The complex epidemiological scenario of West Nile virus in Italy

Abstract

Entomological, veterinary, and human surveillance systems for West Nile virus (WNV) infection have been implemented in Italy since the first detection of the virus in 1998. These surveillance activities documented a progressive increase of WNV activity and spread in different regions and the emergence of new WNV lineages and strains. Italy is a paradigmatic example of the complex epidemiology of WNV in Europe, where sporadic cases of WNV infection, clusters, and small outbreaks have been reported in several regions. In addition, different strains of both WNV lineage 1 and lineage 2 have been identified, even co-circulating in the same area.

Figure 1

Maps of WNV activity and surveillance results in Italy, 1998–2012. In the map of the years from 1998 to 2007, wetland areas under surveillance for WNV infection are represented with circles; areas where seroconversions of anti-WNV IgGs in sentinel horses and/or chickens were detected are highlighted in yellow and the years reported. In the maps of the years from 2008 to 2012, areas involved by WNV activity, i.e., human and equine cases of WNV disease, WNV detection in birds and mosquitoes, and seroconversion in sentinel chickens, are indicated with dots.

Figure 2

Map of Italy showing the areas where different WNV strains were detected in the period from September 2008 to August 2013. WNV lineage 1 strains are indicated in blue; WNV lineage 2 strains are indicated in red.

Figure 3

Molecular phylogenetic analysis by Maximum Likelihood method of Italian WNV strains. The evolutionary history was inferred by using the Maximum Likelihood method based on the Tamura-Nei model [83]. The percentage of trees in which the associated taxa clustered together is shown next to the branches. Initial trees for the heuristic search were obtained by applying the Neighbor-Joining method to a matrix of pairwise distances estimated using the Maximum Composite Likelihood (MCL) approach. The trees are drawn to scale, with branch lengths measured in the number of substitutions per site. All positions containing gaps and missing data were eliminated. Evolutionary analyses were conducted in MEGA5 [84]. GenBank accession numbers of the sequences are indicated near the name of WNV strain. Italian WNV sequences are highlighted in bold. (a) Maximum Likelihood phylogenetic tree of WNV lineage 1 strains for partial NS5 and 3’UTR genomic sequences (659 nucleotides). The tree with the highest log likelihood (−1635.3077) is shown. The analysis involved 29 nucleotide sequences. (b) Maximum Likelihood phylogenetic tree of WNV lineage 2 strains for partial NS5 genomic and 3’UTR sequences (357 nucleotides). The tree with the highest log likelihood (–999.4010) is shown. The analysis involved 18 nucleotide sequences.