Lassa fever: origins, reservoirs, transmission and guidelines

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

Lassa virus is a member of the arenavirus family. The disease 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 endemic in parts of West Africa, particularly Guinea, Liberia, Nigeria and Sierra Leone, where the animal reservoir, the multimammate rat is prevalent.

See WHO map of Lassa fever distribution

There is some evidence of endemicity in the Central African Republic, Mali, Senegal and other West African countries sharing borders with the highly endemic countries.

In 2011, sporadic cases were confirmed for the first time in Ghana and the Democratic Republic of the Congo.

Benin confirmed its first cases in 2014, and reported an outbreak in 2016. Cases continue to be reported.

Togo reported two cases in 2016, both in healthcare workers in northern Togo. Togo had not previously reported confirmed Lassa cases despite being located between known endemic areas. Further cases were reported in 2017.

Burkina Faso reported a case in early 2017.


Lassa fever is endemic throughout Guinea, but most clinical cases have been reported from Kindia, Faranah and Nzerekore regions.

Lassa fever: map of Guinea (JPEG, 1.46MB)


Lofa, Bong, and Nimba counties are regarded as hyperendemic (areas of intense transmission). In 2014 an outbreak was reported at a UN Mission in Kakata, Margibi County.

Lassa fever: map of Liberia (JPEG, 1.01MB)


Lassa fever is endemic in Nigeria, with outbreaks almost every year in different parts of the country. Yearly peaks are observed between December and February. The Nigeria Centre for Disease Control now makes regular updates on Lassa fever. An increase in cases has been recorded since January 2018.

Sierra Leone

Historically, outbreaks were most frequently reported from Kenema and Kailahun districts.

Lassa fever: map of Sierra Leone (JPEG, 355KB)

A change in geographical spread is evident in the recent years, as shown on these maps.

Lassa fever: maps of Sierra Leone outbreaks 2008 onward (PDF, 210KB, 1 page)


Lassa virus is present in wild multimammate rats (Mastomys species), which shed the virus in their urine and droppings. These are common in rural areas of tropical Africa, and often live in or around homes. Once infected, rodents shed virus throughout their life.

Transmission of Lassa virus to humans normally occurs 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 consumed as food.

Person to person transmission occurs through infected bodily fluids, such as blood, saliva, urine or semen. 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.


Infection is mild or asymptomatic in 80% of cases, but can cause severe illness and is fatal in approximately 1 to 3% of patients. The incubation period for disease is usually between 7 and 10 days, with a maximum of 21 days.

The onset of illness is insidious, with:

  • fever and shivering
  • malaise
  • headache
  • generalised aching
  • sore throat

Nausea, vomiting, diarrhoea or cough can accompany these symptoms.

An important diagnostic feature is the appearance of patches of white or yellowish exudate and occasionally small vesicles or shallow ulcers on the tonsils and pharynx.

As the illness progresses the body temperature can rise to 41ºC with daily fluctuations of 2 to 3ºC.

Extreme lethargy and exhaustion can occur in severe attacks, that is disproportionate to the level of fever. During the second week of illness symptoms include:

  • oedema of the head and neck
  • encephalopathy
  • pleural effusion
  • ascites

Renal and circulatory failure may occur, aggravated by vomiting and diarrhoea.

In the severest 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 improves rapidly although tiredness can persist for several weeks. Late complications include sensorineural deafness in around 25% of patients, persisting for life in around a third of those affected.

Infection is fatal in around 15% of hospitalised patients.

Lassa fever is particularly severe in pregnant women in the third trimester; the fetus dies in about 95% of cases.

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.


Clinical diagnosis of Lassa fever is difficult. It can be confused with other infections such as severe malaria, typhoid fever, and other viral haemorrhagic fevers.

In the UK, Public Health England (PHE) has specialised laboratory facilities to provide a definitive diagnosis at the Rare and Imported Pathogens Laboratory, PHE Porton.

RT-PCR for nucleic acid detection, virus isolation or antibody detection methods are used to diagnose Lassa fever.

See VHF sample testing advice


Treatment with the antiviral drug ribavirin is most effective when started early in the course of clinical illness.

Supportive care such as fluid replacement, blood transfusion or other appropriate measures is also essential.


The UK has specialist guidance on the management (including infection control) of patients with viral haemorrhagic fevers, 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 no licensed vaccine for Lassa fever. 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 bodily fluids of an infected patient prevents person to person spread. In healthcare settings these 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 recovered they are only infectious via semen and urine. Patients must avoid sexual intercourse for 3 months.

People living in endemic areas of West Africa with high populations of rodents are most at risk of Lassa fever. Imported cases rarely occur elsewhere in the world. 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.

Imported cases in the UK

Imported Lassa fever cases are extremely rare in the UK. Eight confirmed cases of Lassa fever have been imported since 1980, with no evidence of onward transmission from any of these cases.

Published 5 September 2014
Last updated 16 February 2018 + show all updates
  1. Updated with current epidemiology.
  2. Updated with current epidemiology.
  3. Updated with 2017 data.
  4. Updated with recent outbreaks in Benin and Togo.
  5. Benin and Togo added to affected countries.
  6. First published.