Virulence of Avian Influenza H7 Viruses in Birds: The Impact of the Hemagglutinin Polybasic Cleavage Site on the Virulence of a Recent German H7N7 Virus and Pathogenicity of European H7N7 and H7N1 Viruses in Ducks

Avian influenza viruses of subtype H7Nx cause high economic losses and can evolve, similar to H5Nx viruses, to highly pathogenic viruses from low pathogenic ancestors. Some H7Nx viruses are endemic in Europe and are frequently reported to cause outbreaks in different poultry species. While turkeys and chickens are highly susceptible as indicated by high mortality, ducks are mostly clinically resistant to avian influenza virus (AIV). In 2015 a natural pair of low pathogenic AIV (LPAIV)/ high pathogenic AIV (HPAIV) H7N7 virus was isolated from the same farm in the district Emsland, North West Germany. The genetic comparison between the LP and the HP H7N7 virus showed mutations in all eight segments and a polybasic cleavage site motif in the HP virus leading to high virulence and 100% mortality in chickens. In this thesis virulence determinants of this virus were investigated by reverse genetics and infection experiments in chickens, turkeys and ducks. In the first publication we focused on the impact of the polybasic cleavage site of the H7N7 virus as a main virulence determinant in different bird species. While the polybasic cleavage site increased virulence and endothelial tropism of LPAIV in chickens to levels similar to the HP virus, it was less important but still necessary for high virulence in turkeys, but not in ducks. Pekin, Mallard and Muscovy ducks were susceptible to infection with HPAIV H7N7, but only Muscovy ducks showed mild to moderate clinical signs. Since Mallard ducks shed remarkable amounts of virus, they may play a role in the silent spread of HPAIV H7 viruses. In the second publication, we assessed virulence of different European H7 viruses in Muscovy and Pekin ducks. In Muscovy ducks, the Fowl Plague Virus (FPV) H7N7 from 1927 caused 20% mortality, while the two H7N1 viruses from 1934 from Germany and from 1999 from Italy showed moderate to high virulence with a mortality rate ranging from 20 to 80% after intrachoanal inoculation. Although all viruses were highly virulent in chickens and possess a polybasic CS they exhibit variable virulence in ducks. These findings show the importance of the investigation of virulence determinants of H7 viruses in ducks. Taken together, virulence determinants and pathobiology of HPAIV H7 is different according to the bird species. In chickens, the polybasic CS is the major determinant for virulence and tropism, while other gene segments are important for virulence and transmission in turkeys. Chickens and turkeys are dead-end hosts playing minimal role in the spread of HPAIV, whereas asymptomatically infected ducks can further spread the virus. These findings are crucial to understand the molecular biology of HPAIV in different bird species.