Whirling disease is caused by a microscopic parasite that was introduced to the United States from Europe in the 1950s. In Wyoming it affects trout and is spread through movement of infested fish and water or through mud that may contain the parasitic spores. It affects the cartilage tissue of salmonids, and symptoms of infected fish may include a blackened tail and deformity which may cause the fish to swim in a whirling manner. It ultimately results in death of the fish.
Whirling disease occurs in 23 states, including Wyoming. It was first discovered in Wyoming in 1988 and is present in some of the rivers in the western part of the state along with a handful of waters in southeast Wyoming and the Bighorns. Anglers can help prevent its spread by thoroughly cleaning waders and recreational gear. More information on whirling disease is available on the Wyoming Game and Fish website: http://gf.state.wy.us
source: WGAF press release