Learn more about whirling disease in Wyoming. Whirling disease is caused by a microscopic parasite called Myxobolus cerebralis and can be found in trout and other fish in the salmonid family. Once the parasite infects young trout, it feeds on cartilage in the skull. This affects the fish’s nervous system and equilibrium, which causes fish to swim in circles and appear to chase their tail. Read the full story on the Wyoming Wildlife magazine page of our website. | Wyoming Game and Fish Department