[West Nile virus: a migrating arbovirus of current interest]

West Nile (WN) virus is a common arbovirosis in sub-Saharian Africa. It has occasionally caused epidemics or epizootics in horses in Mediterranean regions and southern Europe. The virus is transmitted to humans by mosquitoes (primarily the Culex species) that are infected by biting viremic birds. Infections in humans are usually asymptomatic. Recently, however, a growing number of cases involving central nervous system manifestations and deaths have been reported in elderly people in Algeria and Romania. Deaths have also been recorded in migrating birds in zones where the virus is emerging. An outbreak of WN virus in an urban area of North America in 1999 underscored the ability of viruses to appear suddenly in unexpected places. Molecular biology techniques are required for positive identification of WN virus. Serological tests alone do not allow differentiation from other flavivirus in the encephalitis group including Japanese encephalitis in Asia and Saint Louis encephalitis in North America. Virological monitoring of sentinel birds should provide a better understanding of epidemiological factors and of the pathological potential of circulating strains.