Variation in Infections of Myxobolus cerebralis in Field‐Exposed Cutthroat and Rainbow Trout in Idaho

We evaluated the severity and extent of infection by Myxobolus cerebralis in replicated groups of same‐age Yellowstone cutthroat trout Oncorhynchus clarki bouvieri and Kamloops rainbow trout O. mykiss exposed to natural river conditions at four sites along a 25‐km reach of the South Fork of the Boise River. Groups of fry were each exposed for 10 d in June, July, or August and then removed from the river and reared in specific‐pathogen‐free water under controlled laboratory conditions for 84 d. At the end of the study, surviving fish were examined for clinical signs of disease, and fish heads were evaluated by histology for the presence of characteristic pathology and then scored for the extent of pathology on a five‐point scale. The mean histological scores and percent of fish testing positive in both species were similar within each site but varied across the study sites from no infection in fish at the uppermost site to 90–100% testing positive at the lowermost site. Cutthroat trout with moderate histology scores had significantly higher prevalence of exophthalmia and cranial deformities than did similarly infected rainbow trout. Habitat characteristics such as water temperature and the combined mean abundance of oligochaetes and chironomids were significantly different among the study sites and were positively correlated with the disease severity detected by histology.