Lassa fever is a highly infectious and sometimes fatal viral disease that occurs in western Africa.
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What Is Lassa Fever?
Lassa fever is an infectious illness caused by a virus. It is named after the town in Nigeria where it was discovered. Most people infected with the virus have only mild symptoms. But one out of five people with Lassa fever becomes very ill. Lassa virus affects approximately 100,000 to 300,000 people in western Africa each year.
Lassa virus is spread to humans by the Mastomys rodent, which is found in the grasslands and forests of tropical Africa, as well as in human homes. A person can catch the virus by touching objects that have been contaminated with the urine and droppings of the rodents. It is also possible to catch Lassa virus by breathing air near rat droppings, or by eating the rats for food. In addition, person-to-person transmission is common in village settings and in hospitals.
Symptoms of Lassa fever may include fever, pain in the chest, sore throat, cough, vomiting, and diarrhea. The virus is so infectious that medical personnel diagnosing the disease must take special precautions. One-third of people with Lassa fever will develop deafness that is sometimes permanent. One percent of people infected with the virus will die from it.
The U.S. and the World
Lassa fever was first identified in 1969.
- Since 1969, Lassa fever has been killing about 5,000 people a year and infecting as many as 300,000 in West Africa, the only region where it is found. Those numbers may underestimate the extent of the disease, because of poor reporting in some countries.
- About 15 to 20 percent of people who are hospitalized with Lassa fever die. In some areas with high rates of Lassa fever, like Sierra Leone and Liberia, approximately 15 percent of all hospital admissions involve people with Lassa fever.
How Is Lassa Fever Treated and Prevented?
Lassa fever can often be successfully treated with an antiviral drug called ribavirin when it is given within the first six days of illness. Because Mastomys rodents are found all over western Africa, however, it is unlikely that the virus can be prevented by getting rid of the rats. More promising methods of prevention include educating people about how to keep their homes free of rodents and developing a vaccine for Lassa fever.
Garrett, Laurie. The Coming Plague: Newly Emerging Diseases in a World Out of Balance. New York: Farrar, Straus, and Giroux, 1994.
The World Health Organization's Communicable Disease Surveillance
and Response division posts a fact sheet about Lassa Fever at its
The U.S. National Institute of Allergy and Infectious Diseases posts a
fact sheet about emerging infectious diseases at its website.