Following are some of the most frequently asked questions about whirling disease in Montana. If you have a question that is not answered here, you can e-mail us and we will try to provide an answer … and maybe we’ll add your question to this FAQ. [Last updated October 25, 2000]

1. What is whirling disease?

“Whirling disease” is a disease of Salmonid fish (trout, salmon, whitefish) caused by a microscopic parasite known as Myxobolus cerebralis. This tiny parasite has a fairly complicated life cycle which involves two hosts; a small worm that lives on the bottom of a stream or other body of water, and a fish. The fish becomes infected after a form of the parasite (the TAM stage) emerges from the worm and enters the water column. The parasite finds a fish, attaches to the fish and penetrates the skin. The parasite eventually finds its way to the cartilage of the fish where it matures into the mature whirling disease spore. It stays there until the fish dies, releasing spores into the water, which ultimately are ingested by the worms and the life cycle starts all over.

In the fish the parasite can affect nerves and cause cartilage damage which results in the symptoms of whirling disease. Whirling disease gets its name from the abnormal whirling or tail-chasing behavior exhibited by some infected fish. This is caused by damage or pressure to nerves caused by the whirling disease parasite. Other symptoms may include a black tail in younger fish. In older fish symptoms sometime include deformities to the head or body.

2. Where is it found?

The parasite that causes whirling disease (Myxobolus cerebralis) is not native to the U.S. but is native to Europe. It has also been reported in New Zealand, South Africa and the former USSR. It was first discovered in the United States in Pennsylvania in 1956. Since then it has been reported in 22 states, including: Alabama, California, Colorado, Connecticut, Idaho, Maryland, Massachusetts, Michigan, Montana, Nevada, New Hampshire, New Jersey, New Mexico, New York, Ohio, Oregon, Pennsylvania, Utah, Virginia, Washington, West Virginia and Wyoming. In a number of these states, however, the parasite has been observed only in isolated cases and has had very little to no noticeable impact. In Montana and Colorado, however, severe impacts on wild trout populations have been documented.

In Montana this parasite has been detected in more than 85 individual waters, mostly in the trout waters of western Montana. It is found on both sides of the Continental Divide, in the headwaters of the Missouri and Clark Fork of the Columbia drainages. The impact of whirling disease has been different on different waters, with little or no impact observed on some waters and moderate to severe impacts measured only on a few waters.

3. Is whirling disease dangerous to humans?

No. The parasite that causes whirling disease has only been found to infect fish and the tubifex worm which serves as the intermediate host of the parasite. It does not infect humans.

4. What species of trout are susceptible to whirling disease?

All species of trout and salmon can be infected with the parasite but may not develop whirling disease. Scientists have found tht the age of the fish when first exposed to the parasite is very important. Very young fish are highly susceptible but after a fish reaches four months old it is fairly resistant to whirling disease. Other members of the trout and salmon family, such as mountain whitefish may be at risk. Rainbow trout appear to be the most susceptible than other trout species. Brown trout become infected with the parasite, but they appear to have an immunity to the parasite and have not been as greatly impacted as rainbow trout.

In Montana, the following species have been found infected with the whirling disease parasite:

Rainbow trout – all strains of rainbow trout tested to date are susceptible

Yellowstone and Westslope cutthroat – susceptible

Mountain whitefish and brook trout – susceptible

Bull trout and brown trout – partial resistance

Chinook salmon – very susceptible
Coho salmon – quite resistant

Grayling – very resistant

5. What is the history of whirling disease in Montana?

The parasite that causes whirling disease was first discovered in Montana in rainbow trout in the Madison River in December 1994. It was discovered in the Madison River during an investigation into the cause of a major decline in the rainbow trout population in the upper Madison River. Prior to December of 1994 this parasite had never been detected in any fish in Montana. Monitoring of hatchery stocks and limited inspections of wild populations to test for this parasite had taken place for many years prior to its discovery in the Madison River. These monitoring efforts increased dramatically after the 1994 discovery of the parasite and a state-wide survey was initiated. As of early 1998 nearly 300 individual waters have been tested and the parasite has been detected in more than 85 individual sites in Montana. Quite likely many of these waters have been infected for several years, but went undetected until the state-wide survey was initiated.

Biologists believe the initial infection of the Madison River most likely occurred in the mid-1980’s. This is based on the population trends through the 1980’s and 1990’s which revealed a rainbow trout decline that started in the early 1990’s.

6. How is it spread?

Because the parasite that causes whirling disease has two distinct forms including a free-floating stage in the water, and involves two hosts, it can potentially be spread in several ways. However, the most likely way the parasite can be spread is through movement of live fish or parts of fish. A single fish can be infected with many thousands of spores (up to a million or more)! This makes infected fish the most dangerous source of infection. In other states movements of live infected fish have been documented as the source of introduction of whirling disease. Because most Montana trout streams, such as the Madison River, rely on natural reproduction and fish are not stocked into these waters, it is difficult to determine the source of infection in most cases.

Birds are suspected as a potential vector of whirling disease. University studies have shown that viable parasites can pass through the digestive tract of birds and mammals and still be infective to fish. Other potential sources are water or mud on boats, trailers, fishing equipment, etc. The mature whirling disease spore, once released from the fish, is very hardy. It can remain live in mud for many years and is a potential source of whirling disease if moved from one body of water to another. Although vectors such as birds, mud and water are potential sources of whirling disease, live infected fish and/or heads or other infected parts of infected fish remain the most dangerous and most likely source of whirling disease infection.

7. Does that mean I should disinfect my boots and waders after I fish?

The known distribution of the whirling disease parasite in Montana is expanding all the time. Since you may not know if the parasite is present in the water you are fishing in, you should assume it is present and be careful. And remember, whirling disease is not the only threat out there. There are plenty of other disease-causing organisms and aquatic pests that are just waiting to be carried to a new environment.

You should clean and dry your boots, waters and other fishing equipment before going from one body of water to another. If you get the mud off and if your equipment is clean and dry you will greatly reduce any threat of spreading whirling disease and other parasites, disease-causing organisms or other unwanted aquatic pests.

Common sense and routine cleaning is probably sufficient to eliminate the threat of spreading disease with your equipment. It may not be necessary to disinfect your boots and equipment with chemicals. However, if you do wish to disinfect your boots and waders, chlorine is probably the best chemical to use. Very light doses of chlorine will kill the waterborne stage of the whirling disease parasite and most other disease-causing organisms. But it takes a fairly strong concentration of chlorine to kill the mature whirling disease spore that may be found in the mud from an infected stream. A 50:50 solution of household bleach (one part water to one part chlorine bleach) will kill whirling disease on contact (you can dip waders into a solution of the bleach or wipe or spray it on). Or, you can use a weaker 10% solution (1 part chlorine to 9 parts water) and soak your equipment for 10 minutes. Either way, make sure you rinse the chlorine off your waders and other equipment after you disinfect it, and dry in the shade. Chlorine is a very strong chemical and can harm your equipment with prolonged exposure. Equally effective is almost boiling water (200�F) poured over your gear and allowed to cool.

8. Well then, as a fisherman, is there anything I can do to help?

Yes. You can help by becoming informed. The more you know about whirling disease, the more you can help spread the word that this disease is present in Montana and that it can have very serious impacts on Montana’s trout wherever it spreads to.

Some specific things you can do to help prevent the spread of whirling disease include:

Never move live fish from one body of water to another.

Don’t dispose of entrails or heads into any stream or other body of water.

Don’t discard entrails or heads of fish down a garbage disposal. Place them in the garbage for disposal into a land fill.

Try to get the mud off your boat, trailer and other equipment before leaving the water where you have been fishing.

9. Is there a cure?

No. Once the parasite is in a stream, there is no way to remove it. However, the impact of whirling disease has been different on different waters, with little or no impact observed on some waters and moderate to severe impacts measured only on a few waters. And there are things that can be done to reduce the impact of the disease. The more that is learned about whirling disease, the better scientists and fisheries managers will be able to deal with it. Whirling disease is here to stay. Now we have to find a way to maintain wild trout fisheries and live with whirling disease. Scientists have already learned much about this parasite. Through this knowledge and future research, we will find the answers and discover ways to help the trout survive whirling disease and maintain the wild fish and fishing opportunities that Montana is famous for.

10. Is there hope?

Absolutely! Montana’s fisheries managers and fisheries scientists are committed to maintaining Montana’s wild trout fisheries. Not all the streams in Montana with whirling disease are (negatively) impacted. The hope lies in the research and our knowledge of this parasite. Field research field studies and scientific research conducted in the laboratory will provide the answers we need to understand and live with whirling disease. Studies are being conducted into all aspects of whirling disease to understand why some fish survive infections better than others, what species or strain of fish has a better chance of surviving in specific situations, what can be done to control the worm host, and the list goes on. Private, state, federal and university researchers are hard at it working to answer these important questions.

11. Will whirling disease eventually wipe out all of the trout in Montana?

No. Whirling disease has had severe impacts on trout populations in some waters. But even in the upper Madison River, after initial declines, the rainbow trout population is now around 1000 fish 10″ or longer per mile. There are still plenty of brown trout to catch in the Madison River and overall the upper Madison River still has as many catchable trout per mile as many of Montana’s other Blue Ribbon trout streams.

The impact of whirling disease has been different on different waters, with little or no impact observed on some waters and moderate to severe impacts measured only on a few waters. More time and research is needed to evaluate the overall impact we can expect from this disease, but it is obvious that some fish in some waters will survive better than others. Through research and possible use of trout with different life histories, we have hope that whirling disease will not eliminate Montana’s trout. Especially hopeful are the results of recent research that indicates that some of Montana’s most prized native fish, the grayling and bull trout, may have some resistance to the disease.

There is still excellent fishing in Montana! Click here for links to some local fishing reports.

12. Is there a chance that the rivers will just naturally rebound in a few years due to natural selection?

Recovery of a trout population that has been impacted by whirling disease will likely not occur in a short period of time. Impact of the disease on fish will likely vary from year to year depending on a variety of environmental factors, but once the parasite is present in a body of water it will always be a threat to the trout in that water. Over time some fish may adapt through survival of certain elements of the population that are better able to cope with the parasite, such as fish that spawn earlier. Here again, research will be the key to recovery of affected fish populations.

13. Can’t you just stock more fish?

Montana is famous for wild trout. Montana’s wild trout are not stocked; these trout fisheries are maintained by self sustaining populations which spawn naturally in our streams. In order to maintain the wild trout fishery we have to understand how and why this parasite affects trout in the wild. Reintroduction of wild trout with a life history better suited to survival in face of whirling disease may prove helpful. Researchers are now evaluating several species and strains of trout that may have a better chance to survive whirling disease. Fish that spawn at a different time or a different area of a stream or in uninfected tributaries could produce fish that can better live with whirling disease. However, stocking fish to maintain the fishery is not a desirable option. Studies in Montana have shown that stocking hatchery fish on top of wild trout populations does not produce better fisheries. Cost is another consideration. It would cost millions of dollars annually to stock fish into Montana’s rivers to maintain trout fisheries. And where would we raise these fish? Montana does not have the hatchery capacity to raise the fish necessary to maintain these fisheries.

Hatchery fish are an important part of Montana’s fisheries management program. They are used in lakes and reservoirs where natural reproduction is limited or does not occur, and hatchery fish can be used to establish fisheries when special or unique fish are required. But Montana Fish, Wildlife and Parks remains committed to the wild trout philosophy and feels it is important to do everything possible to maintain the wild trout resource.

14. How has whirling disease affected the fishing in Montana? Are there still fish in the Madison? How is MY stream impacted?

The impact of whirling disease has been different on different waters, with little or no impact observed on some waters and moderate to severe impacts measured only on a few waters. Whirling disease is responsible for a significant decline in rainbow trout in the upper Madison River. After initial declines, the Madison River rainbow trout population is now around 1000 fish 10″ or longer per mile. And there are still plenty of brown trout to catch in the Madison River and overall the upper Madison River still has as many catchable trout per mile as many of Montana’s other Blue Ribbon trout streams.

Since whirling disease was first detected in the Madison River it has been detected in more than 85 other waters in Montana. The impact on many of these other waters is still not known. It may be too early to tell what the impact will be in waters, such as the Missouri River, where trout numbers are near record highs. Studies of infection rates will help researchers predict impact on specific waters. It is clear that the level of infection varies from river to river. Waters which harbor low level infections, for whatever reason, may not see a decline in fish numbers.

15. Why are sculpins banned as bait in Montana?

The Montana Fish, Wildlife and Parks Commission banned the use of sculpins as bait after it was discovered that sculpins in some waters were infected with a parasite similar to the whirling disease parasite. This parasite did not look exactly like the whirling disease organism, but was close enough to cause concern, especially since people were collecting these sculpins from waters where the whirling disease parasite was known to infect the trout. Recent work indicates that the parasite in the sculpins is not the whirling disease parasite, but since sculpins in infected waters may ingest whirling disease spores and quite often collected sculpins were transported live in potentially infected water, the ban remains a good idea.

16. What is the impact of land management activities such as mining, grazing and timber harvest on whirling disease’s impact and spread?

Some have suggested the hypothesis that grazing and mining may degrade streams, increase sedimentation and thus increase tubifex worm population. This could then increase the level of whirling disease infection. This possibility is the basis for some research proposals that will relate whirling disease infections to environmental quality and impact of land management activities. However, there is not presently sufficient evidence to say for sure what impact these activities are having on fish populations infected with the whirling disease parasite.

17. Are there any regulation changes that go into effect when a stream is found to have whirling disease?

At present there are no blanket regulations that go into effect when a stream is diagnosed as having whirling disease. The need for special regulations that may be necessary to protect the fish and help them survive better is considered on a case-by-case basis.

18. Why not just go to catch-and-release regs any time a stream is found to have whirling disease?

Catch and release regulations may leave a few more fish in the streams than a harvest fishery in some cases. But this is not the answer in most waters. There is no evidence that catch and release regulations can help maintain fish numbers in the face of whirling disease. Here again, each infected stream has different contributing factors and must be evaluated on a case-by-case basis. One of the reasons fish die from whirling disease is that the parasite leaves the fish more vulnerable to other factors, such as stress. Catch and release fishing can be a very stressful situation for fish, especially if the fish are not carefully handled. This stress, or any handling or other stress, can have a negative impact on infected fish.

19. Where can I get more information?

Check out the “clearinghouse” and “links to other sites” at this web site. The clearinghouse offers information on some available references and where they can be obtained. The “links” information will provide you with information on specific sites to obtain whirling disease information. The Whirling Disease Foundation and Montana Fish, Wildlife and Parks also have specific information on whirling disease.

20. What else?

You tell us – are there any other questions we should add to this FAQ? If so, please e-mail us with your suggestions!