Rapid spread of a new West Nile virus lineage 1 associated with increased risk of neuroinvasive disease during a large outbreak in Italy in 2022

Abstract

Background. A new strain of WNV lineage 1 (WNV − 1) emerged in the Veneto Region, northern Italy, in 2021, eight years after the last outbreak of WNV − 1 in Italy. The virus, which co-circulates with WNV-2, has become endemic in the Region, where, in 2022, most human cases of neuroinvasive disease (WNND) reported in Europe have occurred. Methods. Comparative analysis of the epidemiology and clinical presentation of WNV-1 and WNV-2 infection in humans, as well as the temporal and geographic distribution of WNV-1 and WNV-2 among wild birds and Culex pipiens mosquitoes in Veneto, from May 16th to August 21st, 2022, to determine if the high number of WNND cases was associated with WNV-1. Results. As of August 21st, 2022, 222 human cases of WNV infection were confirmed by molecular testing, including 103 with fever (WNF) and 119 with WNND. WNV lineage was determined in 201 (90.5%) cases, comprising 138 WNV-1 and 63 WNV-2 infections. During the same period, 35 blood donors tested positive, including 30 in whom WNV lineage was determined (13 WNV-1 and 17 WNV-2). Comparative analysis of the distribution of WNV-1 and WNV-2 infections among WNND cases, WNF cases and WNV-positive blood donors showed that patients with WNND were more likely to have WNV-1 infection than blood donors (odds ratio 3.44; 95% CI 95% 1.54 to 8.24; p = 0.0043). As observed in humans, in wild birds WNV-1 had higher infectious rate (IR) and showed a more rapid expansion than WNV-2. At variance, the distribution of the two lineages was more even in mosquitoes, but with a trend of rapid increase of WNV-1 IR over WNV-2. Conclusions. Comparative analysis of WNV-1 vs WNV-2 infection in humans, wild birds, and mosquitos showed a rapid expansion of WNV-1 and suggested that WNV-1 infected patients might have an increased risk to develop severe disease.

Next-generation sequencing survey of acute febrile illness in Senegal (2020–2022)

“…While the importance of North American-to-African transmission of WNV is unclear and requires further study, the repeated observations of transmission between Europe and Africa can be explained by yearly bird migrations on the Mediterranean-Sahel flyway ( Hahn et al, 2009 ; Najdenski et al, 2018 ; García-Carrasco et al, 2022 ). Our maximum-likelihood molecular clock phylogeny and accompanying discrete phylogeographic analysis ( Figure 4 , bottom right) also revealed the relatedness of the WNV strains found in Bounkiling with at least 39 strains recently causing a neuroinvasive outbreak in northern Italy ( Barzon et al, 2022a , b ). Disconcertingly, the tree also exposes the need for additional genomic surveillance: the clade containing the Italian and Bounkiling strains (particularly BK160) shared a common ancestor more than 15 years ago, without any other genomic sequences collected during the interim.…”

Human Arboviral Infections in Italy: Past, Current, and Future Challenges

“…Most of the reported cases occurred in the Veneto region, where a new strain of WNV lineage 1 was introduced in 2021 and is currently co-circulating with WNV lineage 2. This strain of WNV lineage 1 seems to be associated with an increased rate of WNND [ 14 ].…”

Surveillance, prevention and control of West Nile virus and Usutu virus infections in the EU/EEA

“…There were multiple genetic lineages of WNV circulating in Europe. Lineage 2 strains (with a recent common ancestor) were well established and became predominant over lineage 1 strains, but lineage 1 strains still caused outbreaks (Spain 2021) or sporadic cases (Italy 2022) [161]. However, there was no evidence on differences in the virulence of the circulating, wild-type, lineage 1 and 2 strains.…”