Whirling disease is caused by a microscopic parasite from Europe (Myxobolus cerebralis) that can infect some trout and salmon; it does not infect humans. It has been detected in 25 states. During the parasite’s life cycle, it takes on two different forms as spores and requires two hosts: a common aquatic worm (Tubifex tubifex) and a susceptible fish. Cutthroat trout are susceptible, especially during the first months of life. The parasite feeds on the fish’s cartilage, and the infection can cause skeletal deformities, a blackened tail, and whirling swimming behavior. Because the fish cannot feed normally and are more vulnerable to predation, whirling disease can be fatal. No practical treatment exists for fish infected with this disease or for the waters containing infected fish.
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