Abstract
A model system for examining resistance mechanisms of interspecific hybrids to a viral disease was designed and tested. Diploid rainbow trout Oncorhynchus mykiss and heat-induced triploid brown trout Salmo trutta female X lake trout Salvelinus namaycush male hybrids (or brake trout) were evaluated for susceptibility and humoral response to infectious hematopoietic necrosis virus (IHNV). The triploid hybrids exposed to waterborne virus had a cumulative mortality of 2% ( 1 49 ) compared to 53% ( 27 51 ) in the pure species diploid rainbow trout. Virus neutralization activity was not detected in serum obtained from control fish, however, all serum pools from surviving rainbow trout exposed to waterborne IHNV had antibody titers significantly higher ( P < 0.05) than pooled sera from brake trout. This suggested that the IHNV resistance of the hybrids occurred at the cellular level that minimized infection and internalization of the virus. To determine if resistance was a function of external epithelial cell attachment and entry, susceptibility was also evaluated to intraperitoneally injected virus. Mortality was also significantly less ( P < 0.001) in hybrid trout (mean weight, 10.2 and 26.2 g) injected with IHNV compared to rainbow trout (mean weight, 22.3 and 90.0 g). The decrease in mortality was associated with a strong humoral response where 70% of the surviving triploid hybrids had antibody titers ≥ 640. Necrosis of the renal and splenic interstitial hematopoietic tissue of rainbow trout was typical of lesions due to IHNV infection, however, the lesions in the hybrids were less extensive and severe. The decreased severity and extent of the lesions correlated with the decreased mortality and high antibody titers demonstrated in the hybrid trout as compared to the matched group of rainbow trout. These results suggest that more than one host factor can affect resistance to IHNV.
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