Species of the protozoa Sarcocystis have an obligatory 2-host life cycle. The intermediate host is usually an herbivore or omnivore. The definitive host is a carnivore.
Sexual Reproduction. The cycle begins when adult male and female parasites sexually reproduce in the definitive host's epithelial cells. The newly created oocysts are sporulated in the host. In passing, the thin-walled sporulated oocysts often rupture, releasing infective sporocysts. The host then sheds both sporocyts and sporulated oocysts in the feces.
Transmission. In the next stage of the cycle, the intermediate host ingests infective sporocysts fecal-orally by contamination of feed or water.
Asexual reproduction. Sporozoites excyst from the sporocysts and invade the intestinal mucosa to reach endothelial cells. An asexual cycle begins, whereby initial reproduction forms schizonts.
Within the schizonts are merozoites, which are released and eventually find their way to muscle tissue, where they form sarcocysts. The merozoites divide into pairs within the tissues of the cyst, forming metrocytes. Metrocytes continue to undergo maturations, until finally forming mature, banana-shaped bradyzoites, which are slowly dividing, and also infective to definitive hosts.
When the muscle tissue containing sarcocysts with infective bradyzoites is consumed by the definitive host (a carnivore), the bradyzoites penetrate the mucosa and transform into macrogametes and micrgametocytes in the cells of the intestinal epithelium. The microgametocytes produce flagellated microgametes, which penetrate the macrogametes. The zygote lays down a resistant wall and sporulates endogenously.
1: Sarcoocyst (longitudinal section). 2:Sarcocyst (cross-section). 3:Immature sarcocyst with metrocytes and bradyzoites. 4:muscle degeneration and myositis
(Arness et al, 1999)
S. Lindemanni in muscle (tongue) (Image, 3)A
Sarcocyst Spp. in the muscle
of a human
Copyright 2004 Stephanie Adams