Phylogeography and epidemiological history of West Nile virus genotype 1a in Europe and the Mediterranean basin

Aim of this study was to reconstruct the temporal and spatial phylodynamics of WNV-1a, the genotype to which the majority of European/Mediterranean viral strains belongs, by using sequences retrieved from public databases. WNV-1a isolates segregated into two major clades: the recent West Mediterranean sequences formed a single monophyletic group within clade A. Clade B included sequences from East Mediterranean and America. Phylogeographic analysis suggested that WNV-1a probably originated in sub-Saharan Africa in the early XXth century, and then spread northwards since the late 1970s, via two routes: one crossing Eastern Mediterranean and the other the Western Mediterranean countries. Our data suggest that the circulation of the virus in a given geographical area usually precedes the onset of the outbreak by one year or more, and underline the importance of the spatial–temporal phylodynamics reconstruction in clarifying the recent epidemiology and in setting up an efficient surveillance system for emerging/reemerging zoonosis.