Virologic findings in selected free-range mule duck farms at high risk for avian influenza infection

Prevalence of avian influenza infection in free-range mule ducks (a cross between Muscovy [Cairina moschata domesticus] and Pekin ducks [Anas platyrhychos domesticus]) is a matter of concern and deserves particular attention. Thus, cloacal swabs were collected blindly from 30 targeted mule flocks at 4, 8, and 12 wk of age between October 2004 and January 2005. They were stored until selection. On the basis of a positive H5 antibody detection at 12 wk of age with the use of four H5 antigens, the samples from eight flocks were selectively analyzed. Positive samples were first screened with a matrix gene-based real-time reverse transcriptase-polymerase chain reaction assay before virus isolation. Eight avian influenza subtypes (H5N1, H5N2, H5N3, H6N2, H6N8, and H11N9) and three avian paramyxovirus type 1 viruses were isolated. All 11 are characterized as low pathogenicity (LP) and avirulent, respectively, by in vivo tests and, when relevant, nucleotide sequencing of the hemagglutinin (or fusion [F]) protein cleavage site. Regarding H5 isolates, all of their eight genes belong to the avian lineage and some particular genetic traits were determined. H5 genes were fully sequenced and phylogenetically analyzed; they all belong to the Eurasian lineage, lack additional glycosylation sites, and do not cluster, suggesting separate introductions from the wild reservoir. None were grouped with the Asian isolates. The NI gene (H5N1 isolate) was very close genetically to an Italian LP-H7N1 gene. Antigenic relationships between these H5 isolates and others were assessed comparatively by crossed hemagglutination inhibition tests. All these data are very useful to control the evolution of H5 viruses at the genetic and antigenic level to better understand the source of new outbreaks (new introductions from wild birds or the result of spread among poultry) and to assess the immunity afforded by available vaccines. These data are useful also to update antigens for avian influenza survey and to choose the most suitable vaccine in the case of preventive vaccination of ducks.