Causes

Lassa virus is zoonotic (transmitted from animals), in that it spreads to humans from rodents, specifically multimammate mice (Mastomys natalensis). This is probably the most common mouse in equatorial Africa, ubiquitous in human households and eaten as a delicacy in some areas. In these rodents, infection is in a persistent asymptomatic state. The virus is shed in their excreta (urine and feces), which can be aerosolized. In fatal cases, Lassa fever is characterized by impaired or delayed cellular immunity leading to fulminant viremia. Infection in humans typically occurs by exposure to animal excrement through the respiratory or gastrointestinal tracts. Inhalation of tiny particles of infectious material (aerosol) is believed to be the most significant means of exposure. It is possible to acquire the infection through broken skinor mucous membranes that are directly exposed to infectious material. Transmission from person to person has also been established, presenting a disease risk for healthcare workers. Frequency of transmission by sexual contact has not been established.