Sarcocystis sp.

Sarcocystis sp. are intracellular, dixenous coccidian protozoa that belong to the phylum Apicomplexa, which parasitize many animals and occasionally man. For their life cycle, all species of Sarcocystis require two hosts: an intermediate omnivore (prey) and a definitive carnivore (predator).

Sarcocystosis is mainly a veterinary problem, since more than 50% of cattle, pigs and sheep are infected by various Sarcocystis species. However, at least two species are pathogenic to man, namely Sarcocystis hominis and Sarcocystis suihominis. Man is the definitive host (predator) that is infected by the ingestion of raw or undercooked meat from intermediate hosts, which are cattle for Sarcocystis hominis and pigs for Sarcocystis suihominis. Besides being the definitive host, man can also act as an intermediate host, through the ingestion of water or food contaminated with oocysts eliminated in the feces of an infected predator.

Once Sarcocystis has been ingested together with contaminated meat, its walls rupture and release bradyzoites that enter the lamina propria of the intestinal epithelium. Here, they start sexual reproduction leading to the formation of a mature oocyst containing two sporocysts which are excreted in feces. Intact oocysts can be observed in stool samples only in the first few days after excretion. The cyst wall is thin, barely visible, with the two adjacent sporocysts containing four sporozoites each. Very often the wall ruptures, releasing the sporocyst: this is often the only stage detected in stool examination.

The sporozoites released from the oocysts are mobile: they migrate through the intestinal epithelium and enter endothelial cells of small arteries where the first of four asexual stages occurs, leading to the formation of many merozoites. Subsequent generations of merozoites develop downstream in the direction of blood flow and then develop the final asexual generation in muscles with sarcocyst formation and maturation. Sarcocysts can form in virtually all striated muscles, including tongue, esophagus, diaphragm, heart and, to a lesser extent, smooth muscle. For microscopic diagnosis, histological examination of muscular biopsy specimens is advocated.

Cases of human muscular sarcocystosis, although described in the literature, are quite rare and mostly reported from Asian countries. Sarcocystosis in man is in fact very frequently asymptomatic but it can cause intestinal symptoms including nausea, stomachache, and diarrhea. Data on the prevalence of human intestinal sarcocystosis is limited but the infection is certainly more frequent in Europe than on other continents. The prevalence of sarcocystosis is probably underestimated since oocysts are hardly detected in stool examination because present in small numbers and only for few days. Infection can be prevented by thoroughly cooking or freezing meat to kill bradyzoites in the sarcocysts.

Oocysts

Size: 15-19 μm long, 15-20 μm wide.

Morphology: the intact oocyst has a very fragile, thin wall, containing two strictly paired ellipsoidal sporocysts (15-19 μm long, 8-10 μm wide), each containing four sausage-shaped sporozoites and a refractile residual body. The prepatent period ranges from 14 to 18 days for Sarcocystis hominis and from 11 to 13 days for Sarcocystis suihominis. Stool examination should be performed using a density-flotation concentration method (e.g. on sodium chloride, zinc sulfate, or sucrose gradients), rather than sedimentation (e.g. with formalin-ether). Sarcocystis autofluoresces under UV light. Because sporocysts of different Sarcocystis species overlap in shape and size, the species cannot be distinguished solely by microscopy. Molecular biological methods involving the PCR amplification of species-specific rRNA may be used for this purpose.

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