Comparative Field Efficacy of Live Infectious Bursal Disease and HVT-IBD Vector Vaccines against the Egyptian Variant Strain A2dB1

The Chinese variant infectious bursal disease virus (nvIBDV, genotype A2dB1), associated with bursal atrophy and lymphocyte depletion, has rapidly emerged in Egypt since 2023, posing a major threat to the poultry industry. Current commercial vaccines, live, vector, and immune complex types, offer incomplete protection against this strain. This study assessed various IBDV vaccination programs by tracking bird performance, bursa-to-body weight ratios, bursal lesion scores, and PCR-based virus detection at different time points. An experimental challenge was also performed at 14 days of age using a local nvIBDV isolate, administering ~10³.⁵ EID₅₀ orally per bird. Two HVT-IBD vector vaccines (A and B) were tested in combination with different live vaccine schedules: Lukert-2 alone on day-1, Lukert-2 on day-1 plus Lukert P on day-11, and vector vaccines at hatch with a Lukert P booster on day-11. The birds were moved to a controlled environment post-vaccination with appropriate controls. The results showed that combining vector vaccines with live boosters improved protection, although not completely preventing infection, as confirmed by PCR and histopathology. The performance metrics remained acceptable. Birds administered vector vaccines with a Lukert P booster on day-11 had higher bursa/body weight ratios and milder microscopic lesions. Notably, after the nvIBDV experimental infection on day-14, the group receiving vector A with a Lukert P booster showed better bursal lesion scores than the group receiving vectored vaccine B. These findings underscore the need to optimize vaccination strategies using HVT-IBD vector vaccines and live boosters to improve protection and maintain bursal health in broilers