Sarcosporidiosis is defined as infection with Sarcocystis, which is an intracellular protozoan parasite. Sarcocystis predominantly infects nonhuman animals but can also infect humans.
Many Sarcocystis species exist, all of which are believed to have a requisite two-host life cycle. This life cycle is based on a predator-prey host relationship.  In the rare occurrence in which a human is the intermediate, or accidental, host, Sarcocystis organisms can be found in human skeletal and cardiac muscle. [2, 3]
Humans can also serve as the definitive host for Sarcocystis. This can occur following ingestion of the cysts in raw or undercooked beef or pork. After this invasion, the infective sporozoites replicate before being eliminated in the stool as sporocysts.  Once shed, sporocysts are typically ingested by an intermediate host (usually a cow or pig) and pass into the small intestine of this animal. Once in the intermediate host, the oocysts release motile sporozoites, which initially migrate into arteries throughout the body. They then become merozoites in the blood vessels and, finally, in muscle. Several noninfectious generations develop before finally maturing to become infectious sarcocysts. 
Sarcosporidiosis in humans has two distinct forms. These two forms are differentiated based on whether the individual is serving as a definitive or intermediate host.  Intermediate hosts are infected following ingestion of water or food contaminated with sporocysts from the feces of a carnivore (eg, dog, wolf). After ingestion, sporocysts penetrate the host’s intestinal wall and proliferate in vascular endothelium before disseminating hematogenously. Dissemination leads to invasion of skeletal and cardiac muscle. Because humans are not typically preyed on, these cysts are not given the opportunity to progress through their typical life cycle and eventually disintegrate within the muscle. Disintegration can be accompanied by vasculitis and fibrosis of the tissue (myositis).
Definitive hosts are infected following ingestion of meat contaminated by infective oocysts. After ingestion, the oocysts sexually reproduce and mature in the intestinal tract. Infective oocysts are then shed via the stool (enteritis). This form of the infection does not involve a systemic phase or a subsequent tissue phase. Humans serve as the definitive host in this infectious form.
Sarcosporidiosis is distributed worldwide. In the United States, more than 60 cases of muscle involvement by Sarcocystis species have been described, mostly in collections of case reports of 5-10 cases.
Given that sarcosporidiosis is often an incidental finding, the disease is probably underreported.  The definitive form of sarcosporidiosis often causes self-limited nonspecific enteritis and often goes clinically unsuspected.
More than 100 species of Sarcocystis have been recognized, and they have worldwide distribution.  Most cases of human sarcosporidiosis have been documented in Southeast Asia, and the disease is predominantly studied there.
Sarcocystis species that are specific for the skeletal-muscle cysts in cattle and pigs are also distributed worldwide, but cultural practices in certain parts of the world (eg, Thailand) lead to higher rates of human infection. One study reported that the incidence of intestinal Sarcocystis infection in Thai laborers was at 23%.  A study of autopsy specimens in patients in Southeast Asia showed a sarcosporidiosis prevalence rate of 21% in 100 consecutive patients evaluated. The seroprevalence of sarcosporidiosis in Malaysia was estimated at 19.8%.
Although sarcosporidiosis can involve the heart, only one death from myocarditis has been linked to Sarcocystis infection. This isolated case involved a 36-year-old woman with focal inflammation and myocyte necrosis, which was found upon examination of the myocardium and it contained a cyst that was morphologically identified as that of a Sarcocystis species. 
More common manifestations affect intermediate hosts and include painful muscle swellings, fever, and weakness.  Intestinal sarcocystosis, or the definitive form, most commonly produces abdominal pain, diarrhea, and generalized myalgias. Sarcosporidiosis has not been associated with chronic diarrhea or a malabsorptive state.
Sarcosporidiosis has no known racial predilection, but most described cases have been from Southeast Asia.
Sarcosporidiosis has no known sexual predilection.
Sarcosporidiosis has no known age predilection; however, because muscle involvement clinically occurs after cyst deterioration, adults are more likely to present with skeletal muscle involvement than are children. 
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