The Stream Doctor

April 18th, 2005


Q. What is whirling disease and its effects on stream ecology?

A. Whirling disease is an infection of trout and salmon caused by a protozoan parasite, Myxobolus cerebralis. The parasite has a two-host life cycle; tubifex (Tubifex tubifex) worms that live in sediments and ingest the spores released by dying, infected fish when they decay, and the trout themselves. The spores, which can survive for up to 30 years in wet or dry sediments, undergo development in the worm's intestine and multiply rapidly, changing into the form of the disease that infects trout. When released by the worm, the water-borne spores infect susceptible fish by attaching to the fish's body, or they are ingested if trout eat tubifex worms.

Young fish are most at risk when heavily infected with the parasite because the cartilage of young fish is not hardened, allowing the parasite to cause deformities. Infected fish may display a distinctive rapid whirling, thus the name. In instances of high infectivity rates, the disease is usually fatal to young trout. Fish that survive carry the spores throughout their lifetime.

Whirling disease has been identified in all western states except Arizona, and several states in the northeast. It has devastated several popular fisheries, notably the rainbow trout in the Madison River and upper Colorado River drainage. Conversely, California has identified the parasite in many streams; yet, they have not noticed declines in any trout populations. Strangely enough, it appears that rainbow trout are more susceptible than other species. Eradicating this parasite is going to be difficult, if not impossible. Many trout hatcheries are severely infected by the parasite; eradicating them from the rearing ponds and the water supply will costs millions of dollars per hatchery. Keeping them clean will add to these costs. Controlling the disease in nature likely will be even more difficult. The tubifex worm is widespread in nature; the spores spread easily in mud on waders, boats, and other equipment; and little is known of the relationships between the parasite and the various salmonids. These facts all point to a long and difficult research effort to eradicate or control this parasite.

Well, that's what whirling disease is. As far as its impact on the ecology of streams it varies from place to place. Where it severely impacts trout populations, it decreases their numbers and could possibly have a positive impact on their prey populations. I know of no such definitive data on this, but one can postulate a cause-and-effect relationship. Reduction of one population (rainbows), however, could decrease competition with other less susceptible species (browns), allowing them to increase in numbers and make up the impact on the prey. So, we frankly don't know much about impacts on overall ecology of streams, but one can guess at many possibilities. ~ Bert

~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584



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