Immunohistochemical stains confirmed the presence of WFBV in thymus and spleen but not in the bursa of Fabricius (Figure 2) The severity of lymphoid depletion was scored as marked if spleen, thymus, and bursa of Fabricius all showed an estimated [greater than or equal to]75% decrease in expected numbers of lymphocytes, moderate if severity of depletion varied between lymphoid organs but was estimated as a [greater than or equal to]50% decrease in lymphocytes, and mild if some lymphoid organs showed very little depletion whereas others showed moderate depletion (Table 2).
Pathologic abnormalities typical of natural WFBV disease were also present in eiders with major co-infections, and immunohistochemical staining confirmed the presence of virus associated with these lesions.
This study determined the pathogenicity of WFBV, a novel orthomyxovirus in the genus Quaranjavirus, in common eiders by observing clinical progression and pathologic changes in experimentally inoculated eider ducklings over a 10-day period.
Virus shedding occurred in only 2 infected eider ducklings very early in the course of infection with WFBV. This finding has practical implications; the diagnosis might easily be missed in naturally occurring infections if testing is confined to virus detection from oropharyngeal or cloacal swabs of dead or sick birds.
Results of this study suggest that immune suppression might play an important role in the pathogenesis of WFBV disease in eiders.
In 2010 in Australia, an orthomyxovirus with genetic and antigenic properties similar to WFBV (designated as Cygnet River virus [CyRV]) was isolated during an outbreak of salmonellosis (Salmonella enterica serovar Typhimurium) in Muscovy ducks (Cairina moschata) on Kangaroo Island, South Australia (10).
WFBV is the first Quaranjavirus to be definitively shown to cause major disease in its host species.
The potential importance of WFBV to common eider populations is not yet clear.