Epidemiological history and phylogeography of West Nile virus lineage 2

- We reconstructed the epidemiological history of West Nile virus lineage 2 (WNV-2).
- Two different WNV-2 datasets, based on E and NS5 gene sequences, were analysed.
- WNV-2 isolates segregated into several highly significant clades.
- Phylogeography suggests at least two events of WNV-2 spread outside Africa.
- Migratory birds may have played a role in the introduction of WNV-2 into Europe.

West Nile virus (WNV) was first isolated in Uganda. In Europe WNV was sporadically detected until 1996, since then the virus has been regularly isolated from birds and mosquitoes and caused several outbreaks in horses and humans. Phylogenetic analysis showed two main different WNV lineages. The lineage 1 is widespread and segregates into different subclades (1a-c). WNV-1a includes numerous strains from Africa, America, and Eurasia. The spatio-temporal history of WNV-1a in Europe was recently described, identifying two main routes of dispersion, one in Eastern and the second in Western Europe. The West Nile lineage 2 (WNV-2) is mainly present in sub-Saharan Africa but has been recently emerged in Eastern and Western European countries. In this study we reconstruct the phylogeny of WNV-2 on a spatio-temporal scale in order to estimate the time of origin and patterns of geographical dispersal of the different isolates, particularly in Europe. Phylogeography findings obtained from E and NS5 gene analyses suggest that there were at least two separate introductions of WNV-2 from the African continent dated back approximately to the year 1999 (Central Europe) and 2000 (Russia), respectively. The epidemiological implications and clinical consequences of lineage 1 and 2 cocirculation deserve further investigations.