Evaluation of Disease Resistance of the Fish Lake–DeSmet, Wounded Man, and Harrison Lake Strains of Rainbow Trout Exposed to Myxobolus cerebralis

Abstract

Several tests evaluated the whirling disease resistance of three stocks of rainbow trout Oncorhynchus mykiss: the Fish Lake–DeSmet (FD), Wounded Man (WM), and Harrison Lake (HL) strains. In one series, FD fish were exposed to Myxobolus cerebralis triactinomyxons (TAMs; one exposure of 1,000 TAMs/fish). Histological scores ranged from 4.28 to 4.95 (MacConnnell−Baldwin scale) among FD fish from three different hatcheries. In addition, FD fish were exposed either acutely (1,000 TAMs/fish in one dose) or chronically (100 or 200 TAMs/fish on each of 10 d). There was no significant difference in mean myxospore counts among the three different exposure treatments. Histological scores ranged from 3.76 to 3.86 among the exposure groups and did not differ among exposure treatments. The correlation between histological scores and myxospore counts was significant (r = 0.57). Three different ages of WM rainbow trout were challenged with 1,000 TAMs/fish; histological scores for 62-d-old fish were significantly higher than scores for fish exposed at 108 or 137 d. Histological scores for WM fish did not significantly differ from those of HL fish, but both sets of scores were well below those of FD fish. The HL fry were exposed to doses of 0, 10, 100, 1,000, or 10,000 TAMs/fish. Histology scores were significantly different among treatments, ranging from 0.06 to 1.28 among the exposure groups. Even at doses of 1,000 or 10,000 TAMs/fish, a high percentage of fish had no or low pathology. Myxospore prevalence increased with dose: 7.5% at 10 TAMs/fish, 44.1% at 100 TAMs/fish, 70.0% at 1,000 TAMs/fish, and 87.7% at 10,000 TAMs/fish. Myxospore counts were significantly different among treatments, ranging from 0 to 540,556 among individual fish and from 514 to 75,380 among the treatment means of exposed groups. Deformities increased significantly with dose and ranged from 0.0% to 21.1%. Overall, FD rainbow trout showed little resistance to whirling disease, whereas the HL and WM strains included resistant individuals. Identification of resistant HL individuals for selective breeding could lead to a resistant broodstock.