الفهرس 
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Abstract
The infectious bursal disease virus (IBDv) is widespread in poultry flocks all around the world. During the period between 2016- 2017, 22 IBD outbreaks were investigated in El-Menoufiya Governorate in different types of chickens (11 broiler, 7 commercial layer pullet and 4 native baladi varieties) farms. All these IBD outbreaks occurred at 23-42 days of age. The mortality ranged between 3-9 % in commercial broilers farms, 5.8-12% in native breed varieties and 5-10 % in commercial layer replacement pullets. The age of suspected infected chicken ranged between 23-33 days for broilers, 33-40 days for native baladi and 36-42 days for replacement layers. Our study was directed to follows up of MDA by ELISA to determine MDA waning and the age at which the chicks become susceptible to experimental vaccination or challenge and assessment of protection of commercial male layer chickens after vaccination with HVT-IBD vector vaccine alone or followed by classic IBDV vaccines (intermediate or intermediate plus) against challenged with vvIBDV local field isolate ”Lay./Men.Egypt/17” at 46th-day of age. A 320 one-day-old commercial male layer chicks divided into 8 groups of 40 birds of each G1 (chicken vaccinated with HVT-IBD vector vaccine at 1 day old of age) and G2 (chicken vaccinated with HVT-IBDV at one-day of age and challenged with vvIBDV local field isolate ”Lay./Men.Egypt/017” at 46-day of age), G3 (chickens vaccinated with HVT-IBDV vaccine at one-day of age followed by intermediate IBDV live vaccine at 18-day of age), G4 (chickens vaccinated with HVT-IBDV vaccine at one-day of age followed by live intermediate IBDV vaccine at 18-day of age and challenged), G5 (chickens vaccinated with HVT-IBDV vaccine at one-day of age followed by intermediate plus IBDV live vaccine at 18-day of age), G6 (chicken vaccinated with HVT-IBDV vaccine at one-day of age followed by intermediate plus IBDV vaccines at 18-day of age and challenged), G7 (non- vaccinated) and G8 (non-vaccinated challenged chickens).