2009 West Nile disease epidemic in Italy: first evidence of overwintering in Western Europe?

For the second consecutive year a West Nile disease (WND) epidemic has affected Italy causing disease in horses and humans. The infection re-occurred in the same places of the 2008 and moved westerly and southerly involving new areas and regions. The whole genome sequence of the Italian 2009 West Nile disease isolate (WNDV) was compared with those responsible for the 2008 WND outbreaks. The epidemiological findings of the two years of epidemic were compared as well. The high identity between 2008 and 2009 WNV strains (>99%), the earlier virus circulation in 2009 and the re-occurrence of the disease starting from the bordering infected areas reached by the infection in the previous year, strongly support the hypothesis of the overwintering of the virus and the endemisation to local host populations.