TWENTY-FIFTH ANNUAL EASTERN FISH HEALTH WORKSHOP MARCH 10-13, 2000     Comparative Susceptibility Of Three Strains Of  Oncorhynchus mykiss  To Whirling Disease     Christine L. Densmore1, Vicki S. Blazer1, Penelope S. Pooler2, Thomas B. Waldrop3, Deborah D. Cartwright4, and W. Bane Schill1   1National Fish Health Research Laboratory, U.S. Geological Survey, Leetown Science Center, 1700 Leetown Road, Kearneysville, WV  25430; 2Aquatic Ecology Laboratory; U.S. Geological Survey, Leetown Science Center, 1700 Leetown Road, Kearneysville, WV  25430; 3Freshwater Institute, Shepherdstown, WV 25443; 4Johnson Controls World Services, Inc., Leetown Science Center, 1700 Leetown Road,  Kearneysville, WV 25430   Three taxonomically distinct groups of Oncorhynchus mykiss (Mt. Lassen strain of domestic rainbow trout, Cayuga Lake strain of wild rainbow trout, and steelhead trout) were exposed to triactinomyxon spores (TAMs) of Myxobolus cerebralis in laboratory trials in order to compare susceptibilities as defined by standard clinical and diagnostic parameters. Fingerling fish (1300 degree-days development) from each group were either sham-exposed or exposed to low level (200 TAMs/fish) or high levels (2000 TAMs/fish) of infective spores. Twenty-two weeks post-exposure, 60 fish per exposure level per group were removed from their holding tanks for observation for clinical signs (whirling behavior, blacktail, and skeletal deformity) and cranial tissue sampling for spore enumeration and histological assessment of lesions. Clinical signs of whirling disease were evident among exposed fish, both low and high levels, for all three groups beginning six to eight weeks post-exposure.  Results indicate that the domestic Mt. Lassen strain of rainbow trout had the highest susceptibility of the three groups examined in terms of all clinical indicators, cranial spore loads, and presence of lesions.   Less consistency was evident among the data for the steelhead and Cayuga Lake groups.  Cayuga Lake rainbow trout had higher cranial spore loads as well as a higher frequency of associated lesions (low exposure level) compared to the steelhead trout.  Clinically, the Cayuga Lake fish also were the more likely to have blacktail, but the steelhead trout had the greater odd of exhibiting whirling behavior and skeletal deformity.     Return to 25th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page