The Pathogenic Effects of Moroccan Very Virulent Infectious Bursal Disease Virus on Lymphoid Organs: A Comparative Study in Conventional Broiler and Specific-Pathogen-Free Chickens

Infectious bursal disease (IBD) is a major immunosuppressive disease affecting young chickens, and causes significant economic losses to the poultry industry. This work represents the first pathogenicity assessment of Moroccan very virulent IBD virus. Molecular characterization and sequence analysis of this isolate previously identified specific substitutions, including seven amino acid substitutions in segment A, and I472L and E688D in segment B, specific and unique to Moroccan vvIBDV strains. Two chicken lines, broiler and specific-pathogen-free (SPF) chickens, were inoculated via the occulonasal route with 0.2 mL of the 10⁵EID₅₀ /mL viral solution of the IB19 vvIBDV strain at 29 days of age. Experimental monitoring was carried out for 10 days post-challenge (dpc). Clinical signs started on the second dpc, with peak severity observed between 3 and 6 dpc. The total mortality rate reached 10% in broilers (group G1) and 93% in SPF chickens (G3). Macroscopic lesions in G1 broilers included marked hypertrophy of the bursa of Fabricius (BF), followed by very pronounced atrophy, while macroscopic examinations of deceased SPF birds (G3) revealed very hemorrhagic BF with a black cherry appearance in 80% of dead birds. The mean Bursa/Body Index (BBI) of challenged broilers (G1) showed a decrease of 46% compared to the control group (G2), indicating bursal atrophy. Microscopic lesions in the BF consisted mainly of inflammation, with severe lymphoid depletion of the follicles in challenged G3 SPF birds. This in vivo study of Moroccan vvIBDV demonstrated a distinctive virulence profile, and confirmed its classification as a very virulent strain with substantial disease-causing potential. It is crucial to obtain comprehensive knowledge of the prevalence, emergence, pathogenicity, and control of Moroccan IBDV strains.