North Central Regional Aquaculture Center In cooperation with USDA What is Whirling Disease? by Mohamed Faisal1 (Michigan State University) and Donald Garling2 (Michigan State University) hirling disease is the called a triactinomyxon (TAM), chinook salmon, coho salmon, and common name for an develops in the worm host. Infection brown trout. Lake trout may be W infection in salmonids occurs when the TAM released from resistant to infection. caused by the protozoan, Myxobolus the worm attach to a fish’s skin, or cerebralis. Diseased fish usually when a fish eats an infected worm. Which age is show signs of circular swimming, Once TAM are in the fish body, the susceptible? hence the disease name “whirling.” parasite settles in the cartilage, In addition, diseased fish may show multiplies, and feeds on its contents. In general, young salmonids are other signs, such as black tail, Pain associated with damaged more vulnerable than adult fish. In skeletal deformities, and shortened cartilage cause swimming distur- studies with rainbow trout, 2-day- gill cover (Figure 1). Because of the bance and deformed appearance in old sac fry were the youngest to erratic, uncontrolled circular heavily infected fish (Figure 3). acquire the infection and develop swimming, the fish are unable to eat Spores can be shed from gills or spores. The severity of infection or escape predators. feces of heavily infected fish. The decrease with increased age of fish. spores are also released to the This increased susceptibility is Myxobolus cerebralis has a two-host environment when infected fish die because the skeleton in young fish life cycle, alternating between and decompose, or through feces of has not yet developed into mature salmonid fish species and a benthic other fish-eating animals. The bony tissue. Ossified bones take organism, the worm, Tubifex tubifex liberated spores are then taken by place gradually with the age. (Figure 2). The worms live in the tubificid worms, develop in their mud of streams. As many as 10,000 guts into TAM, and then released worms can be found in one square back into the water, thus repeating yard of a muddy river bottom. The the life cycle (Figure 2). Unfortu- parasite’s free-swimming form, nately, M. cerebralis spores are resilient in infected sediments and remain alive for more than 20 years. Which salmonids species are susceptible? Of all salmonids, rainbow trout are North Central Regional the most susceptible species to Aquaculture Center whirling disease, followed by—in Fact Sheet Series #113 decreasing order of susceptibility: USDA grant # 2001-38500-10369 December 2004 the sockeye salmon, brook trout, Figure 1. Characteristics of whirling disease. How is whirling Is whirling disease 4. Frequently clean solids from disease spread? dangerous to humans? settling areas to prevent growth of tubificid worms. Most transmission between rivers is The parasite does not infect humans 5. Clean and dry equipment before the result of transplanting infected or predators that eat infected fish. going from one body of water to fish. Transmission within a river Trout and salmon with whirling another. system occurs by spores carried out disease can be safely eaten. by water currents, contaminated How can fish farmers eradicate equipment, boats, and birds that Can whirling disease be eradicated whirling disease? have consumed infected fish. from affected streams? Getting the mud off, and cleaning 1. Remove all infected fish. and drying equipment should Once whirling disease is in a stream, greatly reduce or eliminate the there is no effective way to eradicate 2. Change to a ground water source potential threat of spreading whirl- this infection. However, there are (springs and wells) free of fish or ing disease. measures that can be done to reduce the pathogen. the impact of the disease. The 3. Convert to concrete or lined How did Michigan and other North severity of the problems observed in raceways. Central fish farms and rivers become Montana may be the result of other factors that have contributed to the infected by whirling disease? 4. Restock only fish certified free of decline of the fish including high whirling disease or surface- Whirling disease was first identified rates of catch and release, and disinfect eggs. in Pennsylvania in the 1950s, pre- increased stream sediment loads sumably arriving with frozen fish from grazing, mining and develop- 5. Frequently clean solids from shipments from Europe. Since then, ment of the surrounding area. settling areas to prevent growth it has spread in 23 states. In fall of Increasing sedimentation provides of tubificid worms. 1968, the disease was first reported additional growing areas for tubifi- in Michigan. A Michigan trout cid worms. Many federal, state and 6. Clean and dry equipment before farmer had obtained fish from an university research studies are being going from one body of water to another. Ohio trout farm known to be af- conducted to develop more sensitive fected by the parasite. Later investi- tests for the diagnosis of the proto- How can fish farmers avoid magni- gations showed that three hatcheries zoan in worms, fish, and water. fying infection levels in receiving in Michigan were contaminated with Additional studies are being con- waters if eradication is impossible? this dangerous protozoan. Through ducted to determine why some fish voluntary agreements, all fish in survive infections while other infected ponds were killed and species succumb to the disease. buried, and the facilities thoroughly disinfected. These efforts, however, How can fish farmers were not effective in stopping the avoid whirling spread of the disease. By 1998, disease infection? whirling disease has been reported in several Michigan river systems 1. Use only ground water and a few fish farms. sources (springs and wells) free of fish. Alarming reports came from Colo- rado and Montana. Massive de- 2. Purchase only fish certified clines of young trout were observed free of whirling disease or in the Colorado River at Middle Park surface-disinfect eggs. in 1993. In 1994, a drastic decline in trout populations of Madison River 3. Use only concrete or lined raceways. in Montana were reported. Figure 2. Life cycle of Myxobolus cerabralis. 2 1. Remove mortalities as frequently Lakes states are as possible. developing policies for fish farms that 2. Frequently clean solids from have tested positive settling areas to prevent growth for whirling of tubificid worms. disease. Most states within the North 3. Cease rearing susceptible fishes. Central Region Does Michigan or other require that fish imported into the North Central states state are certified have any regulations free of whirling concerning fish farms disease before an with whirling disease? import permit can be obtained. The Great Lakes Fish Disease Control Policy and Model Program, published by the Great Lakes Fish Commission, prohibits the stocking fish from farms infected with whirling disease in the Great Lakes Figure 3. Deformities and their tributaries. The Michigan associated with whirling Department of Agriculture and disease. Natural Resources and other Great Acknowledgements: Special thanks to Kim Hopkins from the Khaled bin Sultan Living Oceans Foundation for the illustrations. Additional sources of information: http://www.whirlingdisease.org http://water.montana.edu/mwc/programs/fisheries/whirling/default.htm http://hmsc.oregonstate.edu/classes/MB492/whdchelsea/index.htm http://www.montana.edu/commserv/csnews/nmview.php?article=1441 Information provided by: North Central Regional Aquaculture Center and Michigan State University Extension 1 Departments of Pathobiology and Diagnostic Investigation, College of Veterinary Medicine, and Fisheries and Wildlife, College of Agriculture and Natural Resources, Michigan State University 2 Fisheries and Wildlife, College of Agriculture and Natural Resources, Michigan State University Series Editor: Joseph E. Morris, Associate Director, North Central Regional Aquaculture Center. Originally published by Iowa State University, Ames, Iowa Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s), and do not necessarily reflect the views of the United States Department of Agriculture. 3.