الفهرس 
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Abstract
One hundred and eighty 15 days old SPF chickens were used in this study and divided into 6 groups maintained in isolated rooms. Each group contain 30 chicks; group I served as Blank group not infected , group II was intranaslly inoculated with 106 EID50/bird H9N2 avian influenza virus at 38 days old, group III was intranaslly inoculated with 105EID50/bird Infectious Bronchitis virus at 38 days old, group IV was intranaslly inoculated with 106 EID50/bird H9N2 avian influenza virus at 38 days old then 3days later was intranaslly inoculated with 105EID50/bird infectious bronchitis virus at 41 days old, group V was intranaslly inoculated with 105EID50/bird Infectious bronchitis virus at 38 days old then 3 days later was intranaslly inoculated with 106 EID50/bird H9N2 avian influenza virus at 41 days old , group VI was intranaslly inoculated with 106 EID50/bird H9N2 avian influenza virus and 105EID50/bird Infectious bronchitis virus at the same time at (38 days old). Three chicks from each group were sacrificed at the 1st, 3rd, 5 th, 7th, 10th, 14th and 21th day postinfection. Blood samples were collected and used for Serological tests; HI for H9N2 AIV antibodies detection and ELISA for detection of antibodies aginst IBV. Tissue specimens were collected from nasal conchae, trachea, lungs, kidneys, bursa of fabricius , thymus, spleen, cecal tonsils, liver, pancreas, heart, proventriculus, gizzard and brain for histopathological and immunohistochemical studies. Moreover, tracheal samples were collected from different groups at 3 DPI for scanning electron microscopical study to detect the degree of deciliation, mucosal affection and goblet cells hyperplasia. Most observed clinical signs, post mortem changes and histopathological alteration were observed in mixed infected groups with variable mortality rate. The most affected organs were lymphoid, respiratory and urinary organs especially in mixed infected groups with detection of either one antigen or both viral antigens replicating in the organs causing severe viral histopathological alterations.