Whirling disease agent found in northeast Washington trout

Federal and state fish scientists have recently discovered the pathogen that causes whirling disease in trout in northeast Washington.

Although harmless to humans and other wildlife, whirling disease has been blamed for significant declines in trout populations in Montana and Colorado.

Fish pathologists from the U.S. Fish and Wildlife Service (USFWS) and the Washington Department of Fish and Wildlife (WDFW) made the discovery during an ongoing wild fish health survey in Pend Oreille, Stevens and Ferry county streams. Although it's not the first time the whirling disease agent has been found in Washington, it's the furthest upstream occurrence in the Columbia River drainage.

The pathologists stressed that they had discovered the presence of the whirling disease pathogen, not the presence of the disease itself. They also pointed out that although they sampled over 60 sites, the presence of the pathogen was confirmed at only five locations. All of the fish in which the pathogen were confirmed were eastern brook trout.

Whirling disease is a parasitic infection of salmon and trout by the microscopic protozoan, Myxobolus cerebralis. The disease causes deformity and death in some species of trout while other related species, notably salmon and char, seem unaffected. Some of the deformities in affected fish cause them to "whirl" in a tail-chasing pattern. The disease is not passed directly from fish to fish but through an intermediate aquatic host, tubifex worms. The pathogen passes through the worm, changing into an infectious form, before being released back into the water to infect fish and cause the disease. If a fish eats the host worm, spores of the pathogen infect the fish and cause the disease. When the fish dies, spores of the pathogen are released, find the intermediate host, and the cycle begins again. Tubifex worms are found throughout North America including the state of Washington.

"This discovery significantly expands the known geographic range of the pathogen," said WDFW fish health program manager John Kerwin. "It will definitely affect how we handle fish from those areas."

Kerwin said fisheries managers from several agencies and organizations will work cooperatively to develop a plan to prevent the spread of the pathogen and further investigate its presence during the next field season. Field sampling in the area occurs in late summer when the sites are accessible. The samples were sent to the U.S. Fish and Wildlife Service's Idaho Fish Health Center for analysis during the winter.