Effect of age on the pathogenesis and innate immune responses in Pekin ducks infected with different H5N1 highly pathogenic avian influenza viruses

► The pathogenicity of H5N1 HPAI viruses in ducks varies between different viruses. ► Young ducks are more severely affected by the more virulent H5N1 HPAI viruses. ► Infected older ducks had higher body temperatures then younger ducks. ► IFN-α, RIG-I, and IL-6 RNA gene expression was increased spleens of all infected ducks. ► Differences were observed in the levels of up-regulation of some innate immune genes.