Whirling Disease

2. October 2007 by Ranger.

Be sure to visit the Ranger Cache.

Like many large national parks, Yellowstone National Park has management problems that polarize the public and/or threatens to modify the vegetation or the existence of some species. Among those are the reintroduction of wolves in 1995, a bison herd that is too large which regularly leaves the park boundaries and is infected with brucellosis, the existence of Lake Trout in Yellowstone Lake that are voraciously consuming the native Cutthroat Trout, and Whirling Disease (WD).

Myxobolus cerebralis, even the scientific name for Whirling Disease sounds bad, and it is if you are a trout, a salmon or a fisherman. It was originally misnamed cerebralis (referring to the brain); because it was believed that the parasite infected only the brain of fish. We now know that the parasite feeds on fish cartilage. I noticed a major decline of the available trout in the Yellowstone River about five years ago. Yellowstone River is a world class fly fishing river where sixteen inch and up native cutthroats were common. That is, until M. cerebralis wrecked havoc on the native population. Whirling disease was originally identified in the park within native cutthroat trout near the mouth of Clear Creek on the east side of Lake Yellowstone.

The parasitic organism M. cerebralis is native to the Eurasian continent. The first confirmed outbreak of the disease in the literature was described in Germany in 1903. By 1952, it had spread to France and Denmark, and was later confirmed in South Africa, New Zealand, and in at least 23 states of the United States. The first report of the disease in the U.S. was from Pennsylvania during 1956.

The parasite is a microscopic, primitive animal organism that requires two hosts to survive. Tubifex tubifex, or Sludge Worm, is a worm that exists in the sediment of the benthic zone (lake or river bottoms where the water is generally cooler). Spores of M. cerebralis infect the worm which becomes the first host for Whirling disease. Within the worm’s gut the spores develop and multiply to produce the next spore form known as triactinomyxon. These are then released to float freely in the water until they come in contact with a trout or salmon. Once contact is made, triactinomyxon attaches to the fish’s skin and injects the parasite into the body of the fish. Here it travels along the nervous system until it finds cartilage where it feeds and changes once again back to a myxospore. Once the fish dies and decays, the spores are released and free to begin the life cycle over again. Humans are not susceptible to the disease.

Infected fish with Whirling disease may have a black tail because the parasite places pressure on the nerves that control pigmentation. The attack on the cartilage causes deformities in the skeleton of the fish, which cause the fish to whirl uncontrollably. That reaction prevents the fish from feeding normally and the lack of its ability to escape because of the whirling behavior makes it particularly easy to catch by predators.

Whirling Disease was unknowingly spread by fish hatcheries who were stocking fish throughout the various states without realizing that the parasite existed in their hatchery. It was also spread through the natural flow of water, through the digestive system of birds and other fish feeding on infected fish which then spread the spores through their feces. Other methods of spreading the disease may have happened when the spores were carried on the bottoms of boats, wading boots or the release of fish from one waterway to another by fishermen. The spores of M.cerebralis are somewhat indestructible and can endure either freezing or drying. It is believed they can survive in many streams for up to 30 years

Early studies indicate the success of the parasite is closely associated with cooler water temperatures. The parasite tends to be less successful in warmer waters. Rocky Mountain streams are typically cooler waters during most summers which may have initially allowed the parasite to spread rapidly with a high rate of success. However, much of the West and Yellowstone has experienced warmer summers than usual during the past several years. Many rivers in Yellowstone and Montana were temporarily closed to fishing the past two summers because water temperatures rose to as high as 77 degrees F., which is stressful to the trout population as their optimum water temperature is 68 degrees F. or less. Perhaps that was a blessing to slow down the advancement of Whirling Disease.

There is no practical method to eradicate the parasite from a stream, river or lake. As destructive as this parasite is; it is not likely that it will eliminate all trout from an infected stream or river. We can be sure of this because there are still trout in Eurasia and the parasite has been there for at least 100 years or more. Initial infection definitely reduces the population because it easily infects younger fish. It is possible that some trout could be genetically less susceptible to the disease and that resistance could be passed on to future generations. Let us hope so,

Posted in Wildlife Disease | No Comments »