Clinical and Bursal Pathological Protection Following In Ovo Immune-Complex Vaccination Against Heterologous Very Virulent Infectious Bursal Disease Virus Challenge in Broiler Chickens

Background: Infectious bursal disease (IBD) remains a major challenge for poultry production, with very virulent IBD virus (vvIBDV) strains causing severe bursal damage, immunosuppression, and performance losses. This study evaluated the protective efficacy of a commercially available in ovo immune-complex vaccine against heterologous vvIBDV challenge in broiler chickens. Methods: Seventy-two 18-day-old embryonated eggs received either the vaccine or phosphate-buffered saline (PBS) in ovo. At 24 days of age, 10 birds per group were euthanized to assess vaccine-associated alterations. The remaining birds were challenged with 105 EID50 of the vvIBDV strain VG-248 or received PBS, and were necropsied at 6 days post-challenge. Vaccine efficacy was assessed based on clinical signs, body-weight gain, relative bursal (BF) and spleen weights, macroscopic bursal lesions, and microscopic histopathological scores. Inflammatory, edematous, necrotic, and regenerative alterations were evaluated independently of lymphoid depletion. Results: Control challenged birds developed moderate-to-severe macroscopic bursal lesions, increased relative bursal and spleen weights and reduced body-weight gain. Clinical signs were observed in 5/10 control challenged birds but in none of the vaccinated birds (p = 0.014). Body-weight gain was significantly lower in control challenged birds than in vaccinated challenged birds (339.85 ± 59.35 vs. 541.38 ± 67.10 g; p = 4.36 × 10−6), while the BF ratio was significantly higher (4.26 ± 2.95 vs. 0.95 ± 0.30; p = 1.92 × 10−6). Histologically, control challenged birds showed marked inflammatory infiltration, edema, and lymphoid necrosis, whereas vaccinated challenged birds showed mainly lymphoid depletion and plical atrophy, with minimal inflammation, no edema, low necrosis, and early lymphoid repopulation. Conclusions: In ovo vaccination protected against the clinical, performance-related, and acute pathological consequences of heterologous vvIBDV challenge.