In the lake, the infection has spread to about 20% of the cutthroat trout.

It has since been found in the Firehole, Madison, Gibbon, Gallatin, and Lamar rivers and throughout the Yellowstone Lake watershed. Whirling disease was first detected in Yellowstone in 1998 in cutthroat trout from Yellowstone Lake. No practical treatment exists for fish infected with this disease or for the waters containing infected fish. Because the fish cannot feed normally and is more vulnerable to predation, whirling disease can be fatal. The parasite feeds on the fish’s cartilage, and the infection can cause skeletal deformities, a blackened tail, and whirling swimming behavior.

Cutthroat trout are susceptible, especially during the first months of life. 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. Whirling disease is caused by a microscopic parasite from Europe ( Myxobolus cerebralis) that can infect some trout and salmon it does not infect humans.

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