Susceptibility of Selected Inland Salmonids to Experimentally Induced Infections withMyxobolus cerebralis, the Causative Agent of Whirling Disease

Abstract

Laboratory exposures to the infectious stages (triactinomyxons) of Myxobolus cerebralis demonstrated a range of susceptibility to whirling disease among four species of inland salmonids. Replicate groups of each species were exposed to two concentrations of triactinomyxons, a low dose (100-200 per fish) and a high dose (1,000-2,000 per fish). Exposed fish were evaluated for clinical signs, for severity of microscopic lesions at 35 d, 2 and 5 months, and for spore concentrations in the head cartilage at 5 months. A standard strain of rainbow trout Oncorhynchus mykiss matched for age served as a susceptible species control. Rainbow trout, westslope cutthroat trout O. clarki lewisi, Yellowstone cutthroat trout O. clarki bouvieri, and bull trout Salvelinus confluentus were susceptible to M. cerebralis infections. Clinical signs, including radical swimming (''whirling'') and black tails, were observed at 7 weeks postexposure among rainbow and cutthroat trout challenged at 3 weeks of age. Clinical signs were rare among bull trout exposed at an 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 microscopic lesions among infected fish at 5 months postexposure were found among rainbow trout. Cutthroat trout had less severe lesions, bull trout had mild infections, and no evidence of infection was found among Arctic grayling Thymallus arcticus. Mean spore concentrations among infected fish correlated with the severity of microscopic lesion scores. Rainbow trout had mean concentrations 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 Arctic grayling at 5 months postexposure.

Effects of fish age versus size on the development of whirling disease in rainbow trout

“…Each replicate consisted of 40 fish. M. cerebralis triactinomyxons were produced in the laboratory from cultures of infected Tubifex tubifex (Hedrick et al 1999b). Exposures were conducted at the Wild Trout Research Laboratory, Montana State University, Bozeman, MT.…”

“…One head half and the tail from each fish were prepared for histology. Microscopic pathology was categorized according to the MacConnell-Baldwin Scale (Hedrick et al 1999b), whereby cartilaginous tissue was examined for the presence of the parasite and associated lesions. The abundance of parasites, cartilage damage, inflammation, extent of lesions, involvement of other tissues and bone distortion were evaluated and categorized into one of 6 qualitative categories: no infection, minimal, mild, moderate, high or severe.…”

Ultraviolet irradiation inactivates the waterborne infective stages of Myxobolus cerebralis: a treatment for hatchery water supplies

“…Details on the exposure protocols and fish maintenance are described by Hedrick et al (1999a). After 5 wk, 5 fish from each unexposed group and each group receiving high dose triactinomyxon exposures with or without UV treatments were removed and examined by PCR for the presence of Myxobolus cerebralis infection (Andree et al 1998).…”

Correlation of environmental attributes with histopathology of native Yellowstone cutthroat trout naturally infected with Myxobolus cerebralis

“…These results were expected because when a pathogen invades cranial cartilage, the host's inflam- matory response may restrict nerves along the lower brain stem and spinal cord causing the whirling motion observed amongst infected trout and salmon (Hedrick et al 1999b, Rose et al 2000. Though this may be a reasonable hypothesis, lesions in cartilage of the lower jaw also correlated with prevalence of whirling behavior (and may correlate with lesions in cranium, thus not necessarily precluding this mechanism).…”