Abstract
West Nile virus lineage 2 (WNV-2) was mainly confined to sub-Saharan Africa until the early 2000s, when it was identified for the first time in Central Europe causing outbreaks of human and animal infection. The aim of this study was to reconstruct the origin and dispersion of WNV-2 in Central Europe and Italy on a phylodynamic and phylogeographical basis. To this aim, discrete and continuous space phylogeographical models were applied to a total of 33 newly characterised full-length viral genomes obtained from mosquitoes, birds and humans in Northern Italy in the years 2013–2015 aligned with 64 complete sequences isolated mainly in Europe. The European isolates segregated into two highly significant clades: a small one including three sequences and a large clade including the majority of isolates obtained in Central Europe since 2004. Discrete phylogeographical analysis showed that the most probable location of the root of the largest European clade was in Hungary a mean 12.78 years ago. The European clade bifurcated into two highly supported subclades: one including most of the Central/East European isolates and the other encompassing all of the isolates obtained in Greece. The continuous space phylogeographical analysis of the Italian clade showed that WNV-2 entered Italy in about 2008, probably by crossing the Adriatic sea and reaching a central area of the Po Valley. The epidemic then spread simultaneously eastward, to reach the region of the Po delta in 2013, and westward to the border area between Lombardy and Piedmont in 2014; later, the western strain changed direction southward, and reached the central area of the Po valley once again in 2015. Over a period of about seven years, the virus spread all over an area of northern Italy by following the Po river and its main tributaries.
Highlights
West Nile virus (WNV) is a neurotropic mosquito-borne virus belonging to the Flavivirus genus and Japanese encephalitis virus serogroup [1]
We observed a high level of homogeneity of West Nile virus lineage 2 (WNV-2) European strains, which segregated into a main monophyletic clade, with the exception of a small monophyletic group including isolates obtained in eastern Europe and in Italy [15]
In 2011, we published an article describing the phylogeography of WNV-1a in Europe, which highlighted the genetic heterogeneity of the virus in eastern Europe [40]
Summary
West Nile virus (WNV) is a neurotropic mosquito-borne virus belonging to the Flavivirus genus and Japanese encephalitis virus serogroup [1]. WNV is maintained by an enzootic cycle involving birds ( birds of the Passeriformes order and migratory birds) and ornithophilic mosquitoes such as species of the Culex genus. Horses and other mammals are deadend hosts that may be incidentally involved in the enzootic cycle. Most human infections are asymptomatic, whereas fever may occur in 20% of cases and neuroinvasive disease in less than 1% [1]. WNV was first isolated in a febrile woman in Uganda in 1937 [2], and sporadic cases and small outbreaks were documented in Africa, the Middle East and Europe during the 1950s1960s [3]. In 1999, the virus was introduced into New York City, where it caused a dramatic outbreak that spread throughout the entire Western hemisphere in subsequent years {CDC, 1999 #41; [7]
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