الفهرس 
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Abstract
The present work conducted to investigate the variations in pathogenicity and molecular characterization of IBDV in Egypt. Field survey was performed on a total no of 26 chicken farms from the period of June 2015 to April 2016 in 4 Egyptian governorates (12 in Behera, 7 in KFS, 1 in Marsa Matrouh and 6 in Alexandria), they were [14 broilers farms, 7 native baladi farms and 5 layers farms], the capacity of farms ranged from 1500_30000 birds/farm and the age ranged from 18-42 day old.The IBDV was isolated by inoculation of bursal suspensions in 9-11 day SPF ECEs via CAM with daily observation for 7 days. Dead embryos with their CAMs were harvested and homogenized to make embryonic tissue suspension that used for molecular characterization of IBDV. Reverse transcriptase polymerase chain reaction (RT-PCR) was applied for IBDV targeting hyper variable region (HVR) of VP2 gene. Out of 26 tested samples 11 samples were positive with percent of 42.3 %. The percent were {41.6% in Behera; 42.8% in KFS; 100% in Marsa Matrouh and 33.3% in Alexandria}. 11 RT-PCR positive isolates were detected in 2 layers farms (40%), 4 native baladi farms (57.1%) and 5 broilers farms (35.7%).
Sequence analysis of PCR products of positive samples were carried out and revealed that all positive samples were characterized as very virulent (vvIBDV) strains, had amino acids residues (222 A, 242I, 256 I, 279D, 284 A, 294 I, 299S and 253Q) which are highly conserved among vvIBDV strains, and they were highly genetically related and have amino acid identity between each other from (73.8-100%), (80.6-97.5%) identity with Giza 2008 strains but with vaccinal strains they showed (74.4-95.6%) identity.Six selected positive vvIBDV isolates were used to study the pathogenicity of IBDV in 120 one day old SPF chicks. At day 211st the SPF chickens were divided into 8 groups 15 bird/each, the groups 1-6 were experimentally infected naso-ocularly with 0.1 ml of EID50 of 6 selected positive IBDV field isolates. group 7 infected with IBD-Br./Men. Egypt/09 VP2 strain. group 8 kept as non-infected control. Experimental infection induced clinical signs and gross lesions typical for IBDV and mortalities ranged from 6 to 80% as well as bursal/ body weight index lower than 0.7 indicating bursal atrophy. The microscopical examination of bursal samples collected at days 3 and 7 post-infection revealed different degrees of histopathological bursal changes as lymphoid depletion with proliferating interfollicular connective tissue, atrophy of the follicles, stromal edema and associated with inflammatory response. The selected six isolates which used to evaluate the pathogenicity were highly antigenically related and have amino acid identity between each other (83.1-100%), but induced different mortality rates after experimental infection in 21 days SPF chickens, suggesting that other genomic regions, located on segment A and/or segment B of the viral genome, might contribute to IBDV virulence. from the present study we concluded that the circulating strains are highly related to vvIBDV strains. The genotyping of eleven vvIBDV field isolates indicated continuous evolution of IBDV in Egyptian environment and they were closely related to previous isolated strains from Egypt.