Lassa fever is an acute viral haemorrhagic illness caused by Lassa virus, a member of the arenavirus family of viruses. Humans usually become infected through aerosol or direct contact with excreta of infected rodents.
Lassa fever can also be spread between humans through direct contact with the blood, pharyngeal secretions, urine, faeces or other body secretions of an infected person. Person-to-person transmission of Lassa fever has occurred in health care settings after exposure to blood and secretions of infected patients.
The incubation period for Lassa fever ranges from 2 to 21 days. About 80% of human Lassa fever infections are mild or asymptomatic. Fever, general weakness and malaise are usually the first symptoms to appear in symptomatic patients.
Lassa fever is difficult to distinguish from many other diseases which cause fever, including malaria, shigellosis, typhoid fever, yellow fever and other viral haemorrhagic fevers. The overall case fatality ratio is 1% to 15% among hospitalized patients. Ribavirin is effective treatment for Lassa fever is given early in the course of clinical illness.
Lassa fever is known to be endemic in Guinea, Liberia, Nigeria and Sierra Leone, but probably exists in other West African countries as well.