Virulence Comparisons of Infectious Hematopoietic Necrosis Virus U and M Genogroups in Sockeye Salmon and Rainbow Trout.

Infectious hematopoietic necrosis virus (IHNV) is an aquatic rhabdovirus that infects salmonids in the Pacific Northwest of the United States, Europe, and Asia. Isolates of IHNV have been phylogenetically classified into three major viral genogroups, designated U, M, and L. To characterize virulence of IHNV in the context of these three viral genogroups, seven strains of IHNV (three U genogroup strains, three M strains, and one L strain) were compared for their pathogenicity in juvenile sockeye salmon Oncorhynchus nerka, kokanee (lacustrine sockeye salmon), and rainbow trout O. mykiss. Fish were waterborne-exposed to the different viral strains, and virulence was assessed by comparing mortality curves and final cumulative percent mortality (CPM) in both species of fish at 10°C and 15°C. In sockeye salmon and kokanee, the U genogroup virus types were extremely virulent, causing average CPMs of 69-100%, while the M genogroup virus types caused very little or no mortality (CPM = 0-4%). The endangered Redfish Lake sockeye salmon stock exhibited extreme differences in susceptibility to the U and M genogroups. Conversely, in two stocks of rainbow trout, the M genogroup virus types were more virulent, inducing average CPMs of 25-85%, while the U genogroup viruses caused lower mortality (CPM = 5-41%). In both fish species, the single L genogroup strain caused low to intermediate mortality (CPM = 13-53%). Viral glycoprotein sequence comparisons of the seven challenge strains revealed three amino acid sites (247, 256, and 270) that consistently differed between the U and M genogroups, possibly contributing to pathogenicity differences.