Phylogenetic analysis and virulence characteristics for chinook salmon, rainbow trout, and steelhead of L genogroup infectious hematopoietic necrosis virus (IHNV) from the Feather River, California

Infectious hematopoietic necrosis virus (IHNV) is a significant rhabdoviral pathogen of young salmonids. Historically, periodic IHNV epidemics occurred in juvenile Chinook salmon at the Coleman National, Nimbus, and Feather River (FRH) fish hatcheries, the three largest northern California facilities. An unprecedented outbreak in steelhead in 2001 and substantial Chinook salmon mortality in 1998 and 2000--2002 at the FRH prompted examination into the potential causes of these epidemics. Phylogenetic analyses of a variable portion of the glycoprotein gene (mid-G) and serologic comparisons based on neutralization with three anti-IHNV polyclonal rabbit sera placed 82 Feather River watershed (1969--2004) isolates in one of two subgroups within the L genogroup IHNV (maximum divergence = 4.0%) and one of three serologic groups typical of California. The early genotypes (LI subgroup) appeared to give rise to new serologic and genetic types of the larger subgroup LII. Selected historical isolates (LI) were as virulent as IHNV from recent FRH epidemics (LII) in experimental challenges with juvenile Chinook salmon and rainbow trout. While most isolates were equally virulent for both host species, one isolate was more virulent for Chinook salmon. Three different hatchery Chinook salmon stocks at 588 degree days (dd) in age (average weight 0.55 g) were moderately to highly susceptible (cumulative percent mortality [CPM] = 47--87%) to immersion challenge at 12°C with a contemporary LI or LII isolate (10 4-6 PFU/mL). Two of the Chinook salmon stocks tested as older fish (1591 dd; 2.8 g) were low to moderately susceptible (CPM = 10--58%) to the LII isolate (105-6 PFU/mL). In these latter challenges fish groups tested at varied temperatures (10°C, 12°C, and 14°C) showed a significant decrease in mortality at 14°C compared to 12°C for both Chinook salmon stocks, and also a significant decrease in susceptibility for one of the stocks at 10°C. Mortality for two different hatchery steelhead stocks (714 dd; 0.68 g) exposed to immersion challenge with two contemporary LII isolates (104-6 PFU/mL) was significant, but low (CPM = 1.3--33%) with no difference in mortality between stocks. Older steelhead (1260 dd; 2.2 g) were refractory to IHNV. Isolation of IHNV (10 2.4-3.2 PFU/g) from juvenile Chinook salmon skin for as long as 215 d post exposure provided evidence for IHNV persistence. Serial in vivo passage of L genogroup IHNV in Chinook salmon or rainbow trout produced high viral concentrations in the fish (107.9-8.4 PFU/g), but did not change mid-G consensus sequences or viral virulence for Chinook salmon or steelhead.