TWENTY-FIFTH ANNUAL EASTERN FISH HEALTH WORKSHOP MARCH 10-13, 2000     Susceptibility Of Atlantic Salmon, Lake Trout And Rainbow Trout  To Myxobolus cerebralis Under Controlled Laboratory Exposures     Deborah D. Cartwright2, V.S. Blazer1, C.L. Densmore1, P. Peach1, K. Spring1, W.B. Schill1   1National Fish Health Research Laboratory, U.S. Geological Survey, 1700 Leetown Road, Kearneysville, WV 25430; 2Johnson Controls World Services, Inc. National Fish Health Research Laboratory     Recent findings in Montana and other intermountain regions of the United States, suggest that whirling disease may be the cause of population declines in wild trout. Hence, there has been a great deal of interest in species and strain susceptibility of salmonids. For this experiment, we compared susceptibility of lake trout (Salvelinus namaycush), Atlantic salmon (Salmo salar), and Mt. Lassen rainbow trout (Oncorhynchus mykiss) under controlled laboratory conditions. Fish (225 per dose) were exposed to 200 (low) or 2000(high) TAMs/fish for two hours. Following exposure, fish were divided into triplicate tanks with 75 fish per tank.  An additional 150 fish from each species/strain were sham exposed for 2 hours and placed in duplicate control tanks.  Twenty-two weeks post-exposure, 60 fish were sampled from each control and exposure level. Fish were anesthetized, weighed, measured and bled.  Any apparent morphological abnormalities (cranial, opercular or skeletal deformities; shortened opercles) were noted.  Heads, including gills, were bisected longitudinally and half of the head was preserved in Davidson’s fixative and half was bagged and frozen for spore enumeration. Histologic assessment of infection was based on presence of the parasite and associated lesions.  A grading scale of 0-4, was used to score abundance of mature spores and immature stages of the parasite and inflammatory or degenerative changes to cartilage and surrounding tissues.  Spore enumeration on heads was done using a modification of the trypsinization method. Whirling swimming behavior and blacktail were only observed in the rainbow trout. Cranial deformities were observed in a low percentage (8.3% and 3.3% in low and high groups respectively) of exposed Atlantic salmon versus the high percentages observed in exposed rainbow trout (75% and 73.3% in high and low groups, respectively). Cranial deformities were also noted in 5 exposed lake trout, however, no histologic evidence of whirling disease could be found in these fish. We found lake trout to be totally refractory, as determined by histopathology and preliminary spore counts, under the conditions of our exposures.  Atlantic salmon had a lower susceptibility when compared to rainbow trout in terms of clinical signs, histopathology and preliminary spore counts. Atlantic salmon were similar to brown trout, Salmo trutta, in susceptibility and the prominent eosinophilic granular leukocyte response, observed in association with the parasite.     Return to 25th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page