Merck Manual

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Lassa Fever and South American Hemorrhagic Fevers


Thomas M. Yuill

, PhD, University of Wisconsin-Madison

Reviewed/Revised Jun 2023
  • These infections spread from rodents to people, usually when people eat contaminated food.

  • Symptoms may include fever, muscle aches, headache, nausea, vomiting, cough, and sore throat, as well as bleeding from the mouth, nose, or internal organs.

  • To confirm the diagnosis, doctors do blood and urine tests.

  • Treatment includes giving fluids and other treatments to maintain body functions.

Lassa fever has occurred in Nigeria, Liberia, Guinea, Togo, Benin, Ghana, and Sierra Leone. The South American arenavirus hemorrhagic fevers occur in Bolivia, Argentina, Venezuela, and Brazil.

Caused by arenaviruses Arenaviruses Arbovirus, arenavirus, and filovirus are viruses that are spread from animals to people and, with some viruses, from people to people. The animal involved depends on the type of virus. Many... read more , these infections spread from rodents or their urine or droppings to people, usually when contaminated food is eaten. They can spread from person to person through contact with body fluids (such as saliva, urine, feces, or blood).


Symptoms of Lassa fever and South American arenavirus hemorrhagic fevers usually start about 5 to 16 days after people are exposed to the virus. These infections cause fever, a general feeling of illness (malaise), weakness, diffuse body aches, diarrhea, and vomiting. Over the next 4 to 5 days, chest pain, a sore throat, cough, and vomiting may develop. About 80% of cases of Lassa fever are mild and are often not diagnosed. But symptoms are severe in about 20% of people.

If severe, Lassa fever may cause the face and neck to swell, fluid to build up in the lungs, bleeding from the mouth, nose, vagina or digestive tract, and low blood pressure (see World Health Organization [WHO]: Lassa fever). About 20 to 30% of people with Lassa fever lose their hearing. Loss may be permanent.

Bleeding from the mouth, nose, stomach, and intestinal tract is common in South American hemorrhagic fevers. Obvious bleeding is less common in Lassa fever. But bleeding sometimes occurs from puncture wounds, the gums, or the nose and under the skin (seen as small purplish spots). When death occurs, it usually results from shock caused by widespread leakage of fluid from blood vessels.

Recovery or death usually occurs 7 to 31 days after symptoms begin. Approximately 15 to 20% of people hospitalized for severe Lassa fever die from the illness (Centers for Disease Control and Prevention [CDC]: Lassa Fever Signs and Symptoms).

Disease is severe during pregnancy, especially during the 3rd trimester. Most infected pregnant women lose the fetus (CDC: Lassa Fever Signs and Symptoms).


  • Blood tests

These infections are suspected when people who may have been exposed to the virus have characteristic symptoms. The diagnosis of Lassa fever or a South American hemorrhagic fever is confirmed by blood tests to identify the virus or antibodies to the virus.


  • Supportive care, including fluids

  • Ribavirin

The antiviral medication ribavirin does not cure Lassa fever but reduces the risk of death. It may also be beneficial in people with a South American hemorrhagic fever.


Strict isolation is required to prevent spread to health care workers and family members. Health care workers are at significant risk of becoming infected when treating hospitalized Lassa fever patients and should wear personal protective equipment. During an outbreak, quarantining people with symptoms of infection (fever and hemorrhage) has been an effective way to control outbreaks of these infections. Precautions are taken to prevent spread through the air.

No vaccine is available for Lassa fever. An experimental vaccine is effective against Argentine hemorrhagic fever, a South American hemorrhagic fever caused by the Junin virus.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Copegus, Moderiba, Rebetol, RibaPak, Ribasphere, Ribasphere RibaPak, RibaTab, Virazole
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