Other names: Sarcocystis, Sarcosporidiosis, Sarcocystoides, Rice breast disease


Sarcocystosis is a disease of birds and mammals caused by several species of single-celled coccidian protozoa from the Sarcocystis group.


Sarcocystis infection is common in many species of wild and domestic animals. Humans can become infected following ingestion of infected pork or beef. Though there is no evidence that the species of Sarcocystis that infect wildlife can cause disease in humans, hunters are advised not to consume infected animals. Hunters are most likely to encounter this disease in rabbits and ducks. 

Species Affected

Sarcocystis protozoa rely on predator-prey relationships to maintain their life cycle, and each species of protozoa specifically infects their own species of intermediate (prey) and definitive hosts (predator). Many species of mammals can act as intermediate hosts for Sarcocystis including white-tailed deer, mule deer, moose, elk, caribou, rodents, rabbits, pigs, horses, cattle, sheep, and goats. Definitive hosts include foxes, coyotes, wolves, raccoons, opossums, domestic dogs, cats, some species of snake, and certain primates (including humans). Some species of marsupials (such as opossums), primates, and carnivores can act as both intermediate and definitive hosts. Birds that act as intermediate hosts include dabbling ducks, geese, pheasants, herons, egrets, and domestic chickens. Bird definitive hosts include predatory species such as hawks, owls, and kestrels.


Sarcocystis is distributed worldwide, but is more common in areas with livestock.


Sarcocystis requires both an intermediate (prey) and definitive (predator) host to complete its life cycle. The protozoa first form cysts within the muscle tissue of prey. Predators become infected when they ingest prey containing these cysts. The protozoa reproduce within the definitive host and produce eggs that develop into infective sporocysts (protozoa encased in a protective covering), which are then released through the feces. Sporocysts can remain infective in the environment for months. Intermediate hosts become infected when they ingest sporocysts in food or water contaminated by the feces from a definitive host. The protozoa once again form cysts within the tissue of the intermediate host and persist until consumed by a carnivore or omnivore, continuing the cycle of infection.

Clinical Signs

The development of clinical signs often depends on the number of sporocysts ingested and the species of Sarcocystis. In general, most animals do not experience observable clinical signs. Intermediate mammalian hosts may exhibit loss of appetite, lethargy, diarrhea, weight loss, weakness, or muscle twitching. Clinical signs may last several days to several weeks. When pregnant mammals ingest sporocysts they may abort or give birth to a stillborn fetus. Though intermediate avian hosts usually do not show clinical signs, they may exhibit loss of appetite, weakness, difficulty breathing, neurologic signs, and may have blood in the mouth. Animals that ingest a large quantity of sporocysts may die. Definitive hosts typically do not show any clinical signs of sarcocystosis. 

At necropsy, tan to white cysts often resembling parallel grains of rice may be observed throughout the breast muscle tissue and other muscles, including the heart. The affected tissue often feels gritty when cut with a knife. In mammalian intermediate hosts, cysts can vary greatly in shape and may either be grossly visible or microscopic depending on the Sarcocystis species. Grossly visible cysts in mammals may look like tan-white grains of rice, or they may be long and threadlike, or even round. Some animals that die from sarcocystosis will have signs of hemorrhaging throughout the body. 


Sarcocystosis can be diagnosed by identifying the cysts within muscle tissue or by identifying sporocysts in the feces using fecal flotation methods. 


There is no treatment for sarcocystosis in wildlife.


There are currently no efforts to control sarcocystosis in free-ranging wildlife. Good sanitation is important to control this disease in domestic and captive animals. Freezing meat prior to cooking can also help to kill the organisms. Uncooked meat should not be eaten by humans or domestic animals. Severely affected carcasses are condemned as unfit for human consumption. Condemned carcasses should be buried or incinerated, and should not be left out to be eaten by wild scavengers.

Suggested Reading

  • Dubey, J. P., and K. Odening. 2001. Toxoplasmosis and Related Infections. Pages 478-519 in W. M. Samuel, M. J. Pybus, and A. A. Kocan, editors. Parasitic Diseases of Wild Mammals. Iowa State University Press, Ames, Iowa, USA.
  • Greiner, E. C. 2008. Isospora, Atoxoplasma, and Sarcocystis. Pages 108-119 in C. T. Atkinson, N. J. Thomas, and D. B. Hunter, editors. Parasitic Diseases of Wild Birds. Wiley-Blackwell, Ames, Iowa, USA.
  • Michigan Department of Natural Resources. Wildlife Disease. Sarcocystis.
  • Tuggle, B. N., and M. Friend. Sarcocystis. Pages 219-222 in M. Friend, and J. C. Franson, technical editors. Field Manual of Wildlife Diseases: Birds. United States Geological Survey


Cysts in duck breast tissue caused by SarcocystosisRice grain-like cysts within the breast muscle tissue of an American black duck; photo courtesy of USGS by James Runningen