Size-Related Susceptibility of Salmonids to Two Strains of Infectious Hematopoietic Necrosis Virus

Abstract

Infectious hematopoietic necrosis is generally considered a disease of salmonid alevins, fry, and early juveniles. It has been suggested that this disease kills only young salmonids and that susceptibility decreases with increased size and age of the fish. To determine if decreased susceptibility occurs with increasing size, we exposed four different sizes of rainbow trout Oncorhynchus mykiss (mean weight, 0.2–13.1 g) and kokanee (lacustrine sockeye salmon) Oncorhynchus nerka (0.2–7.2 g) to two waterborne strains of infectious hematopoietic necrosis virus. Both species were exposed to four concentrations of a type‐1 virus strain, which was isolated from fish in Oregon, and a type‐2 virus strain obtained from fish in Idaho. Neither strain of virus was consistently more virulent over all exposure concentrations for kokanee at the 0.2‐g fish size, At all other sizes, kokanee were consistently more susceptible to the Oregon strain than to the Idaho strain (sign test, P < 0.002). Conversely, all sizes of rainbow trout were consistently more susceptible to the Idaho strain than to the Oregon strain (sign test, P < 0.001). The results showed that there are differences in virulence for type‐1 and ‐2 strains of virus and that these strains have the ability to produce disease in kokanee and rainbow trout as large as 7.2 g and 13.1 g, respectively.

Development of passve immunotherapy for control of infectious hematopoietic necrosis

“…The half-life of the exogenously supplied immunoglobulin may also be extended because of decreased metabolic activity of fish in cooler water. The salmonid species tested as well as size and age may also affect results because these have been shown to impact IHNV-susceptibility (LaPatra et al 1990). Additionally, dose of viral challenge and susceptibility differences within an experimental group are important parameters to consider when interpreting results for development of passive immunotherapy strategies.…”

“…However, in naturally infected populations, protection against reinfection is generally observed (Busch 1983) suggesting that low antibody titers are sufficient for strong protection. Although decreased susceptibility to IHNV has been reported with increasing fish size and age (LaPatra et al 1990) IHN epizootics have been reported in large (100 to 500 g) rainbow trout (Busch 1983) and Atlantic salmon Salmo salar (Armstrong et al 1993). Because mortality in large fish can have severe economic consequences, passive immunotherapy may be a potential strategy for use on valuable fish.…”

Virulence comparison of different electropheotypes of infectious hematopoietic necrosis virus

“…LaPatra et al (1990a) compared virulence of IHNV electropherotypes 1 and 2 in different sizes of kokanee salmon Oncorhynchus nerka and rainbow trout. The type 1 strain was more virulent for kokanee, whereas, type 2 was more virulent for rainbow trout.…”

“…The host species from which the isolate was obtained also varied but most likely had no effect on virulence because other studies have shown that adult salmonids of different species in the same geographical area can be asymptomatic carriers of identical electropherotypes of IHNV (Hsu et al 1986, Ristow & Arnzen de Avila 1991. Additionally, salmonid species susceptibility studies which compared IHNV isolates of different host origin showed that the host species from which the IHNV isolate was obtained has a negligible effect on pathogenicity for other salmonids (LaPatra et al 1989, 1990a, Chen et al 1990, Traxler et al 1993. Hsu et al (1985) noted that the nucleoprotein (N) molecular weight differences detected for different IHNV isolates (Leong et al 1981) was a unique characteristic.…”

Genetic variation underlying resistance to infectious hematopoietic necrosis virus in a steelhead trout (Oncorhynchus mykiss) population

“…Lapatra et al 1990, Bergmann et al 2003, Becker et al 2005, Ryce et al 2005. However, it is unclear whether there is a relationship between fish length at a specific age and IHNV resistance.…”