What is Whirling Disease?

Whirling disease is a serious health problem for trout and salmon. It is caused by a microscopic parasite called Myxobolus cerebralis. The impacts of the parasite and the resulting disease are variable. However, whirling disease remains a concern for trout in streams and hatcheries of North America. Physical signs of the disease include blackened tail, spinal deformities, and erratic swimming. In severe infections, whirling disease can cause death. Whirling disease is most infective to rainbow trout and cutthroat trout, but can infect all salmonid species. Click here to learn more about the life cycle of the whirling disease parasite.


Tubifex tubifex
Spore TAM Infected fish

What does an infected fish look like?

An infected fish may have no outward signs of infection by the whirling disease parasite, Myxobolus cerebralis. Subclinical carriers are very common. Although these fish appear healthy, they can spread the disease.

When an infected fish develops whirling disease, typical signs include a darkened tail, twisted spine and deformed head (shortened, twisted jaw). Young fish may also swim erratically (whirl). However, other conditions can cause these signs. Whirling disease must be confirmed by microscopic examination or other tests. If you see fish with these signs in an area where whirling disease has not been reported, you should contact your state fisheries agency.


How does whirling disease spread?

Stocking or natural movement of live, infected fish is the primary route by which whirling disease is disseminated. However, there are other ways that the parasite can be spread, including by birds and humans – particularly boaters and anglers.


How do I find out if it's in the waters I fish?

Please look at the contacts page and call the contact for the state you plan to fish in. You can also check the maps page. We don't have maps for every state but we are adding new ones as we get the information.


Is there anything anglers and boaters can do to help prevent further spread?

Anglers, boaters, and others can make a difference in reducing the chances of spreading whirling disease. Distribution of the parasite is expanding rapidly in some areas, so you should assume it is present if you don’t know otherwise. Read below for recommended actions to prevent the spread of whirling disease.

Carefully cleaning and drying all equipment is the best course of action to deal with all aquatic hitchhikers.

Recommended precautions that will help prevent not only the spread of whirling disease, but also other disease-causing organisms and aquatic pests include:

  • Never transport live fish from one water body to another. (This is illegal in many states.)
  • Obtain certified disease-free fish for any private fish stocking projects.
  • Do not use trout, whitefish, or salmon parts as cut bait.
  • Dispose of fish entrails and skeletal parts properly. Never discard fish parts in or near streams or rivers. Because an infected fish may harbor tens of thousands of myxospores, simply disposing of infected fish parts in a clean drainage could provide enough spores to start an infection. Do not discard fish parts in a kitchen disposal. Whirling disease myxospores can survive most wastewater treatment systems. Instead, discard in dry waste that would go to a landfill.
  • Carefully clean all equipment before leaving a site, and allow it to dry. Rinse all mud and debris from equipment and wading gear, and drain water from boats before leaving an infected drainage. This is good practice for preventing transfer of other aquatic hitchhikers as well.

What can state and federal agencies and outfitters do to help?

  • Provide clean water and a hose at boat ramps and popular fishing spots on heavily infected waters for rinsing equipment.
  • Provide some means for brushing boots, for example a simple boot scrubber, near the hose.
  • Post alerts or maps of the known distribution of whirling disease at fishing sites so anglers know if they are either fishing in a heavily infected water or have come from one.
  • Post instructions for preventing the spread of Myxobolus cerebralis and other aquatic nuisance species.
  • Talk to your friends, colleagues, and clients about this issue. Share the information you have learned here.