Influenza A viruses in feral Canadian ducks: extensive reassortment in nature

The current dogma of influenza accepts that feral aquatic birds are the reservoir for influenza A viruses. Although the genomic information of human influenza A viruses is increasing, little of this type of data is available for viruses circulating in feral waterfowl. This study presents the genetic characterization of 35 viruses isolated from wild Canadian ducks from 1983 to 2000, as the first attempt at a comprehensive genotypic analysis of influenza viruses isolated from feral ducks. This study demonstrates that influenza virus genes circulating in Canadian ducks have achieved evolutionary stasis. The majority of these duck virus genes are clustered in distinct North American clades; however, some H6 and H9 genes are clustered with those from Eurasian viruses. Genes appeared to reassort in a random fashion. None of the genotypes identified remained present throughout all of the years examined and most PA and PB2 genes that crossed over into swine were clustered in one phylogenetic grouping. Additionally, matrix genes were identified that branch very early in the evolutionary tree. These findings demonstrate the diversity of the influenza virus gene pool in Canadian ducks, and suggest that genes which cluster in specific phylogenetic groupings in the PB2 and PA genes can be used for markers of viruses with the potential for crossing the species barrier. A more comprehensive study of this important reservoir is needed to provide further insight into the genomic composition of viruses that crossover the species barrier, which would be a useful component to pandemic planning.