Lassa fever (ORPHA:99824)
Lassa fever (LF) is a potentially severe viral hemorrhagic disease caused by Lassa virus and characterized by initial fever and malaise followed by gastrointestinal symptoms and, in severe cases, bleeding, shock and multi-organ system failure.
Epidemiology
LF is endemic in West Africa. Lack of surveillance prohibits accurate estimates of incidence, but estimates range up to 300,000-500,000 infections and 5,000-10,000 cases of LF per year. Up to 80% of infections are thought to be asymptomatic or mild.
Clinical description
After an incubation period of 1-3 weeks (range 3-21 days), patients typically present with the insidious onset of non-specific signs and symptoms including fever, sore throat, malaise, headache, chest pain and myalgia/arthralgia, followed rapidly by gastrointestinal manifestations (vomiting, diarrhea, abdominal pain) and, in some cases, rash. In the second week, severe cases develop neck and facial swelling, bleeding (usually from the nose and mouth), neurologic involvement, shock and multi-organ system failure. Mild-to-moderate leukopenia and thrombocytopenia are often present. Deafness is a sequelae in up to 30% of survivors.
Etiology
Over 25 different viruses cause viral hemorrhagic fever. LF is caused by Lassa virus, a member of the virus family Arenaviridae. Lassa virus is maintained in nature in the multimammate rat (Mastomys natalensis) and humans are infected through exposure to this rodent's excreta. Human-to-human transmission occurs through direct contact with blood or bodily fluids of infected persons.
Diagnostic methods
Common diagnostic modalities include cell culture (restricted to biosafety level-4 laboratories), serologic testing by enzyme linked immunosorbent assay (ELISA) or indirect fluorescent antibody (IFA), and reverse transcription polymerase chain reaction (RT-PCR). Because no commercial assays are presently available, these tests are typically performed only in a few specialized laboratories.
Differential diagnosis
LF is difficult to distinguish from a host of other febrile illnesses, at least during its onset. Other viral hemorrhagic fevers need to be excluded, as well as malaria, typhoid fever, leptospirosis, rickettsial infection (see these terms), and meningococcemia.
Management and treatment
Patients should be isolated and viral hemorrhagic fever precautions (face shields, surgical masks, double gloves, surgical gowns, and aprons) should be used to prevent nosocomial transmission. The nucleoside analogue drug ribavirin should be administered intravenously. Oral ribavirin may also be effective but is less so than the IV form. Otherwise, treatment generally follows the guidelines for severe septicemia. Anti-malarials and broad spectrum antibiotics should be considered until the diagnosis of LF can be confirmed. Persons who had unprotected contact with someone with LF should be monitored and post-exposure treatment with oral ribavirin considered.
Prognosis
The case-fatality rate in hospitalized patients is typically 15-20%. Shock, bleeding, neurological manifestations, high viremia, aspartate aminotransferase (AST > 150 IU/L) and pregnancy confer a poor prognosis.

                            Last update: 
                            December 2012


                            - Expert reviewer(s): 
                            Dr Daniel BAUSCH  -  Andrew BENNETT