sarcocyst

A sarcocyst is a tissue cyst formed by protozoan parasites of the genus Sarcocystis, typically found in the striated muscle of an intermediate host. Each sarcocyst encases numerous bradyzoites, the slowly dividing stages that persist in muscle tissue and serve as the reservoir for transmission to the next host.

Lifecycle and hosts: Sarcocystis species are heteroxenous parasites requiring two hosts. The intermediate host—commonly livestock such as cattle, sheep, or pigs—harbors asexual replication that produces tissue cysts (sarcocysts) in muscle. When a suitable definitive host (a carnivore such as a dog, cat, fox, or sometimes other predators) consumes infected tissue, sexual reproduction occurs in the intestinal epithelium, and oocysts or sporocysts are shed in feces. Sporocysts contaminate the environment and infect new intermediate hosts, continuing the cycle. Humans can be incidental hosts in some life cycles, typically acquiring infection through undercooked meat or, less commonly, exposure to sporocysts.

Structure and diagnosis: Sarcocysts are elongated, cyst-like structures within muscle tissue. The bradyzoites inside are the infective forms for the definitive host. In animals, sarcocysts can be detected by muscle histology, digestion methods, or molecular tests; in meat inspection, visible or microscopic cysts may be assessed. Serology or PCR can aid in species identification and epidemiology.

Clinical features and prevention: In animals, infections are often asymptomatic but can cause muscle inflammation, weakness, or reduced growth in heavy infections. In humans, disease is usually mild or asymptomatic; intestinal sarcocystosis can produce transient gastrointestinal symptoms, while muscular disease is rare. Prevention focuses on cooking meat to safe temperatures, good kitchen hygiene, and interrupting transmission by controlling definitive hosts and preventing contamination with feces.