Comparative efficacy of commercial infectious bursal disease vaccination strategies against emerging novel variant IBDV (Genotype A2d).

Infectious bursal disease virus (IBDV) remains a major threat to poultry production because of its immunosuppressive effects and its impact on flock performance. In 2023, a genotype A2d novel variant IBDV (nVarIBDV) was identified in Egypt and found to be closely related to strains circulating in China, raising concerns about the effectiveness of current vaccination strategies. This study evaluated the protective efficacy of commercial IBD vaccination strategies against challenge with genotype A2d nVarIBDV in specific pathogen-free White Leghorn chickens. A total of 140 chicks were randomly allocated into seven groups. G1 and G2 received hatchery vaccination on day 1 with a recombinant HVT-IBD vector vaccine and an immune-complex vaccine, respectively, whereas G3 and G4 received a live intermediate-plus vaccine and an inactivated oil-emulsion vaccine at 12 days of age. G1-G4 were challenged with nVarIBDV at 28 days of age. G5 and G6 served as non-vaccinated challenged controls for very virulent IBDV and nVarIBDV, respectively, while G7 remained non-vaccinated and non-challenged. Antibody responses, viral shedding, and histopathological lesions were assessed after challenge. Vaccinated groups showed lower viral shedding and less severe tissue damage than non-vaccinated challenged birds; however, none of the evaluated schedules fully prevented viral replication or microscopic lesions. Based on bursal lesion-score criteria, the live intermediate-plus and recombinant vector vaccine groups showed the highest protection rates, whereas the inactivated vaccine group showed only partial tissue protection despite strong antibody responses. Overall, antibody titres alone were insufficient indicators of protection, supporting the use of combined serological, virological, and pathological assessment of IBD vaccine performance.