Lassa Fever: DVRD

Special Pathogens Branch

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Main Index

Disease Information
- What Is a VHF?
- Fact Sheets
   - Arenaviruses
   - Lassa Fever
   - LCM
   - Rift Valley Fever
   - Filoviruses
   - Ebola HF
   - Marburg HF

- All About HPS

Teaching and Prevention Materials

Other Resources

Glossary of Terms

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References

CJ Peters and K Johnson. "Lymphocytic Choriomeningitis Virus, Lassa Virus, and Other Arenaviruses." In  Part III: Infectious Diseases and Their Agents : 1577-1578.

JT Meulen, I Lukashevich, K Sidibe, et al. "Hunting for Peridomestic Rodents and Consumption of Their Meat as Possible Risk Factors for Rodent-To-Human Transmission of Lassa Virus In the Republic of Guinea." American Journal of Tropical Medicine 1996, 55: 661-66.

CJ Peters. "Arenaviruses". In Richman D, Whitley RJ, Hayden FG (eds) Clinical Virology. New York: Churchill Livingstone: 1997: 973-996.

Looking for more information on Lassa fever?  See our Other Resources page.

Plus,  see our "Lassa" online video and Lassa Fever online slide set.

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 of this rodent reservoir impractical.

When caring for patients with Lassa fever, further transmission of the disease through person-to-person contact or nosocomial routes can be avoided by taking preventive precautions against contact with patient secretions (together called VHF isolation precautions or barrier nursing methods).  Such precautions include wearing protective clothing, such as masks, gloves, gowns, and goggles; using infection control measures, such as complete equipment sterilization; and isolating infected patients from contact with unprotected persons until the disease has run its course.

Image, above: Wearing protective clothing -- an important part of practicing barrier nursing methods.

What needs to be done to address the threat of Lassa fever?
Further educating people in high-risk areas about ways to decrease rodent populations in their homes will aid in the control and prevention of Lassa fever. Other challenges include developing more rapid diagnostic tests and increasing the availability of the only known drug treatment, ribavirin. Research is presently under way to develop a vaccine for Lassa fever.

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Special Pathogens Branch
Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases
Centers for Disease Control and Prevention
Public Health Service, U.S. Department of Health and Human Services

National Center for Infectious Diseases
Centers for Disease Control and Prevention

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