Natural reassortants between potentially zoonotic avian influenza viruses H5N1 and H9N2 from Egypt display distinct pathogenic phenotypes in experimentally infected chickens and ferrets

Co-circulation of zoonotic highly pathogenic avian influenza virus (HPAIV) H5N1 and AIV of subtype H9N2 among poultry in Egypt for at least six years should render this country a hypothetical hotspot for the emergence of reassortant, phenotypically altered viruses, yet no reassortants have been detected in Egypt. The present investigations proved that reassortants between Egyptian H5N1 clade 2.2.1.2 and H9N2 of the G1-B lineage can be generated by co-amplification in embryonated chicken eggs. Reassortants were restricted to the H5N1 subtype and had acquired between two and all six of the internal segments of the H9N2 virus. Five selected plaque-purified reassortant clones expressed a broad phenotypic spectrum both in vitro and in vivo. Two groups of reassortants were characterized to have retarded growth characteristics in vitro compared to the parental H5N1 virus. One clone provoked reduced mortality in inoculated chickens, although characteristics of an HP phenotype were retained. Enhanced zoonotic properties were not predicted for any of these clones, which was confirmed by ferret inoculation experiments: Neither the parental H5N1 virus nor two selected clones induced severe clinical symptoms or were contact-transmitted to sentinel ferrets. While the emergence of reassortants of Egyptian HPAI H5N1 viruses with internal gene segments of co-circulating H9N2 viruses is possible in principle, spread of such viruses is expected to be governed by their fitness to outcompete the parental viruses in the field. Eventual spread of attenuated phenotypes, however, would impact syndrome surveillance in poultry farms negatively and might foster enzootic virus circulation.