Molecular characteristic of VP2 gene of infectious bursal disease viruses isolated from a farm in two decades.

Infectious bursal disease virus (IBDV) is a double-stranded RNA virus in the Birnaviridae family. Four pathotypes, attenuated, virulent, antigenic variant, and very virulent, have been identified. In this study, we examined the phylogenetic relationship of 25 field isolates that were collected from a single farm during 1989-2008. A sequence analysis of PCR amplified 714 bp VP2 region showed that all the samples were derived from very virulent infectious bursal disease virus (vvIBDV) and were more closely related to the vvIBDV isolate UK661. From 1999, the isolate XA1999 had amino acids I228 and T394. XA2000, XA2001, XA2002, and XA2003-09 had amino acids E279 and T394. From 2004 to 2008, the isolates had amino acids H320, I349, S375, and R381 while the UK661 virus had T228, D279, Q320, V349, P375, K381, and A394. Such mutations do not change key amino acid residues in the domains which are essential for its virulence. It suggests that a virulent IBDV strain could maintain its virulence for a long period in the same chicken farm and the strain is highly stable under normal environments.