Lassa fever

WHO remains mobilized against Lassa fever as number of cases go down in affected areas

4 March 2019 - While the number of new Lassa fever cases have significantly reduced over the past weeks, intensified response activities are on-going to control the outbreak across Nigeria. To fully contain the outbreak, WHO remains mobilized and is supporting Nigeria to stem further spread of the disease.

“As an influential leader of Etsako East, one of the worse hit areas, I am impressed with the continued sensitization and follow up visits by WHO personnel to ensure that our people are not infected”, says Mr John Onyene of Edo State.

Nigeria's Lassa fever outbreak contained, but continued vigilance needed


10 May 2018 - With six weeks of declining numbers and only a handful of confirmed cases reported in recent weeks, the critical phase of Nigeria’s largest-ever Lassa fever outbreak is under control. However, Nigeria is endemic for Lassa fever and people could be infected throughout the year, making continued efforts to control any new flare ups crucial.

WHO will continue to support the Nigerian government to maintain an intensified response to the current Lassa fever outbreak in Nigeria.

On the trail of Lassa fever in southern Nigeria


22 March 2018 - In Edo state – where the outbreak has been spreading particularly fast – WHO, the Nigeria Centre for Disease Control (NCDC) and the local government are reaching out to communities with a large-scale awareness raising campaign. Sensitization sessions will aim to reach nearly 9,000 community leaders, town announcers, headmasters, herbalists, healthcare workers, clinicians, church leaders, and women who work in local markets.

On the frontlines of the fight against Lassa fever in Nigeria


19 March 2018 – The institute of Lassa Fever Control in Irrua is at the epicentre of Nigeria’s response to the country’s worst outbreak of Lassa fever on record. It is located in Edo state, where more than 40% of the 365 confirmed cases have occurred.

NCDC, WHO and partners have sent staff to support the institute at Irrua. WHO experts are working with healthcare workers to ensure that the infection does not spread within the health facility.

Nigeria battles its largest Lassa fever outbreak on record


1 March 2018 – The current Lassa fever outbreak in Nigeria shows an increasing trend in the number of cases and deaths in recent weeks with 317 confirmed cases reported in 2018 so far.

WHO is supporting the NCDC-led response with a focus on strengthening coordination, surveillance, contact tracing, laboratory testing, clinical management of patients, and community engagement.

WHO/M. Hotowossi, K. Vewonyi

Lassa fever is an acute viral haemorrhagic illness caused by Lassa virus, a member of the arenavirus family of viruses. It is transmitted to humans from contacts with food or household items contaminated with rodent excreta. The disease is endemic in the rodent population in parts of West Africa. Person-to-person infections and laboratory transmission can also occur, particularly in the hospital environment in the absence of adequate infection control measures. Diagnosis and prompt treatment are essential.

Humans usually become infected with Lassa virus from exposure to urine or faeces of infected Mastomys rats. Lassa virus may also be spread between humans through direct contact with the blood, urine, faeces, or other bodily secretions of a person infected with Lassa fever. There is no epidemiological evidence supporting airborne spread between humans. Person-to-person transmission occurs in both community and health-care settings, where the virus may be spread by contaminated medical equipment, such as re-used needles. Sexual transmission of Lassa virus has been reported.

Lassa fever occurs in all age groups and both sexes. Persons at greatest risk are those living in rural areas where Mastomys are usually found, especially in communities with poor sanitation or crowded living conditions. Health workers are at risk if caring for Lassa fever patients in the absence of proper barrier nursing and infection prevention and control practices.

The incubation period of Lassa fever ranges from 2–21 days. The onset of the disease, when it is symptomatic, is usually gradual, starting with fever, general weakness, and malaise. After a few days, headache, sore throat, muscle pain, chest pain, nausea, vomiting, diarrhoea, cough, and abdominal pain may follow. In severe cases facial swelling, fluid in the lung cavity, bleeding from the mouth, nose, vagina or gastrointestinal tract and low blood pressure may develop.

Protein may be noted in the urine. Shock, seizures, tremor, disorientation, and coma may be seen in the later stages. Deafness occurs in 25% of patients who survive the disease. In half of these cases, hearing returns partially after 1–3 months. Transient hair loss and gait disturbance may occur during recovery.

Death usually occurs within 14 days of onset in fatal cases. The disease is especially severe late in pregnancy, with maternal death and/or fetal loss occurring in more than 80% of cases during the third trimester.

Because the symptoms of Lassa fever are so varied and non-specific, clinical diagnosis is often difficult, especially early in the course of the disease. Lassa fever is difficult to distinguish from other viral haemorrhagic fevers such as Ebola virus disease as well as other diseases that cause fever, including malaria, shigellosis, typhoid fever and yellow fever.

Definitive diagnosis requires testing that is available only in reference laboratories. Laboratory specimens may be hazardous and must be handled with extreme care. Lassa virus infections can only be diagnosed definitively in the laboratory using the following tests:

  • 1. reverse transcriptase polymerase chain reaction (RT-PCR) assay
  • 2. antibody enzyme-linked immunosorbent assay (ELISA)
  • 3. antigen detection tests
  • 4. virus isolation by cell culture.

The antiviral drug ribavirin seems to be an effective treatment for Lassa fever if given early on in the course of clinical illness. There is no evidence to support the role of ribavirin as post-exposure prophylactic treatment for Lassa fever.

There is currently no vaccine that protects against Lassa fever.

Prevention of Lassa fever relies on promoting good “community hygiene” to discourage rodents from entering homes. Effective measures include storing grain and other foodstuffs in rodent-proof containers, disposing of garbage far from the home, maintaining clean households and keeping cats. Because Mastomys are so abundant in endemic areas, it is not possible to completely eliminate them from the environment. Family members should always be careful to avoid contact with blood and body fluids while caring for sick persons.

In health-care settings, staff should always apply standard infection prevention and control precautions when caring for patients, regardless of their presumed diagnosis. These include basic hand hygiene, respiratory hygiene, use of personal protective equipment (to block splashes or other contact with infected materials), safe injection practices and safe burial practices.

Health-care workers caring for patients with suspected or confirmed Lassa fever should apply extra infection control measures to prevent contact with the patient’s blood and body fluids and contaminated surfaces or materials such as clothing and bedding. When in close contact (within 1 metre) of patients with Lassa fever, health-care workers should wear face protection (a face shield or a medical mask and goggles), a clean, non-sterile long-sleeved gown, and gloves (sterile gloves for some procedures).

Laboratory workers are also at risk. Samples taken from humans and animals for investigation of Lassa virus infection should be handled by trained staff and processed in suitably equipped laboratories under maximum biological containment conditions.

On rare occasions, travellers from areas where Lassa fever is endemic export the disease to other countries. Although malaria, typhoid fever, and many other tropical infections are much more common, the diagnosis of Lassa fever should be considered in febrile patients returning from West Africa, especially if they have had exposures in rural areas or hospitals in countries where Lassa fever is known to be endemic. Health-care workers seeing a patient suspected to have Lassa fever should immediately contact local and national experts for advice and to arrange for laboratory testing.

Technical information

Technical information resources

A one-stop-shop for all the publications, guidance and other information resources on Lassa fever.

Disease outbreak news

General information

Training and learning

This course provides a general introduction to Lassa fever and is intended for frontline responders engaged in preventing and managing outbreaks. The course takes approximately 1 hour to complete.

Geographic distribution

This map shows the geographic distribution of Lassa fever in West African affected countries, 1969-2018.

Contact information

Department of Pandemic and Epidemic Diseases
World Health Organization
Avenue Appia 20
1211 Geneva 27, Switzerland
Email: edpln@who.int