Whirling disease, a disease of trout and salmon caused by a microscopic parasite, Myxobolus cerebralis, is native to European waters. It first surfaced in the United State in Pennsylvania in1956, and has since spread throughout much of the northern and western part of the country, frequently causing significant population declines. The parasite responsible for the disease has two distinct forms and involves two host species, a sediment dwelling worm (an intermediate host) and fish. It was often spread by the movement of live, infected trout (hatchery stocking is a frequent culprit), but the spore stage may potentially be transported in via fish-eating birds or in the sediment on the feet of human anglers.
The parasite finds its way into the cartilage of an infected fish, and subsequently matures into it's spore stage. In the process, the fish may suffer nerve and cartilage damage which manifests itself as deformities to the head or body, a darkening of the tail in younger fish, and/or an abnormal whirling or tail-chasing behavior that gives the disease its common name. Upon the death of an infected individual, the spores are realeased into the environment, ingested by the worm that acts as the intermediate host, and the process comes full circle when young trout feed on these worms.
While all salmonid species can be infected, fisheries dominated by rainbow trout have seen the greatest declines, with especially dramatic cases occuring in the states of Montana and Colorado. It is the youngest fish in a given population that are at the greatest risk, and high rates of infection can lead to the loss of entire
Although it is unlikely that an infected waterway can ever be ridded of the parasite, research has uncovered much about the organism, and there is reason to be optimistic about the future of our fisheries, even those that have been highly impacted.
Restrictions on future hatchery stocking and an increased awareness on the potential role of anglers in transporting the organism — via mud on boots, waders, and other equipment — are helping to limit the spread of the disease to other systems. Water-filtering techniques have been developed that may have value where infection is limited to smaller tributaries that can be treated before entering a system mainstem. Land distrubing activities that generate higher sediment loads — in turn provided more habitat for the host worm — are subject to even greater scrutiny in areas where the disease in know to occur.
Variations in life history and behavior of trout populations, most especially with regard to newly hatch fish, are known to influence rates of infection, and managers are exploring factors such as the timing of hatching, the timing of mirgration out of infected tributaries, and genetic differences between populations that may impart direct or indirect resistence to infection.