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Whirling Disease and Oregon's Trout and Salmon

Findings

Ongoing ODFW fish health surveys detected the parasite Myxobolus cerebralis, causative agent of whirling disease, for the first time in a hatchery in western Oregon. In fall 2001, during routine sampling of rainbow trout from a private aquaculture site on Clear Creek, a lower Clackamas River tributary, fish at the facility were found to have a light number of spores of the parasite in their cartilage tissue. These fish appeared healthy and were not dying from disease or displaying any whirling disease symptoms. Although not yet confirmed, the parasite may have entered the facility from infected fish in Clear Creek that supplies water for the rearing ponds. Original detection of the parasite in the state occurred in northeast Oregon in 1986 (see History in Oregon below).

Population declines attributed to whirling disease have never been detected in any wild salmon, steelhead or trout in Oregon. The parasite has not been detected in naturally reared or hatchery fish in the Clackamas basin.  Sampling of fish at the Department’s Clackamas Hatchery and at US Fish and Wildlife Service’s Eagle Creek National Fish Hatchery, both located in the Clackamas River watershed, have never detected the presence of this parasite.

What is whirling disease?

Whirling disease (WD) affects trout and salmon and is caused by the microscopic parasite, Myxobolus cerebralis. The parasite attacks the cartilage of the head and spine. Fish with low numbers of spores may not show any visible signs of the disease. If sufficiently affected by the parasite, young fish may become more susceptible to predation and less able to feed and survive disturbances in the environment. When infection is severe in young fish, they may "whirl" when startled due to pressure on the nervous system from inflammation around damaged cartilage. Fish may also develop a "black tail." Biologists have not observed these symptoms in fish in Oregon streams. The long-term effects in naturally reared fish populations continue to be examined.

What is the parasite's life cycle?

Myxobolus cerebralis has a complex life cycle involving two hosts: fish (salmonid species) and tiny bottom-dwelling aquatic worms called tubifex. Worms that ingest the Myxobolus spores and become infected, release a fragile water-born spore stage of the parasite (triactinomyxon stage) that must infect a fish within a few days or perish. Infected fish develop Myxobolus spores that are very persistent and can survive in moist environments for many years. When an infected fish dies and decomposes, spores are released into the water, beginning the cycle again. Spores can also survive passing through the digestive tract of predators and can be transferred from place to place on muddy boots or other equipment.

What fish are susceptible to the parasite?

The most susceptible fish found in Oregon are rainbow trout, steelhead trout, cutthroat trout, chinook salmon and kokanee salmon. Those considered less susceptible include brook trout, mountain whitefish, bull trout, brown trout and coho salmon. The parasite does not infect lake trout, warmwater game fish, sturgeon or nongame fish.

Are humans affected?

No. The parasite does not infect humans or other warm-blooded animals.

What is the geographic range of the parasite?

Myxobolus cerebralis has been in the United States since the early 1950's and is now found in 23 states. In the western U.S., it was found in California (1965), Nevada (1966), Oregon (1986), Idaho (1987), Colorado, (1987), Wyoming (1988), Utah (1991), Montana (1994), Washington (1996) and New Mexico (2000).

What is the history of the parasite in Oregon?

Oregon has not experienced the dramatic fish population declines attributed to the parasite like those reported in Montana and Colorado. This parasite was first found in the Snake River basin in the mid 1960’s in fish that had been planted in the upper Owyhee River and other tributaries of the Snake River in Nevada.

The presence of Myxobolus cerebralis was just confirmed in northeast Oregon in 1986 at a private trout hatchery in the Grande Ronde watershed. Subsequent investigation found infected anadromous and resident fish in the hatchery's water supply. Statewide surveys of wild and hatchery fish from 1987-1989 found the infection was limited to fish in the Grande Ronde and Imnaha basins. Adult chinook and steelhead returning to these same basins were also infected. The same survey indicated the parasite was not established in fish populations in any other Oregon rivers.

In 1987 the first documentation of Myxobolus cerebralis positive stray adult Snake River steelhead occurred in the Deschutes Basin at Warm Springs Hatchery on the lower Deschutes River. From 1997 to the present, an ODFW survey, funded by Portland General Electric, found the WD parasite in stray adult steelhead, spring chinook and sockeye in the river below Pelton Dam. Further surveys of naturally reared and hatchery fish have found no evidence of the parasite in Deschutes River fish populations.

How did the disease get to western Oregon?

While the exact origin is unknown at this time, it is possible that stray anadromous fish carried the parasite from the Snake River basin, where the parasite has been present since the 1980's. Fish from those areas could have strayed into the Clackamas River as they have in the lower Deschutes River.

What has been done?

Upon detection and confirmation of the presence of Myxobolus cerebralis spores, the private fish grower halted all planned transfers of these fish to other locations.

Biologists immediately initiated sampling of wild salmonids above and below the hatchery location and in other tributaries of the Clackamas River for the presence of Myxobolus cerebralis spores. Testing of salmon, kokanee, cutthroat, rainbow, whitefish and steelhead at a variety of sites in the basin resulted in no further evidence of the presence of the parasite. Since the spring of 2002, naturally reared fish surveys have been expanded to other streams in the Willamette River basin, with no evidence of the parasite. Naturally reared fish surveys have been conducted throughout the state with no evidence of the parasite except in the Grande Rhonde and Imnaha basins and stray adults in the Deschutes River.

Department hatcheries, private facilities and STEP sites in the state are routinely surveyed for the presence of Myxobolus cerebralis spores every year.

Testing for Myxobolus cerebralis in fish from all private trout hatcheries in Oregon is part of the normal disease sampling protocol that is required as part of the private aquaculture license. Of the 26 private trout licensed operators in Oregon, fifteen of these raise and sell live fish. Fish from one facility (Clear Creek) tested positive for presence of the spores in 2001, and has since used a different water source resulting in negative results for the parasite.

Is there a cure or a way to eradicate the parasite?

In watersheds where Myxobolus cerebralis is present in naturally reared fish, there is no cure or means to eradicate it. The parasite can be controlled in hatchery environments with careful management. Impacts to natural populations do not appear to have occurred in Oregon but are still being investigated.

What can anglers do to prevent the spread of the parasite?

  • Don't transport live fish between bodies of water without a permit. It's prohibited and illegal.
  • If you observe signs of whirling disease in fish, contact ODFW. If you witness illegal transport of fish, contact Oregon State Police or a local ODFW office.
  • Don't dispose of fish heads, skeletons or entrails in any body of water. Fish parts should be disposed of in the garbage, by deep-burying, or by total burning.
  • Rinse mud from wading boots and boats to reduce the risk of spreading the parasite spores to other waters.
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