Putting a New Spin on Trout

Contact: Susan Rivers, MD DNR | srivers@dnr.state.md.us

Photo: Taken from Leetown Science Center Salmonid Whirling Disease Leaflet by Maria E. Markiw

ANNAPOLIS, MD (March 1, 2008) – Myxobolus cerebralis is a microscopic parasite of trout species that causes a condition known as whirling disease. The parasite was introduced to the United States in the 1950’s from Europe. Since that time it has been found in over 23 states. The parasite was first found in a Maryland trout culture facility in 1995. In 2007, the parasite resurfaced and was found in three state trout facilities and three waterways. Here at the beginning of the spring fishing season, the Maryland Invasive Species Council has chosen Myxobolus cerebralis as the March Invader of the Month.

Whirling disease first had significant impacts on native trout populations in the western United States in the 1990’s. Even though the parasite was present before that time, experts believe that stress caused by drought and water quality problems gave the parasite the upper hand over the trout species.

The parasite forms resistant spores in infected trout. As they die and decompose, these spores are released and are ingested by an intermediate host worm, Tubifex tubifex. In the worms they develop to a free-swimming stage called a TAM (triactinomyxon). The TAMs can rupture the worm, swim to a trout and either penetrate the gill or skin and begin a slow journey to the head and spine. There they begin to ingest cartilage as they keep developing. Eventually when the TAMs mature, they form resistant spores that remain in the bone until the fish dies and the cycle begins again. The TAMs and spores are both microscopic, so signs of damage in trout are an indication that testing needs to be done to see if the parasite is present. In the old days, we had to look for the spores by grinding up fish skeletons, but new DNA technology makes detection quicker and easier.

Myxobolus cerebralis attacks cartilage in trout and damages the brain and spinal column of diseased fish. If trout are infected as babies, they have the most cartilage and are more severely impacted. As the cartilage is destroyed, the head and spinal column collapse, as shown in misshapen heads and kinked backbone. If the collapsing skeleton presses down on nerves, the fish may not be able to swim straight and may spin repeatedly, and the tail may turn black. When this occurs with baby or fry trout, they cannot swim to catch food and usually die.

Whirling disease is usually found in waterways that support the intermediate host Tubifex. Streams with a certain type and amount of mud usually support these worms. Pollution can increase the occurrence and success of the parasite and can decrease the resistance of the trout host.

Whirling disease was brought to the United States at a time when little was known about the parasite and fish health concerns were not an issue. Once established, it can wipe out entire generations of trout species. In the western states, cutthroat and rainbow trout have been hard hit. Here on the east coast, a species of concern, the eastern brook trout, could suffer similar impacts. In order to prevent the potential spread of the parasite the Maryland Fisheries Service destroyed 130,000 trout from three trout rearing facilities a nd is monitoring the waters where the parasite has been found in wild fish.

The key to preventing whirling disease is to not move trout species unless you are sure of the health of the fish. If you fish and move from stream to stream, clean your gear well. Think about not using felt bottom waders that are porous and could give spores a place to hide. When in doubt, DON’T MOVE ANYTHING FROM ONE AREA TO ANOTHER. Whirling disease isn’t the only problem you can spread.

For more information about other Invasive Species of Concern, visit www.mdinvasives.org or call the Maryland Department of Agriculture at 410-841-5920.

For more information on the Internet:

Montana Water Center, http://whirlingdisease.montana.edu/

Trout Unlimited, http://www.whirling-disease.orgphotos available electronically on request.