Genetic and pathogenic characterization of a novel infectious bursal disease virus field strain with natural reassortant and recombinant features from southern China

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Infectious bursal disease (IBD), caused by infectious bursal disease virus (IBDV), poses a major threat to the global poultry industry. In this study, we identified and characterized a novel field IBDV strain NN040124, which exhibits both reassortment and recombination features. Genotyping identified this field strain as A3B1a, with segment A derived from a very virulent IBDV (vvIBDV) strain (A3) and segment B from a classical-like attenuated vaccine (attIBDV) strains (vv-A/att-B IBDV)). Crucially, recombination analysis revealed that segment B is a backbone originated from vaccine strain B87, with the N-terminal part replaced by the homologous region from a vvIBDV strain (Harbin-1). This replacement is implicated in the restored virulence of the strain, as demonstrated by a challenge experiment in 4-week-old commercial Three-Yellow chickens, which resulted in 40% mortality, typical clinical signs, and severe bursal lesions. These findings confirm the emergence of a novel dually reassortant and recombinant IBDV in Southern China, identify the N-terminal region of segment B as a potential virulence determinant in a naturally occurring field strain, and highlight the potential risks of live vaccine use. Our findings underscore the importance of continuous genetic and pathogenic surveillance of IBDV to inform effective prevention and control strategies.

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