Sarcocystis

Sarcocystis is a genus of intracellular protozoan parasites in the family Sarcocystidae, within the phylum Apicomplexa. Members have a heteroxenous, two-host life cycle in which sexual development occurs in the definitive carnivorous host and asexual development in a vertebrate intermediate host. The parasites form tissue cysts known as sarcocysts in the skeletal muscles of intermediate hosts, while the parasite completes development via oocysts or sporocysts shed in the feces of the definitive host.

Most species use canids, felids, or other carnivores as definitive hosts; in many livestock species such as cattle, sheep, pigs, and horses serve as intermediate hosts. Transmission occurs when an intermediate-host animal ingests sporocysts from contaminated food, water, or feed; the parasite multiplies and forms sarcysts in muscle. When a predator consumes infected muscle, sexual reproduction occurs in the intestine and oocysts are released, continuing the cycle.

In livestock, infections are often subclinical but can cause muscle hardening, meat inspection lesions, or abortions in some species and reduce production. In horses, certain species such as Sarcocystis neurona can cause severe neurologic disease (equine protozoal myeloencephalitis). Pathogenicity varies among species and strains; many infections are incidental.

Humans typically act as incidental hosts for a limited number of species; intestinal and occasional muscular sarcocystosis have been reported, usually after consumption of undercooked meat or contact with contaminated material. Diagnosis relies on muscle biopsy showing sarcocysts, serology, histology, or molecular methods; management focuses on prevention and, in animals, control of definitive-host exposure and proper meat inspection.