Molecular characterization of seven Chinese isolates of infectious bursal disease virus: classical, very virulent, and variant strains.

Seven infectious bursal disease virus (IBDV) strains isolated from China have been characterized in this study, including a classical strain CJ801, an attenuated strain GZ911, a variant strain GZ902, and four very virulent strains G9201, G9302, F9502, and HK46. With the use of reverse transcription-polymerase chain reaction, the full-length VP2 genes were amplified and the hypervariable regions were sequenced. Protein sequences of the hypervariable region (a.a. 143-382) of the field isolates confirmed their identities. CJ801 has the highest identity to the classical strains STC and 52/70. GZ902 has the highest identity to the American variant strains A, E, and GLS, and they share unique amino acid residue at positions 249K and 254S, which are not present in standard serotype 1 strains. Attenuated strain GZ911, like other cell culture-adapted strains, has substitutions at positions 279(D to N) and 284 (A to T) as well as in the serine-rich heptapeptide region. Hence, these substitutions may take an important role in the reduced virulence of these strains. The four very virulent strains have the highest identity to the European very virulent strain UK661 and Japanese strain OKYM. These strains share unique amino acid residues at positions 222A, 256I, and 294I, which are not present in other less virulent strains. The very virulent strains isolated in Guangdong (G9201, G9303) and Fujian (F9502) Provinces have one to five amino acid substitutions at the two hydrophilic domains of VP2 comparing with UK661 and OKYM, indicating that new very virulent strains are evolving. Phylogenetic analysis suggests that Chinese very virulent IBDVs and European very virulent strains are derived from similar origin.