Epidemiology of West Nile Virus Infections in Humans, Italy, 2012-2020: A Summary of Available Evidences

Abstract

In Italy, human cases of West Nile virus (WNV) infection have been recorded since 2008, and seasonal outbreaks have occurred almost annually. In this study, we summarize available evidences on the epidemiology of WNV and West Nile neuro-invasive disease (WNND) in humans reported between 2012 and 2020. In total, 1145 WNV infection cases were diagnosed; of them 487 (42.5%) had WNND. A significant circulation of the pathogen was suggested by studies on blood donors, with annual incidence rates ranging from 1.353 (95% confidence intervals (95% CI) 0.279-3.953) to 19.069 cases per 100,000 specimens (95% CI 13.494-26.174). The annual incidence rates of WNND increased during the study period from 0.047 cases per 100,000 (95% CI 0.031-0.068) in 2012, to 0.074 cases per 100,000 (95% CI 0.054-0.099) in 2020, peaking to 0.377 cases per 100,000 (95% CI 0.330-0.429) in 2018. There were 60 deaths. Cases of WNND were clustered in Northern Italy, particularly in the Po River Valley, during the months of August (56.7%) and September (27.5%). Higher risk for WNND was reported in subjects of male sex (risk ratio (RR) 1.545, 95% CI 1.392-1.673 compared to females), and in older age groups (RR 24.46, 95% CI 15.61-38.32 for 65-74 y.o.; RR 43.7, 95% CI 28.33-67.41 for subjects older than 75 years), while main effectors were identified in average air temperatures (incidence rate ratio (IRR) 1.3219, 95% CI 1.0053-1.7383), population density (IRR 1.0004, 95% CI 1.0001-1.0008), and occurrence of cases in the nearby provinces (IRR 1.0442, 95% CI 1.0340-1.0545). In summary, an enhanced surveillance is vital for the early detection of human cases and the prompt implementation of response measures.

Keywords: West Nile Virus; West Nile neuro-invasive disease; spatiotemporal pattern.

Figure A1

Summary of Meteorologic Data (2012–2020) Broken down by Calendar Month.

Figure A1

Summary of Meteorologic Data (2012–2020) Broken down by Calendar Month.

Figure 1

Notification rates percent normalized by maximum year value in Italy, Greece, and Romania compared to the rest of European Union (2012–2019, data were retrieved from ECDC Surveillance Atlas of Infectious Diseases (https://atlas.ecdc.europa.eu/public/index.aspx; accessed on 23 April 2021)).

Figure 2

Number of laboratory diagnosed West Nile neuro-invasive disease (WNND) cases by month of symptom onset, Italy (2012–2020; no.  =  487).

Figure 3

Monthly notification rate of West Nile virus (WNV) diagnoses normalized by maximum value for Italy compared to the rest of European Union (a), of all new diagnoses of WNV compared to corresponding new diagnoses of West Nile neuro-invasive disease (WNND) cases in Italy alone (b).

Figure 4

Italian provinces characterized by human WNV infections between 2012 and 2020. In blue, areas where diagnoses were originally reported in 2012; in red, all provinces with cases reported in 2020; and in yellow provinces where at least one case of WNV infections was reported between 2013 and 2019.

Figure 5

Correlation between the incidence of cases of West Nile virus (WNV) infections in blood samples and West Nile neuro-invasive diseases (WNND) in the corresponding index areas (r = 0.617, p < 0.001).

Figure 6

Correlation between the incidence of cases of West Nile neuro-invasive disease (WNND) in the index areas and: daily average temperatures (r = 0.0982, 95% CI −0.01754–0.2114, p = 0.0916), daily average precipitation (r = −0.1666, 95% CI −0.2768 to −0.0520, p = 0.0045), and daily average relative humidity (r = 0.1376, 95% CI 0.0224–0.2393, p = 0.0194).

Figure 7

Three-dimensional representation of the incidence rate (z-axis), by meteorological factors, represented by daily temperature ((a), x-axis; (b), x-axis), relative humidity ((a), y-axis; (c), x-axis), and daily precipitation rate ((b,c), y-axis).