Pathogenicity, Humoral Immune Response, and Organ Tropism of Novel Variant Infectious Bursal Disease Virus in Commercial Broiler Chickens

Novel variant infectious bursal disease virus (nVarIBDV) infection causes mortality, morbidity, and immunosuppression in chickens worldwide. This study explored the pathogenicity, immunogenicity, and nVarIBDV load in commercial broiler chickens. Sixty 3-wk-old chicks were divided into nVarIBDV (28) and control (32) groups. The nVarIBDV group was inoculated with 1.0 ml (10^6.75 median egg infective dose [EID₅₀]) of nVarIBDV through the oral and ocular routes at 0 days postinoculation (dpi), while the control group was uninoculated. The control group was sampled on 0 dpi, and both groups were sampled on 1, 3, 5, 7, 10, 14, and 21 dpi. Clinical signs were recorded daily. Body, bursa of Fabricius, and spleen weights were measured, and weight ratios were calculated. Blood, bursa, spleen, cecal tonsil, thymus, and bone marrow samples were collected on each sampling day. Infectious bursal disease (IBD) antibody titer was analyzed, bursa tissue was stained with hematoxylin and eosin, and virus copy number was evaluated in the organ samples. The nVarIBDV group recorded lower body weight, bursal atrophy, and bursa:body weight index below 0.7 from 3 to 21 dpi. Moderate to severe bursal lesions, with scores significantly (<i>p</i> < 0.05) higher than the control group, degeneration of lymphoid follicle, infiltration of inflammatory cells into interstitial space, thickened and vacuolated epithelium, follicular cyst formation in the bursa, and splenomegaly occurred from 3 to 5 dpi in the nVarIBDV group. IBD antibody in the nVarIBDV group was higher (<i>p</i> < 0.05) at 5 to 21 dpi and peaked at 10 dpi. The nVarIBDV was detected in all the organs examined, but the highest viral load was in the bursa, followed by the thymus, spleen, cecal tonsil, and bone marrow in decreasing order. The nVarIBDV was pathogenic, immunogenic, and highly infective in broiler chickens and maintained a high copy number in the organs.