Molecular characterization of Indian isolates of infectious bursal disease virus from broiler chickens.

The present study was undertaken to characterize recent field isolates of infectious bursal disease virus (IBDV) by reverse transcription-polymerase chain reaction (RT-PCR) and partial sequencing of VP2 gene. The virus could be detected in 17 of 20 field samples from broiler chickens in Haryana state, India as well as in all the four vaccine strains. Nucleotide sequences of four field isolates and one vaccine strain were compared with 10 reported IBDV strains from different parts of the world. Nucleotide substitutions at 795G, 827T, 833C, 857C, 897A, 905T, 908T, 1011A and 1094G specific for very virulent (vv) strains, were maintained in all the four field isolates. However, unique nucleotide substitutions at 806A-G, 851 C-T, 1010 T-C, 1019T-C and 1082T-C showed further divergence of these isolates from already reported vvIBDVs. Deduced amino acid substitutions at 222P-A, 256V-I, 279N-D, 294L-I and 299N-S specific for vvIBDV strains were also present in all the four isolates. The vaccine strain showed amino acid change 279D-N, a characteristic of attenuated vaccine strains. Phylogenetic analysis showed that all the field isolates in the present study were closely related to reported UK (UK661) and Japan (OKYM) field isolates. All the four field IBDV strains of the present study were closely related to each other but distinct from already reported vvIBDVs of India. On the basis of nucleotide sequencing and phylogenetic analysis, it is very likely that IBD causing strains in this part of India are of very virulent character and are still undergoing changes at genetic level.