Introduction of West Nile virus lineage 2 leads to neuroinvasive cases in humans and horses in Sicily, 2022-2023

In Italy, West Nile virus (WNV) has been consistently causing disease in humans, horses, and birds since 2008; it was initially confined to the northeast of the country and then spread gradually to the western and southern regions. WNV lineage 1 (WNV-L1) virus was the only lineage that circulated in Italy until 2011, but it was progressively replaced by WNV lineage 2 (WNV-L2). WNV disease in humans was not observed in Sicily, southern Italy, until 2016, when the first case of West Nile neuroinvasive disease (WNND) due to WNV-L1 was reported. Our results demonstrate the introduction of WNV-L2 in Sicily via the detection of four equine and five human cases of WNND reported in western Sicily in September 2022. Local WNV circulation was confirmed by virological and serological investigations in horses, birds, and dogs. WNV-L2 was demonstrated to be the strain responsible for both human and equine neuroinvasive cases and continued to circulate in western Sicily in 2023, where it was detected in insects and in a neuroinvasive human case in August.