Highly pathogenic HP avian influenza viruses AIVs are naturally restricted to H5 and H7 subtypes with a polybasic cleavage site CS in hemagglutinin HA and any AIV with an intravenous pathogenicity index IVPI 1.2. Although only a few non H5 H7 viruses fulfill the criteria of HPAIV it remains unclear why these viruses did not spread in domestic birds. ...