الفهرس 
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Abstract
Highly pathogenic avian influenza H5 subtype viruses (H5N1) have become endemic and caused large outbreaks in domestic poultry in Egypt since 2006 with substantial economic losses and frequent human infections. The H5N1 endemic status continues to devastate the local poultry industry. It is important to investigate the current situation of the circulating H5N1 viruses and the efficacy of the applied vaccination programms.
A total of 77 chickens suspected to be infected with avian influenza virus (AIV) of different breeds and ages were collected from 35 flocks, mainly backyard (24 flocks) and 11 commercial flocks at different localities within Sharkia Province during 2009-2011. High mortality up to 100% and clinical picture characteristic of AIV infection were observed among the examined flocks.
The isolation and identification of AI viruses were carried out on ECE using HA, rapid kits of influenza A virus and RT-PCR as well as partial sequence analysis of the hemagglutinin (HA) gene.
Twenty eight hemagglutinating virus isolates with a percentage 80% were detected. Flu detect test strip (Influenza type A antigen test kit) revealed 24 AIV types A with a precentage 85.71%. The 24 influenza A viruses were identified as H5 subtype by RT-PCR, using primers specific to H5.
Four H5-AI viruses of backyard chickens were partially sequenced. Sequence analysis revealed that the HA protein of four isolates harbor multiple basic amino acids (321PQGERRRKKR/G331) at the cleavage region, a characteristic.