Novel variants of infectious bursal disease virus can severely damage the bursa of fabricius of immunized chickens.

In recent years, atypical infectious bursal disease (IBD) with severe immunosuppression has brought new threats to the poultry industry and has caused considerable economic losses. Novel variant infectious bursal disease virus (IBDV) has been identified as the etiological pathogen and for unknown reasons is widespread in poultry on many chicken farms in China that have been immunized with vaccines against very virulent IBDV (vvIBDV). Using immunoprotection experiments in specific-pathogen-free chickens, we first verified that novel variant IBDV could severely damage the bursa of Fabricius of the important immune organ of immunized chicken in the presence of antibodies induced by three types of vvIBDV vaccines, which is a primary reason for the current epidemic of atypical IBD. Monoclonal antibody reactivity patterns and cross-neutralization assays further confirmed the obvious antigenic mismatch between novel variant IBDV and vvIBDV. Sequence analysis of the genome of novel variant IBDV (SHG19 strain) was performed and the key amino acid residues that might be involved in antigenicity and virulence differences of novel variant IBDV compared to vvIBDV were further analyzed. This study not only determined the primary reason for the atypical IBD epidemic, but also remind us of the urgency for developing new vaccines against novel variant IBDV.