Abstract
BACKGROUND AND AIM: Infectious bursal disease (IBD) is an immunosuppressive disease caused by the IBD virus (IBDV), which adversely affects poultry vaccination programs. The novel variant IBDV (nvIBDV) has recently emerged in various regions, including Malaysia, raising concerns about its immunosuppressive potential and impact on Newcastle disease (ND) vaccination. This study aimed to investigate the immunosuppressive effects of the Malaysian nvIBDV strain (UPM1432/2019) in specific-pathogen-free (SPF) and broiler chickens and evaluate its influence on ND vaccine efficacy. MATERIALS AND METHODS: SPF chickens were orally infected with nvIBDV across three passage levels to study virus-induced clinical signs, lesions, and atrophy. Broiler chickens were vaccinated with live and killed ND vaccines and subsequently challenged with nvIBDV to measure ND antibody titers. The genotype of nvIBDV was characterized using sequence analysis of segments A and B. Bursal histopathology and statistical analyses were conducted to evaluate the virus's immunosuppressive effects. RESULTS: Infected SPF chickens displayed no clinical signs but showed significant bursal atrophy and lesions across all passages (p < 0.05). Broilers infected with nvIBDV exhibited no mortality or clinical signs; however, ND antibody titers significantly declined by 14 days post-challenge (1493.0 ± 746.1) compared with the unchallenged group (2975.7 ± 189.5; p < 0.01). Histopathological analysis revealed severe depletion of the bursal follicles, lymphoid cell aggregation, necrosis, and hemorrhage. Genotyping of nvIBDV identified as genotype A2dB1, consistent with strains from other regions. CONCLUSION: The Malaysian nvIBDV strain causes subclinical infections in SPF and broiler chickens, resulting in bursal atrophy and reduced ND vaccine-induced antibody titers. Silent spread and immunosuppressive effects present significant challenges to poultry health management and vaccination efficacy. Enhanced diagnostic and biosecurity measures are crucial for mitigating its impact.