Susceptibility of rainbow trout resistant to Myxobolus cerebralis to selected salmonid pathogens

Laboratory challenges of two rainbow trout Oncorhynchus mykiss strains with Myxobolus cerebralis triactinomyxons confirm the resistance to whirling disease of the Hofer trout strain. Although the number of fish that became infected and developed clinical disease was similar for the Hofer and the Trout Lodge strains at all challenge doses, the median spore numbers were lower at all challenge doses for the Hofer rainbow trout. Parasite challenge doses required to produce lesions of high severity were 10-fold lower for the Trout Lodge strain (100 triactinomyxons) than the Hofer strain (1,000 triactinomyxons). Challenges of the Hofer strain with other common salmonid pathogens; the myxozoans Ceratomyxa shasta and Tetracapsuloides bryosalmonae, the bacterium Yersinia ruckeri, and the viruses Salmonid herpes-like virus type 1 and infectious hematopoietic necrosis virus demonstrate that the susceptibility of the Hofer strain was similar to what would be expected for other strains of rainbow trout, either domestic or wild. These pathogen challenges provide evidence that the Hofer trout present a low risk for introducing any pathogen that might be detrimental to native or established fish populations or further amplifying those that are endemic.