RESUMEN
Epidemiological studies on AI virus H5N1 in different governorates in Egypt [Alexandria, Bohera, Cairo, El-fayom, Gharbia, Giza and Dakahlia] during late 2009 and 2010 were carried out. These studies included seventy five flocks [49 broilers, 17 layer, 4 breeder and 5 duck flocks] showing high mortality rate with respiratory manifestations and diarrhea, through isolation in specific pathogen free eggs and identification by hemagglutination inhibition test using H5N1 antiserum. The data revealed the following: a percentage of 32.7%, 76.5%, 50% and 40% in broiler, layer, breeder and duck flocks respectively were positive for AIV H5N1. The highest incidence was recorded in layer flocks followed by broiler breeder flocks, duck then broiler flocks with total incidence of 44% in all species. Sequencing and phylogenetic tree of hemagglutinin [HA] gene of six positive AIV H5N1 isolates from chickens during 2010 were done. Phylogenetic tree showed that all HA gene sequences blonged to highly diverse clade 2.2.1 viruses according to WHO/FAO/OIE nomenclature. Analysis of amino acid sequences of HA glycoprotein revealed some mutations at the receptor binding site of the HA molecule