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Whirling Disease and Colorado's Trout
Whirling Disease and Colorado's Trout

​​About Whirling Disease 

Brown Trout with Whirling DiseaseMyxobolus cerebralis, a metazoan parasite, can cause a serious affliction in some species of trout and salmon known as whirling disease. The water-borne parasite may not directly kill trout, but severely infected young trout often develop debilitating deformities of the skull and spinal column or display the erratic tail-chasing behavior from which the disease gets its name. Eventually, heavily infected young fish may die.

The parasitic disease probably originated in Europe, where native brown trout have developed a natural resistance to the parasite. However, these fish can still carry and transmit the spore. Rainbow trout are most susceptible to infection and have experienced population level effects due to exposure to the parasite. In addition, all of Colorado's native cutthroat trout subspecies are susceptible to infection, as are brook trout.

Whirling disease does not infect humans. People cannot contract the disease from eating or handling infected fish. 

How Whirling Disease Affects Fish

Whirling disease

Whirling disease has a two-host (fish and worm) life cycle. The triactinomyxon (lower right) is the soft waterborne spore that infects young fish. Hard myxospores are formed in the cartilage of the fish, causing them to develop deformities such as cranial, spinal, and opercular deformities, and blacktail (lower left), as well as causing the whirling motion from which the disease gets its name. When the fish dies, these myxospores (upper left) are released into the water and ingested by the Tubifex worm (upper right). In the worm, myxospores replicate and transform back into the triactinomyxon, which are released by the worm to restart the life cycle.

The whirling disease parasite has a two-host life cycle that involves trout and an alternate host, a common bottom-dwelling tubifex worm. When an infected trout dies, large numbers of hard spores are released. These hard spores are hardy, resist freezing and drought, and can remain viable for months. After release from the host fish, the tubifex worm can ingest the spores. When released from the worm, these water-borne spores, known as triactinomyxons, can infect susceptible fish by attaching to their bodies. The hard spores are then formed in the cartilage of the infected fish, completing the life cycle. 

Most salmonid species native to North America have little or no natural resistance, having only recently been exposed to the parasite. Young fish are at greatest risk because the parasite attacks their soft cartilage, causing nerve damage, skeletal deformities and, in some cases, death. Once a fish reaches three to four inches in length, cartilage forms into bone and the fish is much less susceptible to effects from whirling disease. However, they remain carriers of the parasite.

Whirling Disease in Colorado

Whirling disease was first observed in the United States around 1958. The parasite was accidentally introduced in Colorado in the 1980s through imported trout from a private hatchery. It's now found in at least 20 states, including West Virginia, Pennsylvania, New York, Ohio, Michigan and most western states.

Whirling disease is thought to be a major factor in the declines of wild rainbow trout populations in many  Colorado waters. It's suspected that the outbreak of the disease may be linked to other environmental factors that aren't yet apparent. Affected drainages in Colorado, many of which experienced severe declines in the young-of-year portion of the rainbow trout population following introduction, include the North Platte, South Platte, Upper Arkansas, Rio Grande Headwaters, San Juan, Upper Colorado-Dolores, Gunnison, Colorado Headwaters, and White-Yampa drainages.   

Prevention and Control of Whirling Disease

As of yet, there is no practical cure to treat wild trout infected with the disease. The Colorado Parks and Wildlife has developed strict policies and regulations to help control and prevent the spread of the disease in Colorado.


Eleven  of Colorado's sixteen state hatcheries once tested positive for whirling disease. Capital investments were made to protect the state's hatchery system,  and routine fish health sampling indicates diminishing infections at some sites; the number of hatcheries still considered positive for the parasite has been reduced to six.  Many trout from positive hatcheries will carry few, if any, spores. But as a precaution, Colorado Parks and Wildlife considers these trout "positive" until repeated hatchery tests find no spores.  


A policy implemented in spring 1995 prevents the stocking of trout from hatcheries testing positive into waters where whirling disease has not been found. This includes wilderness areas and streams where native trout may be restored. Only trout from negative testing hatcheries can be stocked into waters where the parasite has not been found. 

Current evidence suggests that stocking of hatchery trout exposed but not necessarily infected with the parasite into waters where whirling disease is known to exist does not increase the level of infectivity. Trout from positive hatcheries will be stocked into waters where the parasite has been found in order to minimize the risk of contaminating other watersheds.

How You Can Help

You can help prevent the spread of whirling disease by taking the following precautions:

  • Thoroughly wash off any mud from vehicles, boats, trailers, anchors, axles, waders, boots, fishing equipment and anything that can hold the spores or mud-dwelling tubifex worms.

  • Drain boats, equipment, coolers, live bait wells and any water holders.

  • Don't transport any fish from one body of water to another, which can help spread whirling disease. It is unlawful in Colorado to move and stock live fish without a special license.

  • Don't dispose of fish entrails or other by-products into any body of water.

  • Never transport aquatic plants. Make sure boats, engine props, anchors, trailers and wheels are cleared of weeds after every use.


As a member of the Colorado Fish Health Board, Colorado Parks and Wildlife is taking the lead on developing ways to insure that the state's aquatic habitat remains healthy. Through public awareness, research and continued fish health programs, the impact of the whirling disease parasite and other pathogens, can be minimized or eliminated in many of the state's waters. Learn more about CPW's whirling disease research studies.

Whirling Disease Resistant Rainbow Trout

Fishery managers may be successful in reestablishing wild rainbow trout by stocking strains of rainbow trout found to be resistant to the whirling disease parasite.

The Hofer rainbow trout strain has demonstrated strong resistance to the parasite. These fish originated from the Kamloops rainbow trout in the Columbia River system in North America. In the late 1800s, these fish were transported to Germany to be grown as food fish in local hatcheries. Because whirling disease originates in Europe, the fish were reared in whirling disease-positive waters. Over time, this rainbow trout strain developed a resistance to the parasite. This resistance has been confirmed in laboratory studies. The Hofer rainbow trout have typical behaviors associated with domestic fish, which include reduced fright response and aggressive feeding. These characteristics of domestication, while beneficial in a hatchery setting, are not advantageous for wild fish. 

To improve wild behavior, researchers bred the resistance of the Hofer rainbow trout into the Colorado River rainbow trout strain. Rainbow trout are not native to Colorado, and the Colorado River rainbow trout is a wild strain that is a result of federal, state and private stocking in the early 1900s in Colorado. This strain did very well in rivers in Colorado until the spread of whirling disease. The Colorado River rainbow trout strain is highly susceptible to the parasite. 

CPW's hatchery program strives to retain the maximum number of wild genes possible in the new broodstock while conferring resistance to whirling disease to this strain. This will help maintain wild behavior in the fish and result in more successful natural spawning and survival. Crosses between the Hofer and Colorado River Rainbow trout strains are now reared by several state hatcheries and are stocked into rivers and streams where rainbow trout populations were severely impacted by whirling disease. 

The Hofer rainbow trout has also been bred with the Harrison Lake strain of rainbow trout in an effort to retain resistance in a wild, lake-dwelling rainbow trout strain. The Harrison Lake rainbow trout strain originates from Harrison Lake, Montana. Although marginally resistant itself, resistance to M. cerebralis was increased significantly when Harrison Lake strain fish were crossed with Hofer strain fish. Crosses between the Hofer and Harrison Lake rainbow trout strains are also now reared by several state hatcheries and stocked into lakes and reservoirs in which the whirling disease parasite is still present.

Current research projects are focused on management of these whirling disease resistant rainbow trout strains, including stocking strategies to increase survival, monitoring long-term survival, natural reproduction, and recruitment, optimizing production of these strains in state hatcheries, and developing and managing wild brood stock locations.

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