Abstract
West Nile virus (WNV) is a zoonotic arboviral pathogen transmitted by mosquitoes in a cycle involving wild birds as reservoir hosts. The virus has recently emerged in North America and re-emerged in Europe. North American WNV outbreaks are often accompanied by high mortality in wild birds, a feature that is uncommon in Europe. The reason for this difference is unknown, but the intrinsic virulence of the viruses circulating in each continent and/or the susceptibility to the disease of Palearctic as opposed to Nearctic wild bird species could play a role. To assess this question, experimental inoculations with four lineage 1 WNV strains, three from southern Europe (Italy/2008, Italy/2009 and Spain/2007) and one from North America (NY99) were performed on house sparrows (Passer domesticus), a wild passerine common in both continents. Non-significant differences which ranged from 0% to 25% were observed in mortality for the different WNV strains. Viremias lasted from 1 to 5–6 days post-inoculation (dpi) in all cases; individuals inoculated with NY99 had significantly higher titres than those inoculated with any of the Euro-Mediterranean strains. Remarkably, host competence was found to be higher for NY99 than for the other strains. Consequently, albeit being pathogenic for house sparrows, some Euro-Mediterranean strains had reduced capacity for replication in -and transmission from- this host, as compared to the NY99 strain. If applicable also to other wild bird host species, this relatively reduced transmission capacity of the Euro-Mediterranean strains could explain the lower incidence of this disease in wild birds in the Euro-Mediterranean area.
Highlights
West Nile virus (WNV, Flaviviridae family, Flavivirus genus) is an arthropod-borne pathogen of humans, horses and some birds [1]
Viruses and virus preparations The four West Nile virus strains used in this work were as follows: NY99-crow-V76/1, a North American WNV strain, isolated from a diseased crow during the WNV outbreak in New York in 1999 (GenBank accession n° FJ151394); GE-1b/B, isolated from a golden eagle that died as a consequence of WNV infection in Toledo, Spain in 2007, (GenBank accession n° FJ766331 [20]; hereafter “Spain/2007”); Italy 15803/08, isolated from a magpie hunted during a pest-control programme in Ravenna, Italy in 2008 (GenBank accession n°: FJ483549; hereafter “Italy/ 2008”); and Italy/2009, isolated from a yellow-legged gull found moribund in Ravenna, Italy, in 2009, along with several other WNV-infected individuals of the same species, that died afterwards in a wildlife rehabilitation centre (GenBank accession n°: JF719067)
The results shown in this work indicate that at least some WNV strains circulating in southern Europe are pathogenic for an avian species as common as the HoSp
Summary
West Nile virus (WNV, Flaviviridae family, Flavivirus genus) is an arthropod-borne pathogen of humans, horses and some birds [1]. In Europe, where the disease is re-emerging, WNV only causes sporadic clinical cases and self-limited outbreaks, with no (or only very limited) wild bird mortality [4,5]. The reason for these observed differences in wild bird mortality is unknown but could be. As a widespread wild bird species, HoSp could potentially represent a useful model for experimental studies on the comparative virulence of WNV strains with different geographical and/ or phylogenetic origins. The fact that an Old World WNV strain is as virulent as the North American prototype strain for HoSp is intriguing since it implies that the perceived mortalities in HoSp should have occurred in the Old World WNV outbreaks when, no such observations have been reported
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