sarcysts

Sarcysts are tissue cysts formed by parasites of the genus Sarcocystis within the striated muscles of their intermediate hosts. Each sarcyst is a cyst-like structure that encloses numerous intracellular predators called bradyzoites, which can persist for long periods and may become infectious when ingested by a suitable definitive host.

Life cycle and hosts

Sarcocystis species have a two-host life cycle that typically involves a carnivorous definitive host and a herbivorous or omnivorous intermediate host. Sexual reproduction occurs in the intestinal tract of the definitive host, producing oocysts or sporocysts that are shed in feces. The intermediate host becomes infected by ingesting sporocysts, after which the parasite encysts in muscle tissue as sarcysts. The cycle continues when a definitive host consumes tissue from an infected intermediate host.

Morphology and species variation

Sarcocysts vary in size and appearance among species but generally appear as elongated or globular structures within muscle fibers under histological examination. The bradyzoites inside are the infectious forms for the definitive host. Different Sarcocystis species have distinct host associations, and the pathogenic potential for humans and domestic animals varies accordingly.

Clinical significance and diagnosis

In wildlife and livestock, infection is often subclinical but can cause muscle inflammation and weakness in heavy infections. In humans, sarcocystosis is rare but can be acquired as intestinal disease (in some species where humans are definitive hosts) or muscular disease with fever and myositis in others. Diagnosis relies on histology of muscle tissue, serology, PCR, or detection of sporocysts in feces for gastrointestinal forms. Prevention focuses on proper meat cooking, good meat inspection, and avoiding contamination of food and water.