CHARACTERIZATION OF INFECTIOUS SALMON ANEMIA VIRUS, AN ORTHOMYXO-LIKEVIRUS ISOLATED FROM ATLANTIC SALMON (SALMO-SALAR L.)

Infectious salmon anemia (ISA) virus is the cause of infectious salmon anemia in farmed Atlantic salmon, The virus has been shown to contain RNA with structural characteristics similar to those of accepted memb ers of the Orthomyxoviridae. Further biochemical, physiochemical, and morphological characterization of ISA virus was undertaken to clarify its taxonomic position, The virus was found to be sensitive to chlorof orm, heat, and low pH and agglutinated erythrocytes from fish, Erythro cytes from mammals or birds were not agglutinated. Receptor-destroying enzyme activity was detected, and the nature of this enzyme was sugge sted to be an acetylesterase. The buoyant density of the virus was 1.1 8 g/ml in sucrose and CsCl gradients, The maximum rate of virus replic ation was observed at 15 degrees C, while no virus was produced at 25 degrees C. Actinomycin D inhibited viral replication, and viral antige n was detected in nuclei by immunofluorescence. The addition of trypsi n to the culture medium during virus replication had a beneficial effe ct on virus replication, ISA virus contains four major polypeptides wi th estimated molecular sizes of 71, 53, 43, and 24 kDa. Electron micro scopy revealed structures closely resembling the nucleocapsids of infl uenza virus, Mushroom-shaped surface projections were a distinctive mo rphological feature, which differed from the rod-shaped hemagglutinin projections of the influenza viruses. The data reported here support t he relationship of ISA virus to the Orthomyxoviridae, although ISA vir us differs from influenza viruses in some morphological characteristic s and in showing restricted hemagglutination, in different specificity of the receptor-destroying enzyme, in different polypeptide profile, in being unable to replicate at temperatures above 25 degrees C, and i n host range.