الفهرس 
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Abstract
In the present study, we selected seven isolates based on phylogenetic analysis of H5 gene (five
classic of 2.2.1 and two variant of 2.2.1.1). Propagation of the selected viruses on SPF ECE was
carried out .Molecular identification and sequence analysis of four genes (HA, NA, NS1 and PB2)
related to virus virulence was carried out. The pathogenicity of those seven isolates of high
pathogenic avian influenza virus (HPAIV) in SPF chickens was studied. Results revealed that the
seven viruses were propagated in ECE and induced egg mortalities within 72 hrs. All viruses
under study contained multiple basic amino acids (PQGEKRRKKRGLF) characteristic to
HPAIV. Mutation in HA cleavage site at position 341 (R341K) in classic viruses and (R341G) in
variant viruses with substitution in NS1 gene in position 48 (S48N) in variant viruses led to
reduction of this viruses virulence. Pathogenicity results revealed that the classical viruses induced
mortalities in SPF chicks 48 hrs earlier than variant viruses In conclusion, the study reporting the
virulence markers in HA and NS1 genes in both classical and variant HPAI H5N1 viruses.