Heterosporis: An Important New Parasite

Heterosporis is a tiny parasite that forms cysts in the muscles of fish. In severe infections, the parasite and its spores may replace most of the muscle tissue. It is often noticed when fishermen dress fish and find that the fillets have opaque white patches and are soft and unpalatable. There is no evidence that the parasite kills fish directly, but the destruction of muscle would be expected to make fish more vulnerable to predators.

Several species of Heterosporis have been reported in ornamental and sport fish. Historically, the most significant problems have been in Europe and Asia where the parasites are found in ornamental fishes and eels. Recently there have been very severe infections of what may be a new Heterosporis species infecting wild fish in the north-central U.S.

In 2000, fishermen in Wisconsin and Minnesota began reporting yellow perch with cloudy white patches of muscle. These lesions are now known to be caused by Heterosporis and scientists have described the parasite as replacing as much as 90% of the normal muscle tissue. Initial reports describing Heterosporis did not cause much concern in Arkansas because the highly susceptible yellow perch is not an important sport or aquaculture species in this state. However, subsequent studies have found natural infections in walleye, northern pike, rock bass, pumpkinseed, burbot, sculpin, and trout perch. More troubling is recent work where experimental exposures have shown that many other species of fish are susceptible to the parasite including trout, salmon, white suckers, mosquito fish, channel catfish, fathead minnows and largemouth bass. It is clear that the Heterosporis parasite has the potential to infect important sport and aquaculture species in Arkansas. Bluegill, sturgeon, smallmouth bass, and golden shiners exposed to the parasite remain uninfected.

The life cycle of Heterosporis is simple and involves only fish with transmission through a resistant spore. Fish become infected by eating an infected fish or by exposure to spores in the water. This means that moving infected fish from place to place has the potential to also move the parasite to new areas. Farmers who import susceptible species like fathead minnows from Wisconsin, Michigan, Minnesota, Lake Ontario, or other areas known to harbor the parasite, risk importing this parasite, especially if the fish have spent time in the wild or have been cultured on farms that do not use controlled ponds and well water.

The actual risk of adverse effects from the introduction of Heterosporis is difficult to predict. It is possible that the parasite will not do well in southern climates. It is also important to remember that successful experimental infection of one fish does not mean that the species will become infected under natural conditions. However, we know that Heterosporis can produce natural infections in a popular sportfish species (walleye), and experimental infections have been proven in some of our most important cultured fish (catfish, fathead minnows, and largemouth bass). Furthermore, the ability to infect other economically important species is still unknown. Indisputably, great caution is required before bringing new fish to Arkansas from states where the parasite is found.

If this exotic parasite is introduced, there are no treatments for infected fish and the resistant spores of the parasite will make control very difficult. The biggest risk right now is the import of wild baitfish from northern states. The import of other susceptible species from farms in regions where Heterosporis is present is also risky, especially if the supplier also deals in wild-caught minnows or uses wild minnows for forage. Many state agencies are concerned about this parasite and any finding of Heterosporis in Arkansas would immediately jeopardize exports of fish.

                              Heterosporis is visible as light patches in about 80 percent of
                             the exposed muscle of this wild-caught yellow perch. Photo by
                                    Dr. Dan Sutherland, University of Wisconsin-La Crosse.

Much of the material in this article is from meeting reports presented in 2005 by Peggy Stelzig, Daniel Sutherland, and Sue Marcquenski of the University of Wisconsin La Crosse and the Wisconsin Department of Natural Resources.

By Andy Goodwin, Extension Fish Pathologist
Arkansas Aquafarming newsletter (Vol. 23, No. 1)
University of Arkansas Cooperative Extension Program

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Heterosporis: An Important New Parasite