Recurrent outbreaks of infectious bursal disease (IBD) in a layer farm caused by very virulent IBD virus (vvIBDV) in India: Pathology and molecular analysis

Abstract

The present study was carried out to investigate and characterize the nature of infectious bursal disease virus (IBDV) involved in the recurrent outbreaks in an experimental layer farm in Bareilly, Uttar Pradesh, India using direct tissue reverse transcription- polymerase chain reaction (RT-PCR) followed by nucleotide sequencing of VP2 gene. A total of three acute IBD outbreaks with the interval of 4-6 months were recorded in the batch of 3-5 weeks-old layer chicks in the year 2012-2013. Tissues collected from necropsied birds were found RT-PCR positive for IBDV VP2 gene. Genetic analysis of the sequenced VP2 gene revealed the IBDV belonged to very virulent (vv) subtype and had amino acids at positions 222A, 256I, 294I, and 299S typical for vvIBDV strains isolated worldwide. It had only one unique amino acid change in the antigenic peak A (210-225 aa) at position 212DàN (AspàAsn), which is not observed in any of the vvIBDVs isolated in India and abroad. Phylogenetic analysis revealed the isolates were more closely related to vvIBDV strains rA and rB (U.S.A.), Gx and HLJ-7 (China), OKYM (Japan), and shared >95% nucleotide homology with them. The VP2 gene shared 96.7% amino acid homology with IBDI+ vaccine strain used in India, comparatively higher among other vaccines strains, suggesting that IBD intermediate plus (IBDI+) vaccine might provide optimum cross protection, also for other vvIBDV strains. The vvIBDV strains remain a threat to poultry industry worldwide, and require regular monitoring and genetic analysis in order to keep track of the appearance and evolution of antigenically different IBDV strains or subtypes.