First report outside Eastern Europe of West Nile virus lineage 2 related to the Volgograd 2007 strain, northeastern Italy, 2014

Abstract

Background: West Nile virus (WNV) is a Flavivirus transmitted to vertebrate hosts by mosquitoes, maintained in nature through an enzootic bird-mosquito cycle. In Europe the virus became of major public health and veterinary concern in the 1990s. In Italy, WNV re-emerged in 2008, ten years after the previous outbreak and is currently endemic in many areas of the country. In particular, the northeastern part of Italy experience continuous viral circulation, with human outbreaks caused by different genovariants of WNV lineage 1, Western-European and Mediterranean subcluster, and WNV lineage 2, Hungarian clade. Alongside the WNV National Surveillance Program that has been in place since 2002, regional surveillance plans were implemented after 2008 targeting mosquitoes, animals and humans.

Findings: In July and September 2014, West Nile virus lineage 2 was detected in pools of Culex pipiens s.l. mosquitoes from northeastern Italy. Whole genome sequencing and phylogenetic analysis of two representative samples identified the presence of WNV lineage 2 related to the Volgograd 2007 strain (99.3 % nucleotide sequence identity), in addition to WNV lineage 2 Hungarian clade.

Conclusions: This is the first evidence of the circulation of a WNV lineage 2 strain closely related to the Volgograd 2007 outside Eastern Europe, where it has caused large human outbreaks. This strain may pose a new threat to animal and human health in Italy.

Fig. 1

Map showing the location of the entomological traps in 2014 in northeastern Italy and the sites of Culex pipiens s.l. mosquitoes positive for WNV lin.2 Volgograd-like (red) and Hungary-like strains (yellow)

Fig. 2

Phylogenetic tree of WNV lineage 2 complete sequences. Legend: Sequence dataset was analysed using MEGA5, the neighbour-joining (NJ) method, and bootstrap analysis (1,000 replicates) based on the ClustalW algorithm. Significant bootstrapping values (>70 %) are shown on the nodes. (●) Italian WNV lineage 2 genomes detected in pools of Culex pipiens s.l. in northeastern Italy