Unraveling VP2 mutations in Infectious Bursal Disease Virus (IBDV) associated with potential vaccine escape in poultry flocks in Shandong, China

Background Infectious bursal disease virus (IBDV) remains a major threat to the global poultry industry, causing significant economic losses. Recent surveillance data indicate that genetically diverse strains of IBDV are circulating in different regions of China. Methods In this study, both commercial and backyard chicken flocks from Shandong Province were investigated for evidence of IBDV circulation using a combination of serological assays and molecular detection techniques. Results Despite routine vaccination, antibody responses were highly variable, with some flocks showing reduced maternal immunity while others exhibiting active seroconversion by 35 days of age. Many birds showed an abnormally low bursa-to-body weight ratio, indicating immunosuppression. RT-PCR detected IBDV in 68 bursal samples, most of which were classified as very virulent strains (vvIBDV) belonging to genogroup A3, while a smaller subset was identified as variant strains within genogroup A2 based on phylogenetic analysis of the VP2 gene. Comparison of the VP2 protein sequence with other Chinese strains revealed variations in key hypervariable region residues critical for antibody binding. Conserved markers of very virulent strains persisted, while the novel substitutions A222V and D279H emerged, neither of which is present in commonly used vaccine strains. Conclusions Our findings highlight emerging IBDV variants with altered VP2 antigenicity, potentially causing vaccine escape and increasing poultry health and economic risks.