Epidemiological characteristics of infectious bursal disease virus (IBDV) in China from 2023 to 2024: Mutated very virulent IBDV (mvvIBDV) is associated with atypical IBD

Infectious bursal disease (IBD), caused by the infectious bursal disease virus (IBDV), threatens the global poultry industry with acute immunosuppression. The non-lethal novel variant IBDV (nVarIBDV) has been identified as the primary cause of a new outbreak of atypical IBD in China. However, since 2023, several poultry farms in China have continuously reported the detection of vvIBDV in chicken flocks exhibiting atypical IBD characteristics. To analyze this abnormal problem, the prevalence of IBDV in major poultry farming areas in China from 2023 to 2024 was monitored. Our epidemiological survey revealed four coexisting IBDV strains in China, nVarIBDV (69.9 %, 65/93) ranked the first, followed by vvIBDV (16.1 %, 15/93), attIBDV (8.6 %, 8/93), and cIBDV (5.4 %, 5/93). Notably, all these 15 vvIBDV strains detected in this study harbored single or combination mutations in the VP2 hypervariable region (HVR) including residue 279: D279N (seven strains), G254D/I256L/D279N (one strain), or A222T/G254D/I256L/D279N (seven strains). Furthermore, one representative strain of IBDV-NMG23-1701 which contained the D279N mutation was isolated. In the phylogenetic tree constructed based on the IBDV genome, IBDV-NMG23-1701 was located in the vvIBDV branch. However, IBDV-NMG23-1701 did not kill chickens directly but induced severe subclinical symptoms of atypical IBD. We named these strains mutated vvIBDV (mvvIBDV), which genotypically belonged to vvIBDV but exhibited atypical IBD symptoms. The mvvIBDV has been detected in several major poultry farming areas in China. This is the first study to identify mvvIBDV as a new pathogen causing atypical IBD, providing a scientific basis for optimizing prevention and control strategies.