Sequence analysis of infectious bursal disease virus isolates from India: phylogenetic relationships

Prevalence of infectious bursal disease (IBD) among chickens in different parts of Tamil Nadu, India, has been studied by collection of bursal samples from suspected flocks and by performing reverse transcription-polymerase chain reaction (RT-PCR) for amplification of a specific product of 474 bp from the variable region of the VP2 gene. Among 53 bursal samples examined by RT-PCR, 40 showed a positive reaction. The amplified products were subjected to nucleotide sequencing and the obtained sequences were compared with those of IBD virus (IBDV) vaccine strain Georgia, the classical virulent strain 52/70 and the very virulent Japanese OKYM strain. Nucleotide homology data indicated that all the Tamil Nadu isolates showed homology ranging from 91 to 99.6% among themselves. When compared with the very virulent Japanese OKYM strain, four isolates grouped with that strain. Majority of the isolates clustered with the very the virulent OKYM strain as evident from phylogenetic analysis performed using the MEGA program. Comparison of the deduced amino acid sequences of IBDV isolates with those of the vaccine strain Georgia, the classical virulent strain 52/70 and the very virulent strain OKYM also revealed the presence of conserved serine-rich heptapeptide sequence in most of the isolates. Results of this study indicate that majority of the IBDV isolates are very virulent, which is evident from heavy mortality that has been reported in few flocks of poultry in spite of regular vaccination.