Lassa Fever

Organism Lassavirus is an arenavirus.

Clinical features There is a gradual onset with fever, malaise, sore throat, cough, vomiting, diarrhoea and general aches and pains. By the second week, lymphadenop-athy, pharyngitis and a maculo-papular rash on the face or body develops. In severe cases, pleural effusion, encephalopathy, cardiac and renal failures can occur with a mortality of 15-20%.

In endemic areas, 80% of cases are mild or asymptomatic so that serological investigation will find a large number of people with past history of infection.

Diagnosis is often made on clinical criteria once the first case has been identified, with, in particular, inflammation of the throat and white tonsillar patches. Confirmatory diagnosis is made by testing IgM or IgG in urine, blood or throat washings, with ELISA, PCR or IFA, using extreme care.

Transmission is primarily through contact with the excreta (urine and faeces) of infected rodents, deposited on floors, beds or other surfaces, or through rat contamination of food or water. The main reservoir is the multi-mammate or grey rat, M. nata-lensis. This is probably the method of spread in the endemic area resulting in a large number of asymptomatic cases. However, in the severe case, all human body fluids are highly infectious so that secondary spread commonly occurs through contact with blood, urine, throat secretions and the aerosol produced by a coughing bout. The semen remains infectious for a considerable period of time so transmission via the sexual route can occur long after the person has recovered from their clinical illness.

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