Disease Information
Viral Hemorrhagic Fevers: Fact
Sheets
Lassa Fever
What is Lassa fever?
Lassa fever is an acute viral illness that occurs in West Africa. The illness was
discovered in 1969 when two missionary nurses died in Nigeria, West Africa. The cause of
the illness was found to be Lassa virus, named after the town in Nigeria where the first
cases originated. The virus, a member of the virus family Arenaviridae, is a
single-stranded RNA virus and is zoonotic, or animal-borne.
In areas of Africa where the disease is
endemic (that is, constantly present), Lassa fever is a significant cause of morbidity and
mortality. While the disease is mild or has no observable symptoms in about 80% of people
infected with the virus, the remaining 20% have a severe multisystem disease. Lassa fever
is also associated with occasional epidemics, during which the case-fatality rate can
reach 50%.
Where is Lassa fever found?
Lassa fever is an endemic disease in portions of West Africa. It is recognized in Guinea,
Liberia, Sierra Leone, as well as Nigeria. However, because the rodent species which carry
the virus are found in other regions outside of West Africa, the actual geographic range
of the disease may extend to other portions of Africa.
How many people become infected?
The number of Lassa virus infections per year in West Africa is estimated at 100,000 to
300,000, with approximately 5,000 deaths. Unfortunately, such estimates are crude, because
surveillance for cases of the disease is not uniformly performed. In some areas of Sierra
Leone and Liberia, it is known that 10%-16% of people admitted to hospitals have Lassa
fever, which indicates the serious impact of the disease on the population of this region.
In what animal host is Lassa virus
maintained?
The reservoir, or host, of Lassa virus is a rodent known as the "multimammate
rat" of the genus Mastomys. It is not certain which species of Mastomys are
associated with Lassa; however, at least two species carry the virus in Sierra Leone: M.
huberti and M. erythroleucus. Mastomys rodents breed very frequently, produce large
numbers of offspring, and are numerous in the savannas and forests of West, Central, and
East Africa. In addition, some species, like M. huberti, prefer to live in human homes.
All these factors together contribute to the relatively efficient spread of Lassa virus
from infected rodents to humans.
Image, right: Mastomys
rodent, also known as the "multimammate rat". Note the hairless tail.
How do humans get Lassa fever?
There are a number of ways in which the virus may be transmitted, or spread, to humans.
The Mastomys rodents shed the virus in urine and droppings. Therefore, the virus can be
transmitted through direct contact with these materials, through touching objects or
eating food contaminated with these materials, or through cuts or sores. Because Mastomys
rodents often live in and around homes and scavenge on human food remains or poorly stored
food, transmission of this sort is common. Contact with the virus also occurs when a
person inhales tiny particles in the air contaminated with rodent excretions. This is
called aerosol or airborne transmission. Finally, because Mastomys rodents are sometimes
used as a food source, infection may occur via direct contact when they are caught and
prepared for food.
Lassa fever may also spread through
person-to-person contact. This type of transmission occurs when a person comes into
contact with virus in the blood, tissue, secretions, or excretions of an individual
infected with the Lassa virus. A person may also become infected by breathing in
small airborne particles which an already infected person may produce by actions like
coughing. The virus cannot be spread through casual contact (including skin-to-skin
contact without exchange of body fluids). Person-to-person transmission is common in both
village settings and in health care settings, where, along with the above-mentioned modes
of transmission, the virus also may be spread in contaminated medical equipment, such as
reused needles (this is called nosocomial transmission).
What are the symptoms of Lassa
fever?
Symptoms of Lassa fever typically occur 1-3 weeks after the patient comes into contact
with the virus. These include fever, retrosternal pain (pain behind the chest wall), sore
throat, back pain, cough, abdominal pain, vomiting, diarrhea, conjunctivitis, facial
swelling, proteinuria (protein in the urine), and mucosal bleeding. Neurological symptoms
have also been described, including hearing loss, tremors, and encephalitis. Because the
symptoms of Lassa fever are so varied and nonspecific, clinical diagnosis is often
difficult.
How is the disease diagnosed in the
laboratory?
Lassa fever is most often diagnosed by using enzyme-linked immunosorbent serologic assays
(ELISA), which detect IgM and IgG antibodies as well as Lassa antigen. The virus itself
may be cultured in 7 to 10 days. Immunohistochemistry performed on tissue specimens can be
used to make a post-mortem diagnosis. The virus can also be detected by reverse
transcription-polymerase chain reaction (RT-PCR); however, this method is primarily a
research tool.
Are there complications after
recovery?
The most common complication of Lassa fever is deafness. Various degrees of deafness occur
in approximately one-third of cases, and in many cases hearing loss is permanent. As far
as is known, severity of the disease does not affect this complication: deafness may
develop in mild as well as in severe cases. Spontaneous abortion is another serious
complication.
What proportion of people die from
the illness?
Approximately 15%-20% of patients hospitalized for Lassa fever die from the illness.
However, overall only about 1% of infection with the Lassa virus result in death. The
death rates are particularly high for women in the third trimester of pregnancy, and for
fetuses, about 95% of which die in the uterus of infected pregnant mothers.
How is Lassa fever treated?
Ribavirin, an antiviral drug, has been used with success in Lassa fever patients. It has
been shown to be most effective when given early in the course of the illness. Patients
should also receive supportive care consisting of maintenance of appropriate fluid and
electrolyte balance, oxygenation and blood pressure, as well as treatment of any other
complicating infections.
What groups are at risk for getting
the illness?
Individuals at risk are those who live or visit areas with a high population of Mastomys
rodents infected with Lassa virus or are exposed to infected humans. Hospital staff are
not at great risk for infection as long as protective measures are taken.
How is Lassa fever prevented?
 Primary transmission of the Lassa virus from its host to humans can
be prevented by avoiding contact with Mastomys rodents, especially in the geographic
regions where outbreaks occur. Putting food away in rodent-proof containers and keeping
the home clean help to discourage rodents from entering homes. Using these rodents as a
food source is not recommended. Trapping in and around homes can help reduce rodent
populations. However, the wide distribution of Mastomys in Africa makes complete control
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