Lassa virus causes Lassa fever, an acute viral haemorrhagic fever (VHF).

Background

Lassa virus is a member of the arenavirus family. Lassa fever, the disease caused by the Lassa virus, was first described in the 1950s, and the virus was identified in 1969 after 2 missionary nurses died from the disease in the Nigerian town of Lassa.

Lassa fever is a high consequence infectious disease (HCID) (https://www.gov.uk/guidance/high-consequence-infectious-diseases-hcid).

Reservoirs

Lassa fever is a zoonotic (animal-borne) infection that is spread by wild rodents (Mastomys natalensis). Once infected, rodents excrete the virus in their urine and droppings throughout their lifetime, therefore spreading infection. These rodents are common in rural areas of West and Central Africa and often live in or around homes.

Epidemiology

Lassa fever is endemic in parts of West Africa, particularly Guinea, Liberia, Nigeria and Sierra Leone, where the animal reservoir, the Mastomys rodent, is prevalent. This animal vector may be present throughout the region, with occasional cases and outbreaks occurring in countries neighbouring endemic areas. Further details can be found in Figure 1 and on the HCID: country specific risk webpage (https://www.gov.uk/guidance/high-consequence-infectious-disease-country-specific-risk), including details of travel-associated cases reported outside of known endemic areas. It should be noted that for any country, the absence of previously reported Lassa fever cases does not mean that locally acquired cases cannot occur in that country in the future, if the necessary transmission factors are present (for example, known vectors or potential reservoirs).

Figure 1. Map of Lassa fever in Africa. Please see the HCID: country specific risk webpage (https://www.gov.uk/guidance/high-consequence-infectious-disease-country-specific-risk) for details of travel-associated Lassa fever cases reported outside of Africa

Transmission

Transmission of Lassa virus to humans usually occurs through ingestion or inhalation of infected materials. Infection can also occur through contamination of broken skin or mucous membranes via direct or indirect contact with infected rodent excreta on floors, home surfaces, in food or water. Transmission is also possible where rodents are caught and prepared for consumption.

Human to human transmission can occur through contact with infected bodily fluids, such as blood, saliva, urine, or semen of an infected human. This can occur in healthcare or domestic settings. Transmission to close contacts usually only occurs while the patient has symptoms. However, a patient can excrete virus in urine for between 3 and 9 weeks after the onset of illness and via semen for up to 3 months.

Symptoms

The incubation period for disease is usually between 7 and 10 days, with a maximum of 21 days. Infection is mild or asymptomatic in 80% of cases, while serious illness occurs in approximately 20% of symptomatic cases. Infection is fatal in around 1% to 3% of cases, although this is higher among hospitalised patients (up to 15% to 20% of symptomatic hospitalised patients died in some settings).

The onset of illness is gradual rather than sudden and usually includes:
- fever and shivering
- fatigue
- headache
- generalised pain
- sore throat
Nausea, vomiting, diarrhoea, or cough may also be present.

In severe cases, the patient can suffer from extreme tiredness and exhaustion disproportionate to the level of fever. In these cases, symptoms in the second week of illness include:
- respiratory distress
- persistent vomiting
- generalised pain
- facial swelling
- neurological impairment
- haemorrhage
Renal and circulatory failure may occur, aggravated by vomiting and diarrhoea.

In the most severe cases, bleeding into the skin, mucosae and deeper tissues occurs, usually leading to death.

In non-fatal cases, the fever subsides and the patient’s condition generally improves rapidly. Late complications include sensorineural deafness in around 25% of patients, persisting for life in around a third of those affected.

The case fatality rate increases further in pregnant women, particularly in the third trimester. The mortality rate in foetuses in infected mothers is estimated to be around 90%.

Symptoms in children are similar to those in adults, but infant infection can result in ‘swollen baby syndrome’ with oedema, abdominal distension, bleeding and often death.

Diagnosis

Clinical diagnosis of Lassa fever can be challenging in endemic settings, as it can be confused with other infections such as severe malaria, typhoid fever and other viral haemorrhagic fevers (VHF).

In the UK, the UK Health Security Agency (UKHSA) has specialised laboratory facilities where real-time polymerase chain reaction assays for nucleic acid detection can be performed in order to provide a definitive diagnosis. These are held at the Rare and Imported Pathogens Laboratory (https://www.gov.uk/government/collections/rare-and-imported-pathogens-laboratory-ripl) (RIPL), UKHSA Porton, which also hosts the Imported Fever Service (https://www.gov.uk/guidance/imported-fever-service-ifs): a clinical advisory and specialist diagnostic service for medical professionals managing travellers who have returned to the UK with fever.

See VHF sample testing advice (https://www.gov.uk/government/publications/viral-haemorrhagic-fever-sample-testing-advice).

Treatment

An antiviral drug called ribavirin has been reported to be useful if given early in the disease, but recently the evidence for this has been increasingly challenged and more work is needed to find the best treatment regimen. Alternative antivirals are in clinical trials.

Supportive therapy is an essential part of care, and if given early can improve survival rates. This includes maintaining good fluid and electrolyte balance, as well as managing diarrhoea, vomiting, fever and pain with medications and blood transfusions as required. Organ support such as dialysis and ventilation can improve survival in severely unwell patients.

Guidelines

The UK has specialist guidance on the management (including infection control) of patients with VHFs (https://www.gov.uk/government/publications/viral-haemorrhagic-fever-algorithm-and-guidance-on-management-of-patients), including Lassa.

It provides advice on how to comprehensively assess, rapidly diagnose and safely manage patients suspected of being infected, within the NHS, to ensure the protection of public health.

Prevention and control

There is currently no licensed vaccine for Lassa fever. People living in endemic areas of West Africa with high populations of the rodent reservoir are most at risk of Lassa fever infection. In endemic areas, rodent control and avoiding contact with rodents and their excreta helps prevent infection. Infection control includes storing food in rat-proof containers.

Avoiding contact with infected humans can help prevent infection. This includes avoiding contact with blood and other bodily fluids of those who are infected, as well as avoiding contact with contaminated items like bedding and medical equipment. Avoiding funeral practices that involve touching the dead body of people suspected of having died from Lassa fever infection also reduces the risk of infection. In healthcare settings, infection control measures include:
- special barrier nursing procedures
- VHF isolation precautions to isolate infected patients
- wearing protective clothing for contact with the patient
Once the patient has fully recovered, they can remain infectious via semen and urine. Patients are usually advised to abstain from sexual intercourse for at least 3 months.

Imported cases rarely occur outside areas of Africa (see Figure 1). Such cases are almost exclusively in persons who work in endemic areas in high-risk occupations such as medical or other aid workers. The risk to tourists is considered to be very low.

Cases in the UK

Imported Lassa fever cases are extremely rare in the UK. Since 1971, there have been 16 confirmed cases of Lassa fever reported in the UK. All were linked to travel or were travel associated.