Detection and Phylogeny of Infectious Bursal Disease Virus (IBDV) during Field Outbreaks in Broilers

Infectious bursal disease (IBD) is economically important disease causing great losses to poultry industry worldwide. Field outbreaks of IBD in 18 different poultry farms in the Chakwal district were confirmed by clinicopathologic examination and PCR. A total of 6 isolates of IBDV from these outbreaks were genetically characterized based on hyper-variable region of the VP2 gene. IBDV strains were grouped into two distinct clusters on the basis of nucleotide sequences of hyper-variable region of the VP2 gene. According to phylogenetic analysis, 5 IBDV strains showed characteristic amino acid signatures in the VP2 gene (A222, I242, I256, I294, S299) and classified as vvIBDV. Furthermore, the sequencing analysis of detected field strains revealed the high similarity and close clustering with vvIBDV strains isolated from neighboring countries, suggesting geographic and temporal relationships among these strains. Interestingly, one IBDV strain clustered togather with vaccinal IBDV strains and showed 99% sequence homology with attenuated vaccine strains. Our study revealed exclusive circulation of wIBDV and these evidences emphasize the need of further detailed and more systemic approaches to study IBDV distribution for the implementation of effective control measures.