MOLECULAR CHARACTERIZATION OF 7 CHINESE ISOLATES OF INFECTIOUS BURSALDISEASE VIRUS - CLASSICAL, VERY VIRULENT, AND VARIANT STRAINS

Seven infectious bursal disease virus (IBDV) strains isolated from Chi na have been characterized in this study, including a classical strain CJ801, an attenuated strain GZ911, a variant strain GZ902, and four v ery virulent strains G9201, G9302, F9502, and HK46. With the use of re verse transcription-polymerase chain reaction, the Ill-length VP2 gene s were amplified and the hypervariable regions were sequenced. Protein sequences of the hypervariable region (a.a. 143-382) of the field iso lates confirmed their identities. CJ801 has the highest identity to th e classical strains STC and 52/70. GZ902 has the highest identity to t he American variant strains A, E, and GLS, and they share unique amino acid residue at positions 249K and 254S, which are not present in sta ndard serotype 1 strains. Attenuated strain GZ911, like other cell cul ture-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 virulen ce of these strains. The four very virulent strains have the highest i dentity to the European very virulent strain UK661 and Japanese strain OKYM. These strains share unique amino acid residues at positions 222 A, 256I, and 294I, which are not present in other less virulent strain s. 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 I, in dicating that new very virulent strains are evolving. Phylogenetic ana lysis suggests that Chinese very virulent IBDVs and European very viru lent strains are derived from similar origin.