Isolation, Identification and Characterization of a Unique Infectious Bursal Disease Variant Virus from a Layer Farm Having Recurrent Vaccine Failure

Abstract

Aim: The present investigation was conducted to evaluate and determine the genotypes of IBDV isolate associated with persistent outbreaks in fully vaccinated layer flocks from West Bengal, India. Study Design: A recurring IBD outbreak was reported in three consecutive batches of approximately 32 to 36 days old chickens in a fully vaccinated layer farm in West Bengal, India. For etiological investigation, the bursal tissue samples were collected aseptically from the chicken assumed to have died due to IBDV infection. Place and Duration of Study: The present study was carried out on a fully vaccinated layer (Babcobb) farm in West Bengal, India in April 2020. Methodology: The bursal tissue samples were collected aseptically from the chicken assumed to have died due to IBDV infection. All the surviving birds were sacrificed and the bursa was collected for histopathological investigation and RNA isolation was done for the sequencing of the VP2 region. Results: As a result, the population contains IBDV isolates with varying genotypic and phenotypic variability. The obtained isolate is denoted hereafter as WB/HBL/0320. The isolate was subjected to amplification and sequencing of the hypervariable region (HVR) of VP2. The previously reported IBDV strains were used as a reference for analyzing the inferred amino acid and nucleotide sequences of WB/HBL/0320. In the present study, the amino acid positions in the VP2 hypervariable region of WB/HBL/0320 isolate are indicative of the virulent strain, including 242Ile, 253Gln, 256Ile,272Ile, 279Asn, 284Ala, 294Ile, 299Ser, and 330Ser, which are very similar to the very virulent and intermediate plus virulent types with only novel substitution present as A222V. Unlike previously reported virulent genotypes, WB/HBL/0320 showed the unique substitutions V225E and Q304H. Conclusion: The molecular analysis based on a nucleotide sequence of the HVR-VP2 showed close clustering of WB/HBL/0320 with previously reported very virulent strains. However, more investigations are needed to decide on the control of the WB/HBL/0320 variant and the effectiveness of available IBD vaccines for IBDV.