Phylogenetic and pathogenic analyses of circulating infectious bursal disease virus strains in China

Infectious bursal disease (IBD) remains one of the most important immunosuppressive diseases in poultry and has been endemic in China for over four decades. In recent years, outbreaks characterized by markedly reduced mortality have emerged. To investigate the current prevalence and pathogenicity of infectious bursal disease virus (IBDV), strains were isolated from vaccinated poultry farms in central and eastern China, and representative isolates of distinct genotypes were evaluated for pathogenicity. Phylogenetic analysis of segment A (HVR) and segment B (B-marker) classified segment A into eight genogroups (A1-A8), with A2 further divided into A2.1 and A2.2 and A3 into six subtypes (A3.1-A3.6). Segment B was grouped into four genogroups (B1-B4), with B3 and B4 subdivided into B3.1-B3.4 and B4.1-B4.2, respectively. The 23 isolates obtained were grouped into three genotypes: A2.2B1 (14/23, 60.9 %), A3.2B3.2 (8/23, 34.8 %), and A3.5B3.3 (1/23, 4.3 %). The A2.2B1 and A3.5B3.3 strains showed only minor amino acid substitutions relative to reference strains, whereas the newly emerged A3.2B3.2 genotype displayed substantial divergence. Pathogenicity evaluation in SPF chickens revealed no mortality after infection with SD/23 (A3.2B3.2), SHX/24 (A3.5B3.3), or WD/22 (A2.2B1). However, all three strains significantly reduced the bursa-to-body weight index (BBIX) to below 0.7 at 5 days post-inoculation and caused severe bursal atrophy. Histopathological examination showed medullary necrosis and lymphocyte depletion in the bursa following infection with SD/23 and SHX/24, closely resembling the lesions induced by WD/22. Moreover, SD/23 and SHX/24 displayed replication dynamics similar to those of the reference strain LX (A3.1B2), with elevated viral detection rates and high viral loads in multiple organs. In summary, multiple IBDV genotypes are co-circulating in China. Despite the reduced pathogenicity of circulating strains, diminished attention to subclinical infections may facilitate viral transmission and cause considerable economic losses.