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Whirling Disease

Barry Nehring, Aquatic Wildlife Researcher (retired), Colorado Division of Wildlife
Barry Nehring, Aquatic Wildlife Researcher (retired), Colorado Division of Wildlife
Dr. Ash Bullard, Associate Professor, Auburn University

What is Whirling Disease?

Whirling disease is a condition caused by a microscopic parasite, Myxobolus cerabralis, that affects trout and salmon species.  Native to Europe, whirling disease was first discovered in the United States in Pennsylvania in the 1950’s.  The disease has since continued to spread and can now found in numerous states including Tennessee, Virginia, North Carolina, West Virginia and most northeastern and western states.

Impacted Species

Once introduced into the water, the parasite infects a common mud-dwelling worm found in lakes and streams.  The parasite multiplies within the worm and is eventually released into the water where it drifts until it comes into contact with a trout.  It then infects the trout’s nervous system, consuming the fish’s cartilage and skeletal tissue.  Trout with whirling disease may develop a black tail, deformities in the head and spine, or display “whirling” or erratic tail-chasing behavior.  Although an infected trout may not always die directly from the disease, it can affect the ability for them swim, eat, and escape predators. Once the fish dies, the parasite is then released back into the environment, and the cycle starts over.

Whirling disease only affects fish in the trout and salmon family.  Fish species other than trout and salmon such as Largemouth and Smallmouth bass, Bluegill, or Walleye cannot get whirling disease.  Other organisms such as humans, mammals, and reptiles cannot be infected.  Eating an infected fish is not known to cause any harmful effects to humans or pets.

Fish in Tennessee that are susceptible to this disease include:  Brook Trout, Rainbow Trout and Brown Trout. Lake Trout have only become infected under laboratory conditions.  Brown Trout (native to Europe, where the disease originated) can become infected and cary the disease but are less susceptible to the clinical symptons (whirling behavior, black tail, etc.) of whirling disease unless heavily infested.  Rainbow and Brook trout are more susceptible to the disease.

Research has found that the age of the fish at first exposure to the parasite is important to survival of the fish.  Adult fish can contract the disease, however newly hatched fish are highly susceptible to the symptoms of whirling disease.

Current Status in Tennessee

Whirling disease has been confirmed in Rainbow and Brown trout on the South Holston and Watauga tailwaters.  Annual routine testing currently indicate that all TWRA hatchery facilities are disease free! 

Tennessee Wildlife Resources Agency biologists will begin collecting trout in tailwaters across the state and within the South Holston and Watauga watersheds to determine the disease’s current distribution.    

Threats to Tennessee

No negative impacts have been observed to date on the South Holston and Watauga tailwaters.  These tailwaters do have natural reproducing populations of Brown Trout and all Rainbow Trout are currently stocked from state and federal hatcheries with adult and juvenile fish.  Even though Brown Trout become infected and can cary the disease, they are less susceptible to the clinical symptons of whirling disease unless heavily infested.

However, there is concern for potential impacts in more vulnerable areas such as wild trout populations where natural reproduction of Brook and Rainbow trout does occur.  Those impacts would depend on a variety of factors such as water temperature, the amount of sediment available for the worm host, and the age and species of trout present. 


There is no known cure to rid whirling disease now that it has been established, so the best way to protect Tennessee’s many trout fisheries is to prevent it from spreading.

What can you do to prevent the spread?

·         CLEAN. DRAIN. DRY.  Clean all equipment, such as waders, fishing gear, boats, trailers, and float tubes of mud before leaving an area when fishing.  Careful cleaning using disenfectants such as bleach (soak in mild bleach solution for 10 minutes then rinse) will kill the parasite and many other aquatic nuisance species.  Drain water from all motor units, live wells and buckets. Allow all equipment to dry for a minimum of 48 hours.

·         DO NOT transport live or dead trout from one water body to another.

·         DO dispose of fish parts carefully when cleaning trout (dry disposal in garbage is best).

·         CONTACT US if you observe signs of whirling disease in fish. – Please provide a location and picture of the fish if possible.