Lassa fever is caused by the zoonotic Lassa virus, a member of the Arenavirus family, one of 4 distinct families of viruses responsible for the viral hemorrhagic fever syndromes. It has been classified by the CDC as a Category A bioterrorism agent due to its ease of aerosol dissemination and ability to infect with very few organisms. One or more of the arenaviruses are known to have been weaponized by the Russians; therefore, any outbreak should be highly suspect for bioterrorism [a new arenavirus called Lujo virus was recently isolated in South Africa; patients develop Lujo-associated hemorrhagic fever, which has a high case fatality rate (80%), after inhalation of dust containing the virus, by ingestion of contaminated foods, or by inoculation with infected body materials]. The most likely method of dispersal of Lassa virus would be by aerosol release, but it is also contagious by contact with infected body fluids and tissues. The mortality rate of Lassa fever is approximately 15%-20% of symptomatic cases, although it can be up to 50% during occasional epidemics.

Whether acquired naturally or as a result of a bioterrorist attack, the symptoms of Lassa fever may be mild or subclinical in up to 80% of cases, although symptomatic cases would likely be higher following a bioterrorist attack due to the higher initial level of exposure. However, symptoms for the more severe cases, which appear gradually after an incubation period of approximately 7 days, include fever, malaise, dizziness, headache, sore throat, cough, nausea, vomiting, diarrhea, myalgia, chest pain, abdominal pain, back pain, cervical adenopathy, proteinuria, flushing of the face and trunk, and conjunctivitis. Small axillary petechiae may be noted. In some cases, the disease progresses to facial edema, neck swelling, pleural effusion, hemorrhages, seizures, encephalopathy, hypotension, and shock. Additionally, hair and hearing loss may occur. Spontaneous abortion occurs in 80% of cases involving pregnant patients.

Rodents of the genus Mastomys (multimammate rats) are the natural reservoir for the virus, and humans are infected when they come in contact with or inhale the dust from rodent droppings, urine, and nests. Incubation can vary from 3-21 days (usually 7-12 days). Because these rodents are sometimes consumed as food, transmission may occur from direct contact when caught or prepared. Person-to-person transmission is possible through direct contact with infected tissues and body fluids or by aerosol inhalation. Transmission may also occur from contaminated medical equipment.

Lassa fever is more prevalent during the dry season. It is endemic to Western Africa, specifically Guinea, Liberia, Sierra Leone, and Nigeria. It is rarely seen in the US or Europe; bioterrorism should be suspected in any case outside of Western Africa.

Precautions: Use airborne and contact precautions such as gloves, gown, hand washing, splash precautions, and HEPA or equivalent mask when cleaning rodent nests or treating patients with suspected Lassa virus infection. Isolate confirmed cases of Lassa fever. Lassa fever must be reported immediately to the local department of health and/or the CDC.