Fact Sheet No 179
Lassa fever is an acute viral illness of one to four weeks duration
caused by Lassa virus, a member of the arenavirus family of viruses. The disease was first
described in the 1950s, although the virus was not isolated until 1969. Consequences range
widely in severity, from asymptomatic infection without illness to extremely severe
illness which may have a fatal outcome.
In severe cases, hypotension or shock, pleural effusion (fluid in the lung cavity),
haemorrhage, seizures, encephalopathy (dysfunction of the brain) and swelling of the face
and neck are frequent. Approximately 15% of hospitalized patients die. The disease is more
severe in pregnancy, and fetal loss occurs in greater than 80% of cases.
- In clinical illness the onset is gradual, with fever, malaise, headache, sore throat,
cough, nausea, vomiting, diarrhoea, myalgia (painful muscles), and chest and abdominal
pain. The fever may be either constant or intermittent with spikes. Inflammation of the
throat and eyes is commonly observed.
- Hair loss and loss of coordination may occur in convalescence. In addition, deafness
occurs in 25% of patients, with only half recovering some function after one to three
months. Immunity to reinfection occurs following infection, but the length of this period
of protection is unknown.
- The clinical syndrome of Lassa fever is difficult to distinguish from severe malaria,
septicaemia (infections of the bloodstream), yellow fever and other viral haemorrhagic
fevers (e.g., Ebola). Inflammation of the throat with white tonsillar patches is an
important distinguishing feature.
- Definitive diagnosis requires testing that is available only in highly specialized
laboratories. Laboratory specimens may be biohazardous and must be handled with extreme
care at the highest level biosafety containment.
Treatment Specific treatment with the anti-viral drug, ribavirin
may be effective if given within the first six days of illness. Ribavirin should be given
intravenously for ten days.
It is transmitted to humans from wild rodents (the multimammate rat, Mastomys
natelensis). Lassa infection in rodents persists and the virus is shed throughout the
life of the animal. Disease transmission is primarily through direct or indirect contact
with excreta of infected rodents deposited on surfaces such as floors or beds, or in food
- Lassa fever occurs in Guinea, Liberia, Sierra Leone and regions of Nigeria.
- Person-to-person and laboratory infections occur, especially in the hospital
environment, by direct contact with blood (including inoculation with contaminated
needles), pharyngeal (throat) secretions or urine of a patient, or by sexual contact.
Person-to-person spread may occur during the acute phase of fever when the virus is
present in the throat.
- The virus may be excreted in the urine of patients for three to nine weeks from the
onset of illness. Lassa virus can be transmitted via semen for up to three months.
- All age groups are susceptible to Lassa infection. The incubation period is 6-21 days.
Control The main methods of control are isolation of cases,
disinfection, surveillance of contacts and rodent control.
Isolation: Hospital transmission has occurred when inadequate infection control
measures were practised. Therefore, strict barrier isolation of cases in a hospital room
and strict procedures for handling of body fluids and excreta should be maintained.
Disinfection: Patient's excreta, sputum, blood and all objects with which the
patient has had contact, including laboratory equipment used to carry out tests on blood,
should be disinfected with 0.5% sodium hypochlorite solution or 0.5% phenol with
detergent, and, as far as possible, by appropriate heating methods such as autoclaving,
incineration or boiling.
Laboratory tests should be carried out in special high-containment
facilities; if there is no such facility, specimen handling should be kept to a minimum
and performed only by experienced technicians using all available precautions such as
gloves and biosafety cabinets. When appropriate, serum may be heat-inactivated at 60°C (140°F) for one hour. Thorough
terminal disinfection with 0.5% sodium hypochlorite solution or a phenolic compound is
adequate; formaldehyde fumigation can be considered.
Surveillance of contacts: Identify all close contacts (people living with, caring
for, testing laboratory specimens from or having non-casual contact with the patient) in
the three weeks after the onset of illness. Close surveillance of contacts should be
established by conducting body temperature checks at least two times daily for three weeks
after last exposure. In case of temperature greater than 38.3°C (101°F), hospitalize
immediately in isolation facilities. The place of residence of the patient during the
three weeks prior to onset should be determined and a search initiated for unreported or
Rodent control: The ideal method of prevention in endemic areas is to prevent
contact between rodents and humans. However, Mastomys rodents are found widely in
Africa with the Lassa virus having been identified in the above-mentioned West African
countries. Closely related viruses are found in Zimbabwe and Mozambique.
Prophylaxis with ribavirin is recommended by some workers for close
contacts, but there is little information about its efficacy. No vaccine is currently
This species of rodent lives in close proximity to man and humans
can be infected by inhalation of tiny droplets (aerosols) of virus-laden rodent excreta,
by ingestion of contaminated foods or water, or through breaks in the skin.
Exposure may occur in the home or during occupational activities such
as agricultural work or mining. As mentioned above, hospital infections and
person-to-person transmission occur, but the number of these cases is small in comparison
to the number of community-acquired infections resulting from contact with rodents.
To the extent possible, people in endemic areas should restrict entry
of rats into their dwelling, isolate food supplies from rodents, eliminate habitats for
rats and minimize activities that produce aerosols containing rodent excreta.
- Previous outbreaks have been reported in Central African Republic, Liberia, Nigeria and
Sierra Leone. Serological evidence of human infection has been found in Democratic
Republic of the Congo, Guinea, Mali and Senegal.
- The most recent outbreak occurred in Sierra Leone. A total of 823 cases, including 153
deaths (18.6%), were reported from January 1996 to April 1997.
As Lassa fever may have a long (up to 21
day) incubation period, it is possible that travellers from endemic areas may be
incubating the disease. It is important that fevers of unknown origin in people coming
from these endemic areas be investigated for the possibility of Lassa fever. However, one
case of Lassa fever entering a non-endemic area should not arouse fear of an epidemic as
long as it is ensured that the correct infection control procedures are followed.
further information journalists can contact the Office of the Press Spokesperson, WHO,
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