Nucleotide sequence analysis of the entire coding regions of virulent and avirulent strains of viral haemorrhagic septicaemia virus.

The complete nucleotide sequences of the six open reading frames (N, P, M, G, NV and L) and the intergenic regions were determined for two virulent freshwater strains and two avirulent marine strains of viral haemorrhagic septicaemia virus (VHSV). Sequence analysis of 10,845 nucleotides revealed > 97.2% nucleotide sequence similarity and > 98.6% amino acid similarity, confirming the close genetic relationship between marine and freshwater strains of VHSV. Moreover, as few as 10 amino acid substitutions were identically substituted between the marine and freshwater strains investigated. The results of this study suggest that only a limited number of amino acid residues may be involved in the determination of VHSV virulence for salmonids and further highlights the potential risk that marine strains may pose to freshwater aquaculture.