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Whirling Disease

A Resource Stewardship Challenge

Note: The following article first appeared in the June 1996 (Vol. 21, No. 6) issue of Fisheries magazine. Author- Philip J. Hulbert. Whirling disease caused by the myxosporean Myxobolus cerebralis has recently emerged as an issue of controversy and concern in portions of North America. This occurred, at least in part, because whirling disease was detected in some prominent wild trout populations in the western United States and was implicated in adverse impacts occurring among those populations. Whirling disease has also been detected in wild trout and hatchery-reared trout in New York since October 1994. The incidence and severity of fish diseases are generally believed to be greater for fish reared in hatcheries than for wild populations (Herman 1970), and whirling disease in particular was regarded as a problem that could be surmounted with careful management of hatchery practices. Has something changed?

Whirling disease was native to salmonids in Europe but was inadvertently introduced to the eastern United States in 1955 via shipments of frozen trout that harbored spores of this myxosporean fish parasite (Markiw 1992). Earthen-bottomed rearing ponds used in many trout hatcheries provided conditions where Tubifex worms, the second host of the parasite, could flourish. Fish transfers probably spread whirling disease to many of the states in which it was eventually detected, including New York. Whirling disease was reportedly found in some private fish hatcheries in New York 10-20 years ago, but was not reported from any Department of Environmental Conservation (DEC) fish hatcheries (link below) (John Schachte, New York State Department of Environmental Conservation, personal communication). Comprehensive fish health inspections, including testing for whirling disease at facilities with earthen ponds containing rainbow trout (Oncorhynchus mykiss), have been conducted at DEC hatcheries annually since 1976. Despite these efforts, 1994 year-class rainbow trout exhibiting clinical signs of whirling disease, later confirmed with Myxobolus cerebralis spores, were detected at DEC's Rome Hatchery on October 19, 1994. This essay summarizes DEC response to this outbreak.

A review of hatchery records and testing for whirling disease indicated our Caledonia Hatchery was the probable source of infected rainbow trout. Wild brook trout (Salvelinus fontinalis) in Spring Brook, the water supply for Caledonia Hatchery, tested positive for whirling disease, as did the 1994 year-class of rainbow trout in the hatchery. Other transfers from Caledonia Hatchery occurred earlier in 1994; thus, two more DEC hatcheries, Chateaugay and Salmon River, contained domestic rainbow trout and Skamania strain steelhead, respectively, that tested positive for whirling disease. No fish except the original suspect lot at Rome Hatchery showed clinical signs of whirling disease (whirling, black tail, cranial or skeletal deformity). We had no basis on which to determine how or when trout at Spring Brook and Caledonia Hatchery were first exposed to whirling disease.

Most infected rainbow trout and Skamania steelhead were scheduled to be stocked in spring 1995. Since no therapeutic treatments eliminate this disease from infected fish, continued rearing or stocking carried a risk of introducing the disease into potentially uninfected hatchery lots or other waters, respectively. Thus, we destroyed all 570,000 infected domestic rainbow trout and Skamania steelhead by November 1, 1994. This eliminated approximately 75 percent of the statewide domestic rainbow trout inventory and all production fish of the Skamania strain of steelhead.

We then developed a comprehensive plan to more fully deal with the outbreak. Major elements of the plan were to gather information on risks of whirling disease to wild salmonids; to test all lots of salmonids in DEC hatcheries for whirling disease; to test all lots of salmonids from private and other governmental aquaculture operations in New York; to survey wild salmonids for whirling disease in selected rivers and lakes across the state; to communicate our findings to technical and executive staff within the agency and to interested parties outside the agency; and to develop recommendations for future actions. Our goal was to minimize the effects of whirling disease on important wild trout and salmon stocks and on overall recreational fishing opportunities in New York.

Technical literature focused on the life history and geographic distribution of whirling disease but provided little information about impacts to wild salmonids. We sent questionnaires to states in which whirling disease was reported (Markiw 1992) to learn of their experiences and whether or not impacts to wild fish had been detected. The resulting input indicated that some states believed whirling disease had harmed wild salmonids but some, such as Pennsylvania, indicated robust wild trout populations were maintained in whirling-disease-positive waters. Others had not investigated this aspect. By fall 1994, some nontechnical periodicals indicated that whirling disease was becoming a controversial issue in Colorado. After initial contacts, DEC staff were invited to meet with fisheries staff of the Colorado Division of Wildlife to review the history of the whirling disease outbreak in that state and their analysis of possible impacts to wild populations in several waters. We concluded that whirling disease had the potential to harm wild salmonids in New York, at lease in combination with other stresses.

Information cited by Markiw (1992) clearly indicated that the disease can be spread by stocking infected fish into suitable uninfected waters. To avoid doing so, DEC adopted an interim policy of not stocking fish from infected lots and began testing all lots at our nine salmonid hatcheries. Lots were either tested one or two times before spring 1995 at a 95 percent detection capability or three to four times before spring at a 99 percent detection capability. The more rigorous testing was required to satisfy concerns stemming from the Great Lakes Fish Disease Control Policy and Model Program, which prohibits stocking any fish from a hatchery where whirling disease has been detected in Great Lakes Basin waters for at least two years after testing negative for M. cerebralis. Since major components of DEC's salmonid stocking program for the Lake Erie and Lake Ontario watersheds are reared at the Salmon River Hatchery, the multimillion-dollar recreational fisheries occurring in those waters would have been harmed if uninfected lots of trout and salmon were not stocked. After meeting with members of the Great Lakes Fish Health Committee, we received concurrence to do so.

On March 17, 1995, the DEC implemented testing and adopting a whirling disease inspection requirement for privately stocked fish to help control the disease's spread. The whirling disease inspection requirement, DEC's first fish health inspection requirement ever, was applied to salmonids imported or reared within the state for stocking. Private aquaculturists were generally unhappy with the whirling disease inspection requirement adopted in Mach 1995 believing it was unnecessary and burdensome. However, testing results were negative for 75 percent and positive for 25 percent of the private hatcheries DEC tested in New York. At least five out-of-state facilities were certified to export trout for stocking into New York in 1995.

The DEC tested salmonids in selected lakes and rivers to help define the geographic extent of the whirling disease infestation. Sampling priorities included wild rainbow trout populations, waters that had been stocked with rainbow trout from Caledonia Hatchery, waters near fish culture sites, and waters supporting wild brook trout or brown trout (Salmo trutta) populations. Analyses from nearly 100 locations yielded positive findings for fish from nine waters. All positive samples were from waters near a fish culture facility or from waters known to be stocked. We detected whirling disease spores in rainbow, brook and brown trout, but not in lake trout (Salvelinus namaycush), Atlantic salmon (Salmo salar), coho salmon (Oncorhynchus kisutch), chinook salmon (Oncorhynchus tshawytscha), or kokanee (Oncorhynchus nerka). Overall, the infection appeared to be present in scattered locations across the state but was not widespread. None of the wild fish collected was reported to exhibit any clinical signs of whirling disease.

Since fall 1994, we have communicated our actions and findings within and outside the agency to provide accurate information, minimize hysteria, and promote cooperation. We have sent press releases, conducted numerous interviews with the media, and given presentations to technical and nontechnical audiences. As a result, most public reaction to DEC's action has been favorable and supportive.

Continued monitoring and testing of salmonids and sampling from additional wild populations are under way. The DEC is providing technical assistance to private aquaculturists to help minimize the occurrence and spread of whirling disease. Also, we are working to increase the availability of whirling-disease-testing services in New York and nearby states because private aquaculturists told us they were concerned that testing services were not widely available. We have also launched research into susceptibility of selected strains of wild trout to whirling disease and surveys to establish the presence or absence of population-level impacts in selected waters.

The preceding actions were taken to protect public resources from possible harm. The recent discoveries of whirling disease in several western states and New York have polarized constituent groups and stakeholders. Some predict dire consequences to wild trout populations and fisheries, while others insist whirling disease poses little or no threat to wild fish or even to aquaculture. At lease some information can be found to support either view. Fisheries professionals do not know all they need to about whirling disease. For example, in New York, the apparent absence of whirling disease from most of the waters sampled in 1994 was surprising because infected trout could have been legally stocked for decades. Wild salmonids are common or abundant in most coldwater streams, and Tubifex worms should be present near sites of organic material enrichment. Since even resistant species, such as brown trout, can be carriers of the parasite, it is possible that New York waters do not provide conditions that favor either a virulent or widespread expression of whirling disease. We simply do not know. A coordinated approach in investigating whirling disease across the country would shorten the learning curve, thereby benefitting public resource stewards and the private sector. It is time to search for answers and knowledge, not scapegoats.


Herman, R.L., 1970. Prevention and control of fish diseases in hatcheries. Pages 3-15 in S.F. Snieszko, ed. A symposium on diseases of fishes and shellfishes. American Fisheries Society Special Publication 5, Bethesda, MD.

Markiw, M.E., 1992. Salmonid whirling disease. US Fish and Wildlife Service, Fish and Wildlife Leaflet 17, Washington, DC.

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