When hunters notice an abnormality or apparent disease in wild game they are preparing for consumption, they naturally have questions regarding their observation – particularly how it might affect their own health or the health of people consuming the meat.
Sarcocystis, which causes very prominent changes in the breast muscles of waterfowl, is one such disease. Wildlife managers are frequently asked about this disease and the risks associated with handling or eating affected birds. This leaflet will answer the most commonly asked questions about Sarcocystis in waterfowl.
Sarcocystis is a parasitic infection caused by a protozoan (single-celled organism). Some species of Sarcocystis can cause illness in certain animals. However, waterfowl affected with this disease usually do not look or act sick and generally the disease is not fatal. Occasionally, severe infections may cause muscle loss with resultant lameness or weakness. Such affected birds may be more susceptible to predation.
Other names for sarcocystis include rice breast disease, rice belly disease, sarcocystosis, and sarcosporidiosis. The most common protozoan infecting waterfowl is Sarcocystis rileyi.
You won't usually see external evidence of sarcocystis infection. However, once you skin the bird, you can easily recognize the visible form of the disease – you will see cream-colored cylindrical cysts running in parallel lines throughout the muscles.
Since cysts are located in muscles, hunters who pluck birds without viewing the meat may miss the disease. Because these cysts resemble rice grains, sarcocystis is commonly called "Rice Breast Disease." The cysts usually occur throughout the skeletal muscles in the breast and thighs, but may also occur in the heart or smooth muscle of the digestive tract. Deposits of minerals around the cysts enhance their visibility and may feel gritty when you cut the muscle with a knife.
Sarcocystis appears to take time to develop visible cysts, and the disease is rarely seen in juvenile birds. In years of poor waterfowl production, hunters may bag more adult ducks and thus be more likely to notice infected birds.
A microscopic form of the disease, which produces no visible evidence of infection, can occur with some species of Sarcocystis. It is more common in non-waterfowl species and certain species of waterfowl.
A wide variety of birds, mammals and reptiles can contract Sarcocystis. Among waterfowl, dabbling ducks (mallard, pintail, shoveler, teal, black duck, gadwall and widgeon) are the species most commonly affected with the visible form of the disease. Diving ducks are only occasionally affected. The microscopic forms of the infection occur frequently in snow geese, Canada geese, and a variety of duck species.
Wild hoofstock (e.g. elk, deer) are commonly infected with other species of Sarcocystis. Domestic animals and livestock can be infected, but treatment and removal or isolation of infected animals usually keeps prevalence low.
Waterfowl and other animals become infected with sarcocystis by ingesting the eggs of the parasite in food or water. The parasite requires a primary host (carnivore0 and a secondary host (waterfowl and other herbivorous animals) to complete it's life cycle.
In the primary host's intestine, the parasite matures and produces microscopic eggs. The eggs pass out in the carnivore's feces, contaminating the environment. Waterfowl ingest the eggs while feeding. When the eggs hatch, the parasites move through body tissues to the skeletal muscles where they form cysts. The cycle is completed when a carnivore consumes prey infected with sarcocystis.
We don't know a lot about the life cycles of most species of Sarcocystis. At present it appears that each type of Sarcocystis prefers specific primary and secondary hosts. This means that different carnivores are involved in the infection of different waterfowl, and may explain why only certain species of waterfowl are infected in some areas.
No methods currently available will control the disease in wild waterfowl, nor do we see a need for control, since sarcocystis rarely kills waterfowl directly. To control the parasite, we would have to interrupt its life cycle, which would require a thorough understanding of specific carnivore/waterfowl species interactions. This information, and the development of control measures, would be more useful in captive waterfowl.
Sarcocystis found in waterfowl presents no known hazard to humans ands is not known to be transmitted to humans. Proper cooking destroys all forms of the parasite; however, hunters usually discard unappetizing carcasses containing large visible cysts. Other species of Sarcocystis (S. hominis, S. porcihominus) infect cattle and pigs, respectively, and can be transmitted to humans who eat undercooked meat. Dogs can be the primary host for at least seven species of Sarcocystis, with elk, mule deer, cattle, horses, pigs and sheep acting as secondary hosts; but infection does not cause disease in dogs or other domestic animals. Sarcocystis species commonly found in waterfowl have not been shown to infect dogs or other domestic animals. However, because dogs are susceptible to at least some species of Sarcocystis, we don't recommend feeding uncooked infected waterfowl to domestic animals. Remember, if you cook the meat, you kill the parasite.
NOTE: This paper was produced by the National Wildlife Health Research Center, U.S. Fish and Wildlife Service in cooperation with the University of Wisconsin-Cooperative Extension.
Thomas J. Rolfe, D.V.M., Ph.D., is supervisory veterinary medical officer with the U.S. Fish and Wildlife Service, National Wildlife Health Research Center, Madison, Wis.
Scott Craven is professor of wildlife ecology, College of Agricultural and Life sciences, University of Wisconsin-Madison; and University of Wisconsin-Extension, Cooperative Extension.
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