Evidence of persistence and multiple genetic modifications of H7N7 low-pathogenic avian influenza virus in wild mallards in Poland provided by phylogenetic studies

Genetic characterization of the whole genome of four avian influenza H7N7 viruses isolated in three successive winter seasons (2007 to 2009) from wild mallards in three cities in Poland was performed. All of the tested strains were of low pathogenicity and no molecular marker associated with an increased adaptation to poultry, mammals or resistance to antiviral drugs was found. The major outcome of the phylogenetic studies was that the isolate A/mallard/Poland/446/09 (detected in December 2009) shared a recent common ancestor with A/mallard/Poland/41/09 (isolated in February 2009) in relation to HA and PB1 genes, with A/mallard/Poland/16/09 (found in January 2009) regarding NA and NS genes, and with A/mallard/Poland/01/08 (recovered in December 2007) as regards the NS gene. Interestingly, A/mallard/Poland/16/09 and A/mallard/Poland/446/09 were isolated at the same sampling site almost exactly 1 year apart, which points to resident population of mallards (and other resident waterfowl) as responsible for the perpetuation of avian influenza virus (AIV) in the given area between successive winters. On the other hand, the ornithological data discussed in detail in the paper strongly suggest that the virus transmission between close sites but located in different urban areas is most probably achieved by migratory birds, a fact additionally supported by a close relatedness between different gene segments of Polish H7N7 and AIV detected in wild birds in Europe. A high heterogeneity of the gene pool found in the study is indicative of frequent reassortment events. Additionally, two H7N7 isolates were shown to possess selected genes closely related to AIV detected in domestic poultry in Italy and the Czech Republic. The present study corroborates the importance of active surveillance in wild birds as a valuable tool for early warning of avian influenza in poultry.