Whirling Disease Impacts on Trout Populations in North Carolina

Last Updated on August 1, 2015 10:57 am

Following confirmation of whirling disease in rainbow trout from the Watauga River in North Carolina, biologists with the N.C. Wildlife Resources Commission are concerned about potential significant impacts the disease may have on other trout populations, in particular native brook trout populations.

The disease, which is caused by a parasite, affects all species of trout and salmon; however, rainbow and brook trout, two species found in North Carolina waters, appear to be the most susceptible. Brook trout is the only trout species native to North Carolina, and it lives mainly in colder waters, which is also the preferred habitat of the parasite.

“The parasite that causes whirling disease has a highly complex life cycle, requiring two hosts in order to spread,” said Doug Besler, the mountain region fisheries supervisor for the Commission. “One of those hosts is a tubifex worm that thrives in colder water, which unfortunately, is the preferred habitat for brook trout, particularly our southern Appalachian brook trout. If the disease showed up in one of those streams, the impacts could be damaging to those local brook trout populations.”

Despite their concern, biologists acknowledge that the presence of whirling disease doesn’t necessarily equate to a dramatic loss of fish, given that other states where the disease has been present in waters for decades have been able to manage the disease so that impacts on both wild and stocked trout haven’t been nearly as devastating as previously thought.

“In the 1990s, whirling disease was relatively new to many states and there was broad uncertainty about trout population impacts from whirling disease,” Besler said.  “Some western states, such as Montana, had substantial impacts from whirling disease early on, but many of those populations have since rebounded. On the other hand, some eastern states, such as Pennsylvania, do not appear to have experienced broad scale population level impacts from whirling disease.”

“In waters where whirling disease is found, how an outbreak affects trout populations depends on many factors in addition to water temperatures, such as the species of trout present and the quality and quantity of the substrate where the intermediate host resides,” he added.

Biologists, however, aren’t taking a wait-and-see approach to how whirling disease will impact North Carolina trout populations. They are taking every precaution to limit the spread of the disease now.

While the diseased fish came from a trout stream that was not stocked with trout raised at one of three Commission-owned hatcheries, Commission staff have suspended trout stockings until they have tested hatchery fish and determined that they are free of the disease. Commission staff are currently collecting trout from the Watauga River and tributary streams to test for whirling disease and to determine its distribution in the watershed.  The N.C. Department of Agriculture and Consumer Services and N.C. State University are working with the Commission to sample commercial aquaculture operations in the area where the infected trout were found.

The disease is spread mainly by infected fish and fish parts. However, it also can be transmitted by birds as well as by anglers who may transfer the microscopic parasite that causes the disease, Myxobolus cerebralis, on their fishing equipment, boots and boats. Anglers moving infected fish from one water body to another may also transmit the disease.

“Anyone stocking fish in North Carolina is required to have a stocking permit from the Commission, and we encourage anyone considering stocking trout to obtain one,” Besler said. “The primary purpose of that permit system is to allow our biological staff the opportunity to review the stocking application for potential negative impacts to the environment, including the potential to spread invasive organisms. Unauthorized stockings have a much higher potential for serious environmental consequences.”

Besler added that preventing the introduction of unwanted aquatic invasive organisms into North Carolina is the single best approach because control options are often very limited or ineffective once an introduction has occurred.

Younger Fish are More Susceptible

Whirling disease affects trout and salmon by damaging the nerves and cartilage, which may result in abnormal whirling or tail-chasing behavior — hence its name. Other signs of whirling disease are a black tail and deformities to the head or body. These abnormalities in behavior and in the body make the fish more susceptible to predation and make it more difficult for the fish to find food.

A fish’s age can affect the severity of the disease as well. “Biologists have learned that the age of the fish when it is first exposed to the parasite is very important,” Besler said. “Very young fish are highly susceptible with high mortality rates in infected fish; however, older fish are more resistant to the disease.”

There is no known cure for fish infected with the whirling disease parasite. Once it is present in a river system, the parasite is almost impossible to eradicate.

Whirling Disease – A Complex Life Cycle

The parasite that causes whirling disease requires two hosts to reproduce and spread. The first is a small worm, the other a fish. Without these two hosts, the parasite cannot complete its life cycle and will die without multiplying. The worm host of the parasite, called a tubifex worm, is very small, about ½-inch in length and very common in lakes and streams with abundant fine sediment and rich organic material.  It is the only worm that is host to the whirling disease parasite.

While in the worm host, the parasite multiplies, transforming into a spore form that is eventually released into the water, where it floats until it comes in contact with a fish host. The fish host is confined to the salmonid family, which includes trout and salmon species. The spore attaches to the fish’s skin and injects the parasite into the fish’s body, where it travels along the nervous system until it finds the cartilage, which is its food source. Inside the fish, the parasite changes form again. When the fish dies and decomposes, the parasite is released into the environment, and the cycle begins again.

Despite the effects whirling disease has on trout and salmon, the disease does not affect other fishes like bass, pike and catfish, nor does it affect mammals, like dogs and cats. Likewise, the disease does not affect humans, and eating fish infected with whirling disease is not known to cause any harmful effects.

Preventing the Spread of Whirling Disease

The parasite that causes whirling disease is native to Europe and was introduced into North America, likely through frozen fish imported from Europe. It was first discovered in 1956 in Pennsylvania and, since then, it has been reported in more than 20 states and continues to spread.

Along with testing fish at its hatcheries, commercial aquaculture operations, and trout streams, the Commission is asking the public to help prevent the spread of the disease by:

  • Cleaning and drying equipment, clothing and anything else that comes into contact with water;
  • Never moving fish or aquatic life from one body of water to another without first obtaining a permit from the Commission;
  • Disposing of fish parts carefully after cleaning fish by putting fish parts in the garbage, burying them deeply or burning them completely.

Anglers are asked to contact the Commission if they observe deformities, strange swimming behaviors, or other signs of disease in trout.

For a list of frequently asked questions on whirling disease, to learn more about whirling disease and its effects on trout, and to report signs of disease in trout visit the Commission’s dedicated webpage, The page will be updated as test results become available.  The Commission also will post information on its Facebook page, Twitter page and enewsletter, N.C. Wildlife Update.

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