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Mapping of putative virulence motifs on infectious salmon anemia virus surface glycoprotein genes

Biao Qian^1,

Shebel Hariharan^1,

¹ Department of Pathology and Microbiology, Atlantic Veterinary College, University of Prince Edward Island, 550 University Avenue, Charlottetown, PE C1A 4P3, Canada
² Department of Computer Science and Information Technology, University of Prince Edward Island, 550 University Avenue, Charlottetown, PE C1A 4P3, Canada
³ New Brunswick Department of Agriculture and Aquaculture, PO Box 6000, Fredericton, NB E3B 5H1, Canada

Correspondence
Frederick S. B. Kibenge
kibenge{at}upei.ca

Infectious salmon anemia virus (ISAV) is classified in the genus Isavirus of the family Orthomyxoviridae. Although virulence variation of ISAV can be demonstrated experimentally in fish, virus strain identification is ambiguous because the correlates of pathogenicity and/or antigenicity of ISAV are not well defined. Thirteen ISAV isolates characterized for their ability to kill fish were used to search for markers of virulence on the virus surface glycoprotein genes; haemagglutinin-esterase (HE) and fusion (F) protein genes. A single amino acid change N¹⁶⁴D in the putative globular head of the HE protein, and a deletion/insertion of 13 aa with the presence of a specific motif ³⁵²FNT³⁵⁴ in the highly polymorphic region spanning residues ³³⁷V to M³⁷² in the HE protein stalk, in combination with a specific motif ²⁶⁵YP²⁶⁶ very close to the trypsin-cleavage site ²⁶⁷RA/G²⁶⁸ of the precursor F₀ protein were correlated with reduced cytopathogenicity and reduced virulence for Atlantic salmon. Phylogenetic analysis suggests that the original ancestral ISAV was virulent. The virulence of the North American genotype has not changed much, whereas the European genotype evolved into two genogroups, the real-European genogroup that is still virulent and the European-in-North America genogroup, which is of lower virulence. A novel phylogenetic software program, BACKTRACK, estimated that the North American and European genotypes diverged between 1879 and 1891, whereas the European-in-North America genogroup diverged from the real-European genogroup between 1976 and 1988. This direction of evolution supports insertion of specific motifs in the HE protein, resulting in ISAV attenuation.

Present address: 17 Rosedale Drive, Charlottetown, PE, Canada.

Present address: Rolling Hills Pet Hospital, Chula Vista, CA, USA.

A supplementary table listing the oligonucleotide primers used is available with the online version of this paper.