Myxobolus cerebralis infection in rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) exposed under natural stream conditions

From early April into mid-June 1977, sequential groups of juvenile rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) were each expose d for 10 days to the parasite Myxobolus cerebralis by immersion in a stream inhabited by infected wild trout. Following incubation in a M. cerabralis- free facility, trout were subsequently killed, and heads and gill arches we re examined by routine histologic methods. A grading scale to quantify lesi on severity was developed and applied. Percentage infected, lesion severity scores, effects of water temperature and flow rates on percentage infected and lesion severity scores, and resulting pathology were determined for ea ch species at each exposure period. The percentage of rainbow trout infecte d with M. cerebralis was significantly higher than the percentage of brown trout infected for each exposure period. The percentages of rainbow trout i nfected in exposure periods later in the calendar year were significantly h igher than those in earlier periods. The percentages of brown trout infecte d were not significantly different among exposure periods. Overall average lesion severity scores were significantly higher in rainbow than in brown t rout. Lesion severity scores in rainbow trout increased over time (a positi ve correlation with exposure period). Lesion severity scores were not signi ficantly different for brown trout among exposure periods. A significant co rrelation existed between water temperature and percentage of rainbow trout infected; a significant correlation also existed between water temperature and lesion severity scores in rainbow trout. Similar correlations did not exist for percentage of brown trout infected or accompanying lesion severit y scores. In rainbow trout, ventral calvarium was the most common site of M . cerebralis replication, followed by gill arches. In brown trout, lesions were virtually confined to gill arches. Early lesions consisted of foci of cartilage necrosis with small numbers of M. cerebralis developmental stages . More advanced lesions consisted of multifocal areas of cartilage necrosis with numerous M. cerebralis developmental stages and/or mature myxospores bordered and/or infiltrated by mono- and multinuclear leukocytes. Lesions i n brown trout were smaller and had fewer associated leukocytes and M. cereb ralis developmental stages and/or mature myxospores. Higher infection rates , lesion severity scores, and differences in lesion location in rainbow ver sus brown trout explain in part why numbers of rainbow but not brown trout have fallen in western rivers inhabited with M, cerebralis-infected trout.