ABSTRACT
The 99323 Egyptian isolate of infectious bursal disease (IBD) virus (IBDV) was identified during an international survey of acute IBD cases. Its unique antigenicity was characterized by a markedly reduced binding of neutralizing monoclonal antibodies 3, 4, 5, 6, 8 and 9 in an antigen-capture enzyme-linked immunosorbent assay. Nucleotide sequencing of the genome region encoding the VP2 major immunogenic domain in 99323 revealed amino acid changes occurring at positions critical for antigenicity, but phylogenetic analysis demonstrated that 99323 was related to typical, very virulent IBDV (e.g. isolate 89163). Protection experimentally afforded by an antigenically classical live IBD vaccine was investigated in specific pathogen free chickens challenged with 99323 or 89163. Both viruses were similarly controlled, as evaluated by clinical signs, growth retardation, bursa-to-body weight ratios and histological lesions of the bursa after challenge. These results document that an active antibody response to a classical live antigen may clinically control infection by an antigenically atypical very virulent IBDV.