Lassa fever: Epidemiology, clinical features, and social consequences

Lassa fever, a viral haemorrhagic fever transmitted by rats, is endemic in west Africa and may kill tens of thousands of people each year. Peak incidence was thought to be in the dry season (January to March), but data collected in Sierra Leone shows peaks in the overlap with the wet season (May to November). Many infections are subclinical; a high index of suspicion, given the difficulties of clinical diagnosis, is needed when travellers from west Africa present with a fever of unknown origin, with symptoms appearing up to 21 days after leaving the endemic area. The virus is excreted in semen for three months after infection; we do not know how frequently it may be transmitted through sexual intercourse. Attempts are being made to produce a vaccine using the yellow fever virus as a vehicle. The possibility that Lassa virus could be used as a biological weapon has raised the profile of the need for greater understanding of Lassa fever and for more effective control and treatment programmes.