An improved scheme for infectious bursal disease virus genotype classification based on both genome-segments A and B

Infectious bursal disease (IBD) is caused by infectious bursal disease virus (IBDV), which has a genome consisting of two segments of double-stranded linear RNA. IBDVs have been traditionally divided into four phenotypes based on their pathogenicity and antigenicity, including classic, variant, very virulent, and attenuated IBDV. With the emergences of divergent molecular characteristics of novel strains produced by continuous mutations and recombination, it is increasingly difficult to define new IBDV strains using the traditional descriptive classification method. The most common classification scheme for IBDV with segmented genome is based solely on segment A, while the significance of segment B has been largely neglected. In this study, an improved scheme for IBDV genotype classification based on the molecular characteristics of both VP2 (a viral capsid protein encoded by segment A) and VP1 (an RNA-dependent RNA polymerase protein encoded by segment B) was proposed for the first time. In this scheme, IBDV was classified into nine genogroups of A and five genogroups of B, respectively; the genogroup A2 was further divided into four lineages. The commonly used phenotypic classifications of classic, variant, very virulent, and attenuated IBDVs correspond to the A1B1, A2B1, A3B2, and A8B1 genotypes of the proposed classification scheme. The novel variant IBDVs including the strains identified in this study were classified as belonging to genotype A2dB1. The flexibility and versatility of this improved classification scheme will allow the unambiguous identification of existing and emerging IBDV strains, which will greatly facilitate molecular epidemiology studies of IBDV.