Salmonids Resistant toCeratomyxa shastaAre Susceptible to Experimentally Induced Infections withMyxobolus cerebralis

Abstract

Two strains of rainbow trout Oncorhynchus mykiss and one strain of steelhead (anadromous rainbow trout) previously shown to demonstrate resistance to the myxosporean pathogen Ceratomyxa shasta were found to be susceptible to experimentally induced infections with the myxosporean Myxobolus cerebralis. Laboratory exposures to waterborne infectious stages (triactinomyxons) of M. cerebralis of both strains of rainbow trout and the steelhead resulted in clinical signs of whirling disease, including the characteristic tail chasing and black tail, approximately 42–49 d after exposure at a water temperature of 15°C. At 5 months postexposure, the severity of microscopic lesions and spore concentrations in the head cartilage of all salmonids resistant to C. shasta were evaluated. The Iron Gate steelhead and the Pit River rainbow trout that were resistant to C. shasta had lesion scores and spore counts similar to those of control rainbow trout known to be susceptible to C. shasta. Evidence of some resistance to M. cerebralis among the Deschutes River strain of rainbow trout was demonstrated by lower lesion scores and mean spore counts than were seen in the control rainbow trout strain. In general, however, the mechanisms of resistance that have developed among certain salmonid populations to one myxosporean, C. shasta, do not extend to protection or resistance to a second myxosporean, the causative agent of salmonid whirling disease.