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

Fig. 1. IBDV-specific antibody titers in broiler chickens from six farms: IBDV-specific antibody levels were quantified by ELISA at 18 and 35 days of age across Farms A to F. At 18 days, birds from Farms A and F exhibited significantly higher antibody titers than at 35 days, with titers declining below the positive cut-off value of 396 at 35 days, suggesting waning immunity or viral clearance. In contrast, Farms B, C, D, and E showed significant increases in antibody titers between 18 and 35 days (****p < 0.0001), reflecting active seroconversion and recent IBDV infection. Data are presented as mean ± standard deviation

Fig. 2. Bursa-to-body weight ratios in broiler chickens from six farms: Bursa-to-body weight (BBW) ratios were measured at 18 and 35 days of age to assess bursal atrophy associated with IBDV infection. Significant reductions in BBW ratios were observed in Farms A, B, C, D, and E between 18 and 35 days (****p < 0.0001), consistent with immunosuppression due to bursal damage. Conversely, Farm F showed no significant change in BBW ratio (p = 0.1232), indicating stable bursal development and absence of active IBDV-related damage during the study period. Data are expressed as mean ± standard deviation

Fig. 3. Phylogenetic tree of IBDV based on VP2_HVR sequences: The tree was constructed using the maximum likelihood method with 68 newly sequenced VP2_HVR genes and 50 reference sequences from GenBank. Sixty-six field isolates clustered within genogroup A3, including four from backyard chickens (IBDV-ZWT-1 to -4), and two isolates grouped under A2. The Virgo7 and B87 vaccine strains clustered within A3 and A8, respectively. Bootstrap values > 70% are shown at major nodes. The scale bar represents the number of nucleotide substitutions per site

Fig. 4. Amino acid residue substitutions in the hypervariable region of the VP2 protein: Alignment of deduced amino acid sequences of the VP2 hypervariable region from representative IBDV isolates in this study, compared to vaccine and reference strains, showing amino acid substitutions and their predicted phenotypic effects. Amino acid residues highlighted in light blue indicate positions linked to known phenotypic features of the virus. Residues highlighted in light grey correspond to our isolates and their predicted phenotypic characteristics. Unique amino acid residues observed in our isolates are shown in red