Identification of putative motifs involved in the virulence of infectious pancreatic necrosis virus.
Infectious pancreatic necrosis viruses (IPNVs) belonging to the family Birnaviridae display a high degree of antigenic variability, pathogenicity, and differences in outbreak mortality in salmonid species. To determine if virus isolates of Sp serotype differ in virulence, fry of Atlantic salmon (Salmo salar L.) were challenged with nine different field strains. These viruses caused either high mortality and severe pathological changes or low mortality and no lesions. To study the molecular basis for the variation in virulence of IPNV, complete nucleotide sequences of segment A of all these strains as well as segment B of three selected strains were determined. All viruses tested had a unique genome sequence. Only minor differences were noted in the genes encoding VP1, VP3, and VP4 proteins, whereas most changes were observed in the gene encoding the VP2 protein. A high level of variation was found in the small open reading frame (ORF), which encodes a 15-kDa nonstructural (NS) polypeptide also known as VP5. One of the strains lacked the initiation codon for this protein, whereas the other four could encode a truncated version of the NS protein. Additional data obtained by sequencing of the NS and VP2 genes directly from diseased fish demonstrated changes in the VP2 gene after two passages in cell culture, which could possibly be associated with attenuation. Comparison of the deduced amino acid sequences of the NS and VP2 genes reveals that the virulent strains possess a 12-kDa coding NS gene and have residues Thr, Ala, Thr/Ala, and Tyr/His at positions 217, 221, 247, and 500 of the VP2 gene, respectively-the motifs identified in this study to be involved in the virulence of IPNV.