The tiny parasite enters the body of the tubifex worm in the streambed. Fish become infected after the parasite leaves the worm and enters the water. The parasite attaches to a fish and grinds its way down into the fish's cartilage where it matures into the whirling disease spore. It lives there until the fish dies and the whole cycle is repeated when the spores are released back into the water. Usually, the disease affects young fish by damaging their cartilage and nerves.
Some infected fish demonstrate an abnormal whirling or tail-chasing pattern.
Other abnormalities in young fish include a black tail and adult fish sometimes have a deformed head or body. It has also been found in Yellowstone Lake in Yellowstone National Park. Studies show rainbow and cutthroat trout are more susceptible to the disease than brown trout, which appear to be immune to whirling disease. Studies in Montana and at the University of California-Davis show grayling and bull trout are also very resistant to the parasite infection. Research on all aspects of whirling disease is on going.