Molecular characteristics of segment B of seven very virulent infectious bursal disease viruses isolated in China.

Infectious bursal disease virus (IBDV) is a birnavirus that causes immunosuppressive disease in chickens. Segment B of IBDV encodes the RNA-dependent RNA polymerase VP1, which is involved in virulence. We sequenced and analyzed segment B from seven Chinese IBDV isolates, all belonging to very virulent IBDV (vvIBDV), and clustered into Branches II and III. Phylogenetic analysis suggested that segment B of the HLJ isolates in Branch II might have originated from an unidentified host, and HuB-1 might have originated in Europe. Eight aa (4V, 61I, 145T, 287A, 508K, 511S, 646S, and 687P) were conserved in Branches II and III, and may contain potential segment B virulence determinants. Five aa (146D, 242E, 390M, 562P, and 695R) were found only in Branch III, and may be origin characteristics. Moreover, 55T and 63A in the 5'-untranslated region (UTR), and 2786C in the 3'-UTR were conserved in vvIBDV and may function in UTR secondary structure.