Lassa fever is a viral haemorrhagic fever endemic in West Africa that is predominantly asymptomatic or results in mild febrile symptoms (approximately 80% of cases). Severe symptoms (e.g., haemorrhage, respiratory distress, repeated vomiting, facial swelling, shock) occur in around 20% of cases. The overall mortality rate is around 1%. However, the observed case fatality rate among hospitalised patients is reported to be up to 65% to 70%.
Lassa fever mainly presents with non-specific symptoms, such as fever and malaise, which can make clinical diagnosis difficult. In the late stages of illness, cough, chest pain, pharyngitis, and facial oedema may develop that can help distinguish Lassa fever from other illnesses.
Symptoms of Lassa fever can be particularly difficult to distinguish from malaria and typhoid; therefore, each of these potentially serious illnesses should be considered in individuals returning from endemic areas with appropriate histories.
Management involves early recognition of infection, effective isolation and infection control, early initiation of the antiviral drug ribavirin, and supportive care in a hospital setting. Ribavirin given orally is occasionally used as post-exposure prophylaxis, and only under certain circumstances.
Diagnosis is entirely dependent on an accurate history and understanding of the geography of disease, supported by laboratory investigations.
Lassa fever is an acute zoonotic viral haemorrhagic illness caused by infection with the Lassa virus. The Lassa virus, first identified in 1969, is a single-stranded RNA virus that belongs to the Arenaviridae family of viruses (genus: Mammarenavirus ). It is named after the town in Nigeria where the first cases occurred. The natural reservoir of the Lassa virus is the multimammate rat ( Mastomys natalensis ), a rodent found commonly in rural areas of tropical Africa that often colonises in or around human homes where food is stored. Transmission of the virus from rodent to human occurs via human ingestion of urine or faeces from an infected rodent, butchering and/or eating infected rodents, or inhaling air contaminated with infected rodent excretions.   Human-to-human transmission occurs via direct contact with blood, tissue, secretions, or excretions of an infected individual, mainly in the hospital setting. Human-to-human transmission is less common than rodent-to-human transmission.
Lassa fever is considered endemic in Sierra Leone, Liberia, Guinea, and Nigeria according to the US Centers for Disease Control and Prevention (CDC),  and additionally in Benin, Mali, and Ghana according to the World Health Organization (WHO).  The clinical disease is variable, ranging from mild non-specific symptoms (e.g., fever, malaise) to haemorrhagic fever and death. The overall case fatality rate among people infected with Lassa virus is 1%.   However, the observed case fatality rate among hospitalised patients is reported to be up to 65% to 70%.   Lassa fever is part of a group of conditions known as viral haemorrhagic fevers (VHFs). VHFs can be caused by viruses from several different families, all of which have the potential to cause disease with a haemorrhagic diathesis. 
Clinical Lecturer in Infectious Diseases/Virology
University College London
CH declares that she has no competing interests.
Consultant in Tropical and Travel Medicine
Hospitals for Tropical Diseases
London School of Hygiene and Tropical Medicine
RB is providing expert testimony on behalf of the Crown, to the courts, on Q fever.
Assistant Professor of Medicine & Pediatrics
Tulane University School of Medicine
JSS is a paid reviewer and author of articles on UpToDate.
Hospital for Tropical Diseases
University College London Hospitals
MB is Clinical Lead of the Hospital for Tropical Diseases, a contributing partner to the PHE Imported fever service that co-ordinates testing of samples from UK patients for viral haemorrhagic fevers.
Professor of Medicine
Perelman School of Medicine at the University of Pennsylvania
SG declares that he has no competing interests.
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