Centre national de référence des arbovirus et des fièvres hémorragiques virales, Unité de biologie des infections virales émergentes, Institut Pasteur, 25-28, rue du Docteur-Roux, 75015 Paris
West Nile (WN) virus infection, is usually non symptomatic in human and equine, but severe cases are reported. The viral cycle of transmission involves mosquitoes and birds. Usually outbreaks occur in temperate countries during the summer period, and mostly in humid regions where large colonies of birds are present. Since a few years, West Nile appears to be a re-emerging virus with several outbreaks in the Mediterranean region and Eastern Europe but especially with the discovery of the virus in North America in 1999. The phylogenetic analysis of several WN isolates, including those responsible for the recent outbreaks, demonstrates a repartition in two lineages. Isolates from Africa are present in both lineages, whereas isolates from Europe, Mediterranean Basin and India are exclusively in lineage I and isolates from Madagascar are in lineage II. The reasons of the absence of lineage II isolates outside Africa and Madagascar remain unknown. A better knowledge of the different factors involved in the WN virus transmission cycle is needed, specially the possible persistence of viral infection in birds and the possible role of ticks in WN virus dissemination. This would lead to a better understanding of the epidemiology of this complex arbovirus.