الفهرس 
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Abstract
The current study was aimed to investigate some aspects related to epizootiology of severe outbreaks of infectious bursal disease (IBD), which occurred among vaccinated chicken flocks. The current study revealed that 20 IBDV outbreaks occurred at 26-45 days of age and characterized by typical clinical signs and hemorrhagic gross lesions. Moreover, the mortality rates in these flocks’ range between 5-8.2% (average 6.6%) in commercial broiler, 8-16% (average 12%) in commercial layer replacement pullets and 7.6 - 25 % (average 16.3%) in native balladi varieties, IBDV were detected in 100% of examined flocks by AGPT and by RT/PCR.
In the pathogenicity study the mortality percents were observed for 7-days post challenge 6.7%, 13.3, and 10 in commercial male layer groups challenged with vvIBDV local field isolates (1) “vvIBDV/Br-Menoufia/Egypt-/2018”, (2) “vvIBDV/NB-Behera/Egypt /2020” and isolate no (3) vvIBDV/Lay/-Behera/Egypt-/2019, respectively, at 52 days of age. The bursal body weight ratio (B/BR) of commercial male layer chickens challenged with three vvIBDV local field isolates (1, 2 and 3) at 52 days of age were 1.08, 1.36, and 0.89, respectively compared to 3.31 in non-infected control group. The bursal body weight index (B/BI) of commercial male layer chickens challenged with three vvIBDV local field isolates (1, 2 and 3) at 52 days of age were 0.33, 0.41, and 0.27, respectively compared to 1 in non-infected control group (G4). The lymphocytic necrosis of commercial male layer chickens challenged with three vvIBDV local field isolates (1, 2 and 3) were 3.6, 3.7, and 4, respectively. Moreover, the lymphocytic depletion values were 4, 3.5, and 4, respectively in commercial male layer chickens challenged with three vvIBDV local field isolates (1, 2 and 3) at 52 days of age versus 0 in G4 control non challenged. Finally, the three vvIBDV isolates 1, 2 and 3 are pathogenic and cause bursal atrophy and affect other lymphoid organs.