This illness was first documented in the town of Lassa, Nigeria, in 1969 and is confined to sub-Saharan West Africa (Nigeria, Liberia and Sierra Leone). The multimammate rat, Mastomys natalensis, is known to be the reservoir. Humans are infected by ingesting foods contaminated by rat urine or saliva containing the virus. Person-to-person spread by body fluids also occurs. Only 10-30% of infections are symptomatic.

Clinical features

The incubation period is 7-18 days. The disease is insidious in onset and is characterized by fever, myalgia, severe backache, malaise and headache. A transient maculopapular rash may be present. A sore throat, pharyngitis and lymphadenopathy occur in over 50% of patients. In severe cases epistaxis and gastrointestinal bleeding may occur - hence the classification of Lassa fever as a viral haemorrhagic fever. The fever usually lasts 1-3 weeks and recovery within a month of the onset of illness is usual. However, death occurs in 15-20% of hospitalized patients, usually from irreversible hypovolaemic shock.

Diagnosis

The diagnosis is established by serial serological tests (including the Lassa virus-specific IgM titre) or by culturing the virus from the throat, serum or urine. Molecular diagnosis by means of the reverse transcriptase polymerase chain reaction has become available and this provides a sensitive and reasonably rapid diagnostic test.

Treatment

Treatment is supportive. In addition, clinical benefit and reduction in mortality can be achieved with ribavirin therapy, if given in the first week.

In non-endemic countries, strict isolation procedures should be used, the patient ideally being nursed in a flexible-film isolator. Specialized units for the management of Lassa fever and other haemorrhagic fevers have been established in the UK. As Lassa fever virus and other causes of haemorrhagic fever (Marburg/Ebola and Congo-Crimean haemorrhagic fever viruses) have been transmitted from patients to staff in healthcare situations, great care should be taken in handling specimens and clinical material from these patients.