Predominance of reassortant infectious bursal disease viruses in Turkish poultry flocks.

Infectious bursal disease (IBD), caused by the infectious bursal disease virus (IBDV), is an acute, highly contagious viral infection that severely immunosuppresses poultry, threatening global poultry health and production. The co-circulation of field and vaccine strains of IBDV in poultry flocks facilitates genetic diversification through mutation and recombination, leading to the emergence of novel variants. To assess this dynamic, we characterized the genetic diversity of IBDV strains in vaccinated Turkish flocks during 2024-2025. RT-PCR detected IBDV in 91.4 % of broiler and 90.2 % of layer samples. Genetic analysis of the VP2 and VP1 genes from 35 strains identified three co-circulating genotypes. Notably, reassortant A3B1 strains were predominant, while classical (A1B1) and very virulent (A3B2) strains were less common, highlighting reassortment as a key driver of current IBDV evolution. The majority (n = 30) clustered within the A3 genogroup, sharing 89.38-99.57 % VP2 identity with reference strains. Critically, amino acid analysis confirmed that all A3 strains carried the key amino acid markers of very virulent IBDVs (222A, 242I, 253Q, 256I, 279D, 284A, 294I, 299S). In contrast, the single A1 strain exhibited a classical signature (222P, 249Q, 286T). This study represent a comprehensive report on the emergence and dominance of reassortant A3B1 strains in Turkish poultry, providing critical insights into the genomic and antigenic evolution of IBDV. These findings indicate that novel preventive and vaccination strategies for emerging reassortant IBDVs are needed.