Comparative pathogenicity of very virulent and novel variant infectious bursal disease virus Egyptian strains in...

Infectious bursal disease (IBD) is an important viral disease that causing severe immunosuppression in young chickens. This current study compared the pathogenicity of very virulent IBDV (vvIBDV) and Chinese novel variant IBDV (nVarIBDV) strains in Specific Pathogen-Free (SPF) chickens. Chickens infected with the nVarIBDV strain exhibited no clinical signs or mortality, with mild edema and swelling in bursas during early infection, followed by progressive atrophy by the seventh- and fifteenth-days post-infection (dpi). In contrast, vvIBDV displayed severe systemic disease, with early onset ruffled feathers, hemorrhages on the thigh muscles. While both strains caused bursal atrophy, the vvIBDV strain induced more severe systemic pathology, characterized by hemorrhages in bursa, renal and hepatic degeneration, with 50% mortality. The vvIBDV induced severe reduction in Bursa-to-Body Weight (B/BW) ratio, reaching 1.042 ± 0.302, indicating significant immunosuppressive effect. While, the nVarIBDV-infected group showed marked reduction in the B/BW ratio to 1.269 ± 0.269, showing 70% decrease compared to the control group. The results of histopathology showed a mild bursitis, lymphoid depletion, and interfollicular connective tissue proliferation in the nVarIBDV-infected chickens. While, vvIBDV-infected chickens induced a severe bursitis with necrosis of lymphoid follicles and interfollicular fibrous tissue proliferation indicating lymphoid depletion. Moreover, the challenge with nVarIBDV and vvIBDV alter the immune response of a trivalent inactivated vaccine either before vaccination or after vaccination. This study provides valuable data about the pathogenicity of two cocirculating IBDVs in Egypt and highlighted the need for strict routine monitoring for nVarIBDV infection in chicken flocks as it has no clinical signs. Further research may be required to assess the efficacy of the currently available IBD vaccines in Egypt against these strains.