Abstract
Infectious bursal disease virus (IBDV) induces severe immunosuppression in chickens, leading to significant economic losses in the global poultry industry. This study investigated 52 chicken flocks, including commercial broilers, layers, and baladi, from various Egyptian governorates in 2023. These flocks exhibited symptoms of depression, along with kidney and bursa lesions, indicative of IBDV infection. Pooled Bursal homogenates were tested using RT-PCR with VP2-specific primers, revealing that 20 flocks tested positive for IBDV. Six representative samples were selected from 20 positive flocks for isolation in embryonated chicken eggs. The embryonic lesions observed included haemorrhage, skull swelling, and liver necrosis with a pale-yellow appearance, in addition to congestion and thickening in the chorioallantoic membrane (CAM). Partial amplification of the VP2 gene from the harvested embryo suspensions of the six IBDV isolates was performed for sequencing. Phylogenetic analysis of the sequences revealed that five IBDV isolates (VV4, VV5, VV6, VV10, and VV16) belonged to the very virulent strain group A3 cluster, whereas one isolate (VV2) clustered with Chinese Variant strains in the A2d group. Sequence analysis of the hypervariable region (HVR) of VP2 compared to that of Egypt-USC-IBD-1-2019 and vvIBDV/Beh21/Egypt/18 highly virulent IBDV strains revealed several amino acid mutations. The VP2 HVR of all isolates maintained the serine-rich heptapeptide sequence SWSASGS, which is adjacent to the major hydrophilic peak B and serves as a virulence marker. Histopathological examination revealed that bursae from chickens infected with vvIBDV exhibited marked interlobular oedema and lymphoid depletion. In contrast, bursae from chickens infected with Variant IBDV showed massive lymphoid depletion, with hyperplasia of the bursal capsule. These findings highlight the circulation of both virulent and Variant IBDV strains in Egyptian chicken flocks, complicating disease control. Consequently, there is a need to update vaccination programs and vaccine strains for IBDV in Egypt.