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

Matteo Riccò,Simona Peruzzi,Federica Balzarini

doi:10.3390/tropicalmed6020061

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.

Highlights

West Nile virus (WNV) is a mosquito-borne RNA virus belonging to the genus Flavivirus
In Northern Italy higher population density coincides with highly irrigated agriculture areas, a condition that in turn is favorable to the ecology of mosquitoes, including vectors competent for WNV [26,27]
The majority of new cases of West Nile neuro-invasive disease (WNND) occurred in August (56.7%), followed by September (27.5%), July (14.0%), and October (1.4%), with only one case reported in June and November (Figure 2)

Summary

Introduction

West Nile virus (WNV) is a mosquito-borne RNA virus belonging to the genus Flavivirus (family Flaviviridae). In Europe, WNV is usually carried by species of the genus Culex (mainly C. pipiens, C. peregrinus, and C. modestus) and Aedes [1,2,3], being sustained in an enzootic “amplification” cycle between birds and mosquitoes. Even though WNV can infect large mammalians (e.g., horses), including humans, the latter represent rather incidental and dead-end hosts [2]. Infection in humans is usually asymptomatic, but a mild influenza-like syndrome (i.e., West Nile fever, WNF) may be observed in around 20% of all cases [4,5], while less than 1% of infected subjects develop a neuro-invasive disorder, i.e., West Nile neuro-invasive disease (WNND), which typically affects elderly, chronically ill, and immunocompromised people. Even though horse vaccines are currently available, no human vaccines or specific antiviral treatments have been to date licensed [8]

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