Myxobolus cerebralis, the parasite from Europe that causes whirling disease in some salmonid species, has been detected in 25 states. During the parasite’s life cycle, it requires two hosts: a common aquatic worm (Tubifex tubifex) and a susceptible fish. Cutthroat trout are susceptible, especially during the first months of life; grayling and lake trout may carry the parasite but appear immune to the disease. The infection can cause skeletal deformities, a blackened tail, and whirling behavior. Because the fish cannot feed normally and are more vulnerable to predation, whirling disease can be fatal. No practical means has been found to eradicate M. cerebralis from a body of water in which it has become established.