Fact sheet N°179
Updated March 2015
13 March 2015
Lassa fever is an acute viral haemorrhagic illness of 1-4 weeks duration that
occurs in West Africa.
The Lassa virus is transmitted to humans via contact with food or household
items contaminated with rodent urine or faeces.
Person-to-person infections and laboratory transmission can also occur,
particularly in hospitals lacking adequate infection prevent and control measures.
Lassa fever is known to be endemic in Benin (where it was diagnosed for the first
time in November 2014), Guinea, Liberia, Sierra Leone and parts of Nigeria, but
probably exists in other West African countries as well.
The overall case-fatality rate is 1%. Observed case-fatality rate among patients
hospitalized with severe cases of Lassa fever is 15%.
Early supportive care with rehydration and symptomatic treatment improves
Though first described in the 1950s, the virus causing Lassa disease was not
identified until 1969. The virus is a single-stranded RNA virus belonging to the virus
family Arenaviridae .
About 80% of people who become infected with Lassa virus have no symptoms.
One in five infections result in severe disease, where the virus affects several organs
such as the liver, spleen and kidneys.
Lassa fever is a zoonotic disease, meaning that humans become infected from
contact with infected animals. The animal reservoir, or host, of Lassa virus is a
rodent of the genus Mastomys, commonly known as the “multimammate rat.”
Mastomys rats infected with Lassa virus do not become ill, but they can shed the
virus in their urine and faeces.
Because the clinical course of the disease is so variable, detection of the disease in
affected patients has been difficult. However, when presence of the disease is
confirmed in a community, prompt isolation of affected patients, good infection
protection and control practices and rigorous contact tracing can stop outbreaks.
Symptoms of Lassa fever
The incubation period of Lassa fever ranges from 6-21 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 greater
than 80% of cases during the third trimester.
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
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 control practices.
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; and many other diseases that cause fever, including malaria, shigellosis,
typhoid fever and yellow fever.
Definitive diagnosis requires testing that is available only in specialized 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
antibody enzyme-linked immunosorbent assay (ELISA)
antigen detection tests
reverse transcriptase polymerase chain reaction (RT-PCR) assay
virus isolation by cell culture.
Treatment and vaccines
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 and control
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 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
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.
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