<P> This parasite was originally a mild pathogen of brown trout in central Europe and other salmonids in northeast Asia, and the spread of the rainbow trout has greatly increased its impact . Having no innate immunity to M. cerebralis, rainbow trout are particularly susceptible, and can release so many spores that even more resistant species in the same area, such as Salmo trutta, can become overloaded with parasites and incur mortalities of 80 to 90 percent . Where M. cerebralis has become well - established, it has caused decline or even elimination of whole cohorts of fish . </P> <P> The parasite M. cerebralis was first recorded in North America in 1956 in Pennsylvania, but until the 1990s whirling disease was considered a manageable problem only affecting rainbow trout in hatcheries . It eventually became established in natural waters of the Rocky Mountain states (Colorado, Wyoming, Utah, Montana, Idaho, New Mexico), where it is damaging several sport fishing rivers . Some streams in the western U.S. lost 90 percent of their trout . Whirling disease threatens recreational fishing, which is important for the tourism industry, a key component of the economies of some U.S. western states . For example, in 2005 anglers in Montana spent approximately $196,000,000 in activities directly related to trout fishing in the state . Some of the salmonids that M. cerebralis infects (bull trout, cutthroat trout, and anadromous forms of rainbow trout--steelhead) are already threatened or endangered, and the parasite could worsen their population decline . </P> <Dl> <Dt> New Zealand mud snail </Dt> </Dl> <Dt> New Zealand mud snail </Dt>

When does a rainbow trout become a steelhead