Large human outbreak of West Nile virus infection in north-eastern Italy in 2012

Abstract

Human cases of West Nile virus (WNV) disease have been reported in Italy since 2008. So far, most cases have been identified in north-eastern Italy, where, in 2012, the largest outbreak of WNV infection ever recorded in Italy occurred. Most cases of the 2012 outbreak were identified in the Veneto region, where a special surveillance plan for West Nile fever was in place. In this outbreak, 25 cases of West Nile neuroinvasive disease and 17 cases of fever were confirmed. In addition, 14 WNV RNA-positive blood donors were identified by screening of blood and organ donations and two cases of asymptomatic infection were diagnosed by active surveillance of subjects at risk of WNV exposure. Two cases of death due to WNND were reported. Molecular testing demonstrated the presence of WNV lineage 1 in all WNV RNA-positive patients and, in 15 cases, infection by the novel Livenza strain was ascertained. Surveillance in other Italian regions notified one case of neuroinvasive disease in the south of Italy and two cases in Sardinia. Integrated surveillance for WNV infection remains a public health priority in Italy and vector control activities have been strengthened in areas of WNV circulation.

Figure 1

Map of human West Nile virus (WNV) cases of WNV infection confirmed in North-eastern Italy, 2010–2012. WNND: cases of West Nile neuroinvasive disease; WNF: cases of West Nile fever; Donor: blood, tissue, and organ donors with confirmed WNV infection.

Figure 2

Cases of West Nile infection by week of symptom onset confirmed in North-eastern Italy, 2010–2012. WNND: cases of West Nile neuroinvasive disease; WNF: cases of West Nile fever; Donor: blood, tissue, and organ donors with confirmed WNV infection.

Figure 3

Molecular Phylogenetic analysis of WNV lineage 1 whole genome sequences by Maximum Likelihood method. The evolutionary history was inferred by using the Maximum Likelihood method based on the Kimura 2-parameter model [43]. The percentage of trees in which the associated strains clustered together is shown next to the branches (only for values ≥ 80). The tree is drawn to scale, with branch lengths measured in the number of substitutions per site. The analysis involved 35 nucleotide sequences. All positions containing gaps and missing data were eliminated. There were a total of 10,385 positions in the final dataset. Evolutionary analyses were conducted in MEGA5 [44].