Whirling disease finding highlights importance of fish transport rules

OLYMPIA -The recent identification of the pathogen that causes whirling disease in a rainbow trout brought into Southwest Washington by a private landowner in Battleground (Clark County) highlights the importance of rules aimed at keeping diseased fish from being brought into the state, say Washington Department of Fish and Wildlife (WDFW) officials.

The presence of Myxobolus cerebralis, the causitive agent for whirling disease, was confirmed last month by the U.S. Fish and Wildlife Service's Lower Columbia Fish Health Center lab, in Underwood, Wash. The lab tested the fish at the request of WDFW, which collected it in a broader sampling effort last summer.

"The confirmation of the pathogen in this fish underscores the importance of following state permitting requirements," said John Kerwin, WDFW fish health manager. "These rules are intended to keep fish diseases from being spread here and to insure the safety of our native fish populations."

Under state law, persons purchasing and releasing fish are required to first obtain a stocking permit and a transport permit from WDFW. The department maintains a list of certified fish growers that meet WDFW's fish health requirements, including regular testing to ensure stock is free of specific diseases. The purchaser of the fish that tested positive for M. cerebralis had not obtained the required state permits before releasing the trout into a contained pond on his property.

"We will work with the property owner to ensure that any fish which might remain in his pond do not pose a risk to native fish," said Kerwin.

The department also plans to collect additional samples of trout in the area this year, he added.

State biologists collected trout samples last year in southwest Washington after learning that a number of individuals in the area had purchased fish from a private fish farm near Portland, Ore., a portion of which was later closed by the Oregon Department of Fish and Wildlife after M. cerebralis was found in the facility's fish.

Whirling disease is a parasitic infection which leads to deformities that cause fish to "whirl" in the water in a tail-chasing pattern. It has been blamed for significant declines in trout native populations in Montana and Colorado.

Although it is fatal to trout and can affect some other species of fish including salmon, the pathogen or parasite is harmless to humans and other wildlife. The pathogen or parasite is not passed directly from fish to fish, but instead through an intermediate aquatic host, the tubifex worm. Birds or mammals ingest affected fish and the infectious agent is passed through the bird feces back to the tubifex worm. The tubifex worm releases the infectious agent that infects the fish. Tubifex worms are found throughout North America, including Washington.

The recent finding represents the first time M. cerebralis has been detected in the lower Columbia River basin in western Washington. The parasite has been found previously in western Oregon's Willamette River basin, and was found over a decade ago in southeast Washington. In 2001 and 2002 it also was detected in a number of streams in the upper Columbia River watershed of northeast Washington.