Early start of seasonal transmission and co-circulation of West Nile virus lineage 2 and a newly introduced lineage 1 strain, northern Italy, June 2022

Abstract

In spring 2022, Europe faced an unprecedented heatwave, increasing the risk of West Nile virus (WNV) outbreaks. As early as 7 June 2022, WNV was detected in Culex mosquitoes in northern Italy, and – in the following days – in two blood donors, a patient with encephalitis, wild birds and additional mosquito pools. Genome sequencing demonstrated co-circulation of WNV lineage 2 and a newly introduced WNV lineage 1, which was discovered in the region in 2021.

Letter to the editor: Atypical weather is associated with the 2022 early start of West Nile virus transmission in Italy

“…To the editor: In the recent study by Barzon et al [1], the authors report an unusually early onset of West Nile virus (WNV) transmission in Italy in 2022. The virus was detected in mosquitoes on 8 June and in humans on 18 June, about one month earlier than usually detected.…”

Rapid increase in neuroinvasive West Nile virus infections in humans, Italy, July 2022

“…In 2018, as in other European countries [6], Italy experienced an exceptionally early and intense WNV transmission [3] with over 600 confirmed human infections [4]. In 2022, until 24 August, most cases (76%) and deaths (71%) associated with WNV infections in the EU and European Economic Area (EEA) were reported by Italy [7], where the WNV transmission season started as early as in 2018 [5]. We observed an increase in the number of human WNND cases in July that exceeded what was observed in the same period of 2018.…”

“…Co-circulation of WNV lineages 1 and 2 was detected [5], while only lineage 2 circulation was documented in 2018 [3]. However, based on available data, there is no definite evidence of lineage-associated increased disease severity in humans.…”

“…The first detection of WNV circulation in 2022 occurred in early June [5] in mosquitoes and birds, with human cases detected from 19 June and rapidly increasing in July (Figure 1). Strains of WNV lineage 1 and 2 are cocirculating [5].…”

West Nile Virus spread in Europe - phylogeographic pattern analysis and key drivers

“…Nevertheless, considerable differences have been observed between successive years, with for instance particularly severe regional WNV outbreak involving both humans and equids in 2018 12 detected in Italy, Serbia, and Greece 13 . Since the beginning of the 2022 transmission season, over 60 outbreaks among birds (mainly in Italy) have been reported and over 800 human cases of WNV infections were reported in seven countries, mainly in Greece and Italy 7 . Unlike in the United States, where the initial WNV (WNV-1) incursion in 1999 led to rapid dispersal of the virus throughout the continent 14 , patterns of spread have remained somewhat patchy in Europe and involved both WNV-1 and WNV-2.…”