A unified genotypic classification of infectious bursal disease virus based on both genome segments.

Infectious bursal disease virus (IBDV) of chickens is a birnavirus with a bi-segmented double-stranded RNA genome, the segments designated as A and B. We performed phylogenetic analysis using a 366-bp fragment of segment A (nt 785-1150) and a 508-bp fragment of segment B (nt 328-835) of IBDV. A total of 463 segment A and 434 segment B sequences from GenBank, including the sequences of eight recent Bangladeshi isolates, were used in the analysis. The analysis revealed eight genogroups of segment A under serotype 1, designated as A1 (classical), A2 (US antigenic variant), A3 (very virulent), A4 (dIBDV), A5 (atypical Mexican), A6 (atypical Italian), A7 (early Australian) and A8 (Australian variant), and a single genogroup under serotype 2, designated as A0. On the other hand, segment B could be categorized into five genogroups irrespective of serotype, these being B1 (classical-like), B2 (very virulent-like), B3 (early Australian-like), B4 (Polish & Tanzanian) and B5 (Nigerian). Segment B of serotype 2 strains clustered within genogroup B1. With the bi-segmented genome of IBDV, these differences would allow for a total of 45 possible assortments. Based on the combinations of segment A and segment B genogroups observed in 463 IBDV strains, a total of 15 genotypes could be recognized. Recent Bangladeshi IBDV strains, isolated in 2016, appeared to be segment reassortants having segment A of genogroup A3 (very virulent) and segment B of genogroup B3 (early Australian-like). An extended system of nomenclature of IBDV strains is proposed.