Molecular Epidemiology, Genogroup Classification and Diversity of Infectious Bursal Disease Virus

Viral diseases are a major cause of economic losses in the poultry industry. Among these is infectious bursal disease (IBD), which poses a threat to global poultry production. The epidemiology of IBD is changing, while very virulent IBDV (vvIBDV) remains endemic and represents the dominant clinical threat in regions such as Africa and parts of South America, the regional transition towards novel variants (nVarIBDVs) and reassortants (A3B1) is particularly pronounced in Asian and European poultry production, respectively. This review synthesises global IBD epidemiological data reported up to December 2025 and analyses the evolution of molecular genogrouping and fitness. Ongoing IBDV evolution is accelerating, with Asia becoming the epicentre of nVarIBDV (genogroup A2d), which circumvents maternally derived antibodies (MDA) and classical vaccine protection. In Europe, the A3B1 reassortant has become the dominant field IBDV strain. The review highlights a nomenclature conflict regarding the A9 designation, which currently refers to both attenuated strain and pathogenic variant from Portugal. Considering the 9% divergence threshold, the attenuated IBDV strain should be returned to the A1 genogroup lineage "b" (A1b), while A9 denotes Portuguese variants. Furthermore, phylogenetic analysis reveals that the evolution of segment B (VP1) is independent of segment A (VP2), necessitating a bi-segmental approach. The rapid antigenic drift in VP2, combined with the crucial role of the conserved segment B and the RNA-dependent RNA polymerase (RdRp) in viral pathogenicity, necessitates a shift to a standardised dual-segment phylogenetic classification system (A#_xB#_y binomial nomenclature) for effective global surveillance.