Susceptibility of selected inland salmonids to experimentally induced infections with Myxobolus cerebralis, the causative agent of whirling disease

Laboratory exposures to the infectious stages (triactinomyxons) of Myxobolu s cerebralis demonstrated a range of susceptibility to whirling disease amo ng four species of inland salmonids. Replicate groups of each species were exposed to two concentrations of triactinomyxons, a low dose (100-200 per f ish) and a high dose (1,000-2,000 per fish). Exposed fish were evaluated fo r clinical signs, for severity of microscopic lesions at 35 d, 2 and 5 mont hs, and for spore concentrations in the head cartilage at 5 months. A stand ard strain of rainbow trout Oncorhynchus mykiss matched for age served as a susceptible species control. Rainbow trout, westslope cutthroat trout O. c larki lewisi, Yellowstone cutthroat trout O. clarki bouvieri, and bull trou t Salvelinus confluentus were susceptible to M. cerebralis infections. Clin ical signs, including radical swimming ("whirling") and black tails, were o bserved at 7 weeks postexposure among rainbow and cutthroat trout challenge d at 3 weeks of age. Clinical signs were rare among bull trout exposed at a n age of 4 weeks and absent among rainbow and cutthroat trout exposed at 3 months posthatch. Most rainbow, cutthroat, and bull trout were found to be infected when examined at 5 months postexposure. The most severe microscopi c lesions among infected fish at 5 months postexposure were found among rai nbow trout. Cutthroat trout had less severe lesions, bull trout had mild in fections, and no evidence of infection was found among Arctic grayling Thym allus arcticus. Mean spore concentrations among infected fish correlated wi th the severity of microscopic lesion scores. Rainbow trout had mean concen trations of spores in head cartilage reaching 10(6), whereas more resistant species such as bull trout had 10(4) spores; no spores were found among Ar ctic grayling at 5 months postexposure.