الفهرس 
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Abstract
Infectious bronchitis virus (IBV) has a significant impact on the poultry industry worldwide and affecting all chicken ages, types and breeds of IBV. Molecular characterization is based mainly on the S1 gene. However, several studies were focused only on the S1 gene hypervariable regions (HVRs).In this study, six Egyptian IBVs viruses were characterized genetically, and antigenically, along with, pathotyping study. The genetic characterization was conducted by studying the HVR3; however the antigenic characterization were assessed via cross haemagglutination inhibition using trypsinized treated IBV antigens and poly-specific antisera of the six tested IBV viruses. Moreover the pathogenicity index of the IBV isolates was assessed based on severity of kidney and trachea lesion scores of both survival and dead infected birds. The phylogenetic analysis revealed that the six IBV viruses could be clustered within two distinct groups: the IBV classic group resembling the GI-1 genotype and the variant II group of the GI-23 genotype. However the five variant II viruses were showing high identity % but they were showed clear genetic diversity and poor antigenic relatedness to the classical strain. Furthermore variable pathogenicity indexes were recorded among the tested IBV viruses. The obtained results indicate continuous genetic evolution and antigenic variation of the circulated IBV virus. Hence, continuous follow-up of the current vaccine strategy is highly recommended for better control and prevention of infectious bronchitis virus in the poultry sector in Egypt.