Co-infection of LP influenza viruses with other respiratory pathogens is a cause of high mortality and great economic loss. Co-infection of H9N2 AIV with IBV promotes AI propagation, increases the pathogenicity and extends the period of shedding in broiler chickens causing high mortality rates. Increasing severity of AI clinical signs and gross lesions result in the death of infected birds. It is well known that trypsin-like proteases are necessary for the cleavage of HA and thus play a key role in viral pathogenicity. Also, mixed infections of H9N2 with NDV and H5N1 associated with severe economic loss and mortality have been reported among the Egyptian poultry sectors. The evolutionary relationship of such viruses is based on isolation and antigenic and genetic characterization. Accumulated antigenic changes in H5N1 and H9N2 avian influenza viruses have been reported in infected birds in Egypt, leading to antigenic evolution that greatly affects the Egyptian control strategy for AI viruses. Indeed, an increased prevalence of cases of mixed infection of AI viruses with other respiratory viruses like IBV and NDV or mycoplasma and bacteria like E. Coli, has been reported over the last 5 years, causing severe respiratory disease complexes with high mortalities and losses to the poultry industry. Continuous genetic evolution has been reported, particularly in mixed infections with Egyptian H9N2 viruses, with the consequent emergence of new variants which are adapted for transmission to other species and greatly affect vaccination strategy.

Ali Hussein Ahmed HUSSEIN
Professor of Virology Department at Veterinary Medicine, Cairo University, Egypt

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^{Information from the Avian Flu Forum hosted by Boehringer on April 2017}