Abstract
Low pathogenic avian influenza virus (AIV) serotype H9N2 affected many commercial flocks in the Middle East including Egypt beginning from the last decade. Therefore, the present study was an attempt to better understand the situation of H9N2 virus in Egypt as well as to assess the pathogenicity of Egyptian H9N2 avian influenza virus in experimentally infected commercial broiler chickens. Out of 100 examined broiler flocks, 32 flocks were detected to be infected with LPAI H9N2 (32%). The phylogenic analysis revealed that, all the H9N2 virus isolates in this study were belonging to viruses from G1 lineage with more close relationship to the Israeli strains. After challenge, respiratory manifestations (rales, sneezing with nasal and ocular discharge), whitish to greenish diarrhea, depression and anorexia were noticed with 20% mortality. Furthermore, the virus caused remarkable decrease in the average body weight (290g) in the infected group compared to the uninfected group. High Viral RNA was detected in tracheal swabs through 1 to 15 days' post infection (dpi) by RT-qPCR, in which, the copy number was 1.152 x 104 copies, 7.89 x 103 & 5.2 x 102 copies / 200 µl dilutions at 4th day, 11th and 15th day post challenge, respectively. On histopathological examination, lymphocyte depletion in the thymus, follicular atrophy and cystic follicles in the bursa of Fabricius were seen which lead to immunosuppression. This study indicated that the H9N2 virus isolated from a natural outbreak was of low pathogenicity under experimental conditions. Continuous shedding by the infected birds by oral and cloacal routes without any clinical signs might cause undetected spread of the virus under field conditions.

Key words: H9N2, broiler, pathogenicity, Egypt.