Molecular Characterization of Infectious Bursal Disease Virus Circulating Among Broiler Chickens Flocks in Qalubia and Mounofia Governorates, Egypt

Infectious bursal disease virus continues to cause significant economic losses in Egypt despite extensive vaccination efforts. The emergence of genetically and antigenically diverse lineages particularly very virulent strains and the recently reported A2d/nVarIBDV variants has contributed to vaccine failure due to vaccine virus mismatch. This study investigated IBDV circulation in 19 commercial broiler farms in Qalubia and Mounofia governorates employing different vaccination schemes, including commercial live attenuated and recombinant vaccines. Clinical signs, gross lesions, and preliminary AGPT testing suspected IBDV presence in 10 flocks. Molecular detection targeting the VP2 gene identified 9 RT-PCR–positive samples, from which two representative isolates per governorate were sequenced and analyzed. Phylogenetic examination of the VP2 hypervariable region revealed that all strains clustered within the A2d subgroup of the Chinese variant genogroup, showing nucleotide identity (96.4–96.9%) to the reference strain SHG19 and (99.4–99.7%) among themselves. In contrast, a marked genetic divergence from classical vaccine strains (D78, Bursine, Winterfield 2512) was observed, accompanied by amino acid substitutions in key antigenic loops associated with immune escape. These findings indicate ongoing evolution of nVarIBDV under field and vaccine pressure, likely contributing to reduced vaccine efficacy and persistent outbreaks. The study highlights the urgent need for continuous molecular surveillance, antigenic monitoring and the development of genotype-matched or updated vaccines. Thus, it was emphasized the strategic value of vaccination of breeder flocks with inactivated vaccines incorporating both variant and classic IBDV strains, thereby enhancing the robustness and reliability of IBDV control measures in Egypt.