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Whirling Disease

Myzobolus Cerebralis
Myzobolus Cerebralis

Whirling disease was first found in the Middle Fork of the Snake River and in 1990 it showed up in Yellowstone National Park following the fires of 1988, where it may have entered on fire equipment water tanks. In Montana, whirling disease was first identified in 1994, when it was responsible for a drastic decline in fish in the Madison River. It has since spread down the Missouri River and was identified in Yellowstone Lake and on the west side of the Continental Divide in the Columbia River Basin.

Whirling disease is caused by a microscopic parasite, Myxobolus cerabralis, the spore form of a protozoan. It is eaten by a small inch-long tubifex worm and in its gut the spore opens, releasing a second form of whirling disease parasite which in four months transforms into a TAM (Triactinomyxon gyrosalmo). The TAM has three grappling hook arms and survives in the water for 96 hours. When the vulnerable infected fish die or are eaten, spores produced by the TAMs are introduced into the water, where they can lie dormant for up to 30 years before starting the cycle again. Dual hosts -- the tubifex worm and the fish - serve to make the unusual life cycle more complicated. Once in the wild trout system, it is not easily eradicated. The tubifex worm favors constant temperatures and constant flow, thermal relief and soft muddy stream bottoms. It does not do well in fast flowing, free-stone streams.

Myxobolus cerebralis causes whirling disease only in a certain stage of its life cycle when it's known as a TAM. TAMs are fatal only to newly hatched fish and are present in the rivers from May 15 to July 15. Rainbow trout, spawning in spring, are particularly vulnerable to the disease. Research suggests that increasing water flows to dilute the TAMs and minimize exposure of the young trout at a critical stage would be beneficial. The TAMs attack young trout only during the first 3 months of its life before cartilage hardens to bone. Rainbows and native cutthroats seem to be the most susceptible to the disease. Brown trout are resistant because they co-evolved with the parasite in northern Europe. Flow levels on the Madison River below Hebgen Dam may be modified in order to mitigate the impact of whirling disease. Other solutions include introducing whirling disease resistant strains of rainbow trout and prompting fish to spawn earlier so the fry matures before TAM levels become lethal.

To date there is no cure against whirling disease and the disease has spread into nearly every river drainage in Montana. During drought years when water levels are low, the parasites are more concentrated. Also, the parasite is more active in water temperatures between 48 and 60 degrees Fahrenheit. Twenty three states have whirling disease which came 50 years ago in a shipment of processed fish from Denmark. The research is expensive: the federal government gave $5 million, private, corporate donors contributed $2 million and the State of Montana gave $1 million for research. ~

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