Whirling disease is a malady of trout and salmon caused by a microscopic parasite that produces a spore. The water-borne parasite (Myxobolus cerebralis) may not directly kill trout, but fish heavily infested can become deformed or exhibit the erratic tail-chasing behavior from which the disease gets its name. Eventually, heavily infected young fish may die. The parasitic disease probably originated in Europe, where native brown trout have developed a natural resistance to the parasite. Resistant fish can still carry and transmit the spore.
The whirling disease parasite has a two-host lifecycle that involves trout and an alternate host (a common bottom-dwelling tubifex worm).
When an infected trout dies, large numbers of hard spores are released.
These hard spores are hardy, resist freezing and drought, and can remain viable for decades.
After release from the host fish, they can be ingested by the tubifex worm.
The worms are then parasitized by the organism, the end result of this phase being a delicate, water-borne spore.
When released from the worm, these water-borne spores can infect susceptible fish by attaching to their bodies, or when fish eat infected worms.
Most native species have little or no natural resistance to these infections, having only recently been exposed to the parasite.
Whirling disease was first observed in the United States around 1958. The parasite was accidentally introduced in Colorado in the 1980s through imported trout from a private hatchery. It's now found in at least 20 states, including West Virginia, Pennsylvania, New York, Ohio, Michigan and most western states.
Whirling disease is thought to be a major factor in the declines of wild rainbow trout populations in certain Colorado waters. It's suspected that the outbreak of the disease may be linked to other environmental factors that aren't yet apparent. The parasite has been confirmed in 13 of Colorado's 15 major river drainages, including the Colorado, South Platte, Gunnison, Arkansas and Rio Grande rivers, as well as in a number of state hatcheries. Once the disease parasite is established in the wild, it can persist indefinitely, depending on environmental conditions. Efforts to reduce the parasite in hatcheries are proving successful.
Colorado Parks and Wildlife has developed strict policies and regulations to help control and prevent the spread of the disease in Colorado.
Hatcheries - Eight of Colorado's state hatcheries have tested positive for whirling disease. In some cases, this has amounted to only two spores. For reference, more than 4 million spores can fit on the head of a pin! Routine fish health sampling indicates diminishing infections at some sites, a result of measures to reduce or eliminate whirling disease in hatcheries. Many hatchery trout will carry few, if any, spores. As a precaution, the Colorado Parks and Wildlife will still consider these trout "positive" until repeated hatchery tests find no spores. Additional steps, including the installation of an ultraviolet system at the Roaring Judy Hatchery to kill spores that cause whirling disease, are underway.
Stocking - A policy implemented in spring 1995 prohibits the stocking of trout from hatcheries testing positive into waters where whirling disease has not been found. This includes wilderness areas and streams where native trout may be restored. Trout from positive hatcheries will be stocked into waters where the parasite has been found to minimize the risk of contaminating other watersheds. Only trout from negative testing hatcheries can be stocked into waters where the parasite has not been found.
The Colorado Parks and Wildlife is developing a comprehensive policy to determine where and when stocking will occur. This policy is being developed by a group composed of anglers, federal land management agencies, business interests, tackle manufacturers, private trout growers and others. The goal is to safeguard the aquatic resource while continuing to provide quality recreational fishing for trout and other fish in Colorado.
Research - An exhaustive Colorado Parks and Wildlife research project has greatly increased knowledge about whirling disease. Colorado is coordinating its resources with other states to study the parasite. In February 1996, Colorado Parks and Wildlife (then Division of Wildlife) and the U.S. Fish & Wildlife Service hosted a national conference of fishery biologists in order to share information on the disease and develop a comprehensive strategy for future research. This cooperation will maximize all efforts, avoid duplication of research and hopefully result in a better understanding of how to control the spread of the whirling disease parasite.
As a member of the Colorado Fish Health Board, Colorado Parks and Wildlife is taking the lead on developing ways to ensure that the state's aquatic habitat remains healthy. Through public awareness, research and continued fish health programs, the impact of the whirling disease parasite and other pathogens can be minimized or eliminated in many of the state's waters.
Clean: Remember that the tubifex worm can hold the whirling disease parasite. Thoroughly wash off any mud from vehicles, boats, trailers, anchors, axles, waders, boots, fishing equipment and anything that can hold the spores or mud-dwelling worms.
Drain: The parasite can persist in water. Drain boats, equipment, coolers, live bait wells and any holder of water. Make sure you don't inadvertently help spread whirling disease by cleaning all equipment after use in lakes or streams.
Don't Transport: Avoid moving any fish from one body of water to another, as this can help spread whirling disease. It is unlawful in Colorado to move and stock live fish without a special license.
Dispose Correctly: Don't dispose of fish entrails or other by-products into any body of water.