Pathogenicity, immunogenicity, and multi-organ tissue tropism of a novel variant infectious bursal disease virus in specific pathogen-free chickens

Background and Aim: Novel variant infectious bursal disease virus (nVarIBDV) occurs worldwide, causing significant morbidity and bursal atrophy in chickens, leading to immunosuppression and heavy economic losses. This study aimed to evaluate the pathogenicity, immunogenicity, and virus load of nVarIBDV in organs of specific-pathogen-free (SPF) chickens. Materials and Methods: Sixty, 21-day-old SPF chickens were divided into Groups A and B. Birds in Group A were inoculated with 106.75 EID50/ mL of nVarIBDV (UPM1432/2019) while Group B served as a control. Four birds from Group B were sacrificed at 0 days post-inoculation (dpi). Four birds from each group were sacrificed at 1, 3, 5, 7, 10, 14, and 21 dpi. Clinical signs and serum samples were collected. Body, bursa, and spleen weights, and gross lesions were recorded. Bursa samples were collected for histological examination, while the bursa, spleen, caecal tonsils, thymus, and bone marrow were collected for the virus load determination. Data was analyzed using Student’s t-test at 95% confidence level. Results: Watery diarrhea and ruffled feathers, bursal atrophy, yellowish stain, decreased folds, firm consistency of bursa, and splenomegaly were observed in Group A. Bursal follicles were atrophied with bursal lesion scores of 4 to 5. IBD antibody titer in Group A, ranging from 6921 -13869 ELISA units at 5-21 dpi, was significantly higher (p<0.05) than in Group B at all timepoints. The viral load was highest in the bursa and lowest in the bone marrow and was detected from 1-21 dpi in the bursa, spleen, caecal tonsil, and thymus, and up to 7 dpi in the bone marrow with a copy number ranging from 7.111 - 12.414 log10. Conclusion: The nVarIBDV was highly pathogenic, immunogenic, and highly infective in the organs of SPF chickens. It could cause immunosuppression in chickens, exposing them to secondary infections with resultant heavy economic losses, and this report of the detection of nVarIBDV in these organs up to 21 dpi is novel. Keywords: infectious bursal disease, nVarIBDV, pathogenicity, specific pathogen-free chicken, bursa of Fabricius, IBD antibody, viral loads.