In sarcocystosis, the muscles and other soft tissues are invaded by single-celled organisms, called protozoans, of the genus Sarcocystis. Although their final hosts are predators such as dogs and cats, these organisms form cysts in various intermediate hosts, including cattle, pigs, humans, rodents, and reptiles. Some cysts are visible to the naked eye, but others are too small to see. Their size depends on the species of the host and the species of Sarcocystis. A dog can develop sarcocystosis after eating undercooked beef or pork containing sporocysts or after eating food infected with sporocysts from another animal's feces. Infected dogs often have no signs, although a mild diarrhea may be seen.

As noted above, humans may serve as intermediate hosts and can develop inflammation and soreness of muscles and blood vessels. This condition is rare, and the source of human infection has never been determined. Signs include nausea, abdominal pain, and diarrhea lasting up to 48 hours. The extent of human illness caused by infected meat has not been documented.

Because most adult cattle and sheep and many pigs harbor cysts in their muscles, dogs should not be allowed to eat raw meat, edible organs (such as heart, liver, tongue, and brains), or dead animals. Supplies of grain and feed should be kept covered. Dogs should not be allowed in buildings used to store feed or house animals. There are no available vaccines. Experiments have shown that infected pork can be made safe for consumption by cooking at 158°F (70°C) for 15 minutes or by freezing at 25°F (- 4°C) for 2 days or 4°F (- 20°C) for 1 day.

Last full review/revision July 2011 by Russel R. Hanson, DVM, DACVS, DACVECC; Dale A. Moore, MS, DVM, MPVM, PhD; Joerg A. Auer, DrMedVet, Dr h c, MS, DACVS, DECVS; Joseph Harari, MS, DVM, DACVS; Sheldon Padgett, DVM, MS, DACVS