Abstract
Despite intensive control efforts, infectious bursal disease virus (IBDV) continues to circulate throughout the Egyptian poultry sector, leading to serious economic losses even in vaccinated flocks. This situation strengthens the urgent need to monitor and identify the circulating strains and also to assess their antigenic concordance with the vaccine strains. Hence, the present study investigated the genetic characteristics of IBDV in vaccinated broiler flocks in Beheira (n=20) and Menoufia (n=14) provinces, Egypt in 2025. Out of 34 investigated flocks, 15 (44.1%) tested positive by qRT-PCR assay targeting VP2 gene of IBDV. Based on VP2 hypervariable region (VP2-HVR) analysis, the 15 isolates identified herein were characterized as variant (n=10) and vvIBDV (n=5) strains. The variant strains possessed the residues that are common and characteristic for novel variant strains, including T222, K249, I286, and D318, T336 and P343. Also, they harbored the residues K221, I252, N254, A270 and S299, which are thought to be associated with immune escape. On the other hand, the vvIBDV strains showed the key residues A222, I242, I256, I294 and S299 that constitute a molecular signature of virulence. Interestingly, one of our vvIBDV strains harbored the I256L mutation, which might affect antigen-antibody binding, probably contributing to antigenic drift. Notably, the herein described strains displayed a wide range of antigenic diversity (3.7-12.3%) compared to IBD vaccine strains commonly used in Egypt. These findings raise serious concerns about the efficacy of the current vaccination policies and underscore the urgent necessity for continuous epidemiological investigations to keep tract of new viral threats and ensure effective control measures against evolving IBDV strains.

Key words: Infectious bursal disease virus, variant, very virulent, chicken, Egypt