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

Abstract

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.

Differential virulence mechanisms of infectious hematopoietic necrosis virus in rainbow trout (Oncorhynchus mykiss) include host entry and virus replication kinetics

“…1). Consistent with previous data (Garver et al, 2006), immersion challenge with U IHNV resulted in low mortality (18 % final mean CPM). I.p.…”

“…Rainbow trout were more susceptible to IHNV strains now known to be in the M genogroup, whilst kokanee salmon (landlocked O. nerka) were more susceptible to strains now presumed to be U. More recently, this host-specific virulence of U and M IHNV was clearly demonstrated by Garver et al (2006). In rainbow trout, immersion challenge with three M genogroup isolates caused high mortality, whilst challenge with three U genogroup isolates resulted in low mortality.…”

Inhibition of an Aquatic Rhabdovirus Demonstrates Promise of a Broad-Spectrum Antiviral for Use in Aquaculture

“…Our model virus, IHNV, preferentially infects trout at 15°C (25), and we demonstrated that LJ001 was more stable at these temperatures (Fig. 4A), with an inhibitory half-life of ϳ1.9 days (R 2 ϭ 0.83).…”

In Vivo Fitness Associated with High Virulence in a Vertebrate Virus Is a Complex Trait Regulated by Host Entry, Replication, and Shedding

“…Since our study goes beyond an analysis of within-host replication to assess fitness traits throughout a single viral infection cycle, we use the term "infection cycle fitness" to refer to the data we present. We examined infection cycle fitness for two IHNV genotypes previously characterized to have high (HV) and low (LV) virulence in their natural rainbow trout host (Oncorhynchus mykiss) (25,59). This provided information on how viral traits at each infection stage might individually be associated with virulence, thus offering insights on possible selection on virulence in IHNV.…”

“…The virus regularly causes epidemics in hatchery-reared, farmraised, and wild fish throughout its host range, frequently re-sulting in significant levels of acute disease-induced mortality due to necrosis of the host kidneys, liver, and spleen (9). Hundreds of North American field isolates of the virus have been typed, giving rise to well over a hundred unique genotypes, many of which differ in virulence (25). Longitudinal studies indicate complex movement and evolution of the virus in the field, with multiple genotypes cocirculating in some situations and new genotypes sometimes emerging and displacing resident ones (3,56,57).…”