Molecular and pathological characterization of very virulent infectious bursal disease virus (vvIBDV) strains in Kashmir, India.

Infectious Bursal Disease (IBD), or Gumboro disease, is an acute, highly contagious viral infection that primarily affects young chickens, causing immunosuppression and mortality. This study aimed to characterize the genetic and pathological features of recent IBD outbreaks in Kashmir, India. A total of 32 suspected outbreaks were investigated from 2021 to 2023 across three districts: Srinagar, Ganderbal, and Budgam. Bursal tissues were collected for both histopathology and molecular detection. RT-PCR targeting the hypervariable region (HVR) of the VP2 gene confirmed IBDV infection in 29 samples (90.63%), yielding a 912 bp amplicon. Four representative strains (GDK1-GDK4) were sequenced, showing 99.89% nucleotide identity. Phylogenetic analysis grouped these isolates under Genotype A3, closely related to the Indian vvIBDV strain MF142522 and distinctly separate from commonly used vaccine strains (Genotype A1), suggesting possible vaccine mismatch. Clinically, affected birds exhibited depression, anorexia, diarrhea, and ruffled feathers. Gross lesions included hemorrhages in the thigh and breast muscles, and severe bursal enlargement with hemorrhagic folds. Histopathological findings revealed hemorrhages at interfollicular junctions, lymphoid depletion, heterophilic infiltration, interfollicular fibroplasia, and cystic degeneration of follicles. Besides the bursa, lesions were also noted in the kidneys, thymus, and caecal tonsils. These findings confirmed the involvement of vvIBDV strains in the outbreaks. The study highlights the genetic divergence of circulating field strains from vaccine strains and emphasizes the need for continuous molecular surveillance and vaccine reevaluation to control the disease effectively in this region. The distinct ecological and poultry trade dynamics of Kashmir may contribute to the emergence and persistence of such virulent strains.