Sarcocystis spp. in human infections

Abstract

Sarcocystis species are intracellular protozoan parasites with an intermediate-definitive host life cycle based on a prey-predator relationship. Asexual stages develop in intermediate hosts after they ingest the oocyst stage from definitive-host feces and terminate with the formation of intramuscular cysts (sarcocysts). Sarcocysts in meat eaten by a definitive host initiate sexual stages in the intestine that terminate in oocysts excreted in the feces. Most Sarcocystis species infect specific hosts or closely related host species. For example, humans and some primates are definitive hosts for Sarcocystis hominis and S. suihominis after eating raw meat from cattle and pigs, respectively. The prevalence of intestinal sarcocystosis in humans is low and is only rarely associated with illness, except in volunteers who ingest large numbers of sarcocysts. Cases of infection of humans as intermediate hosts, with intramuscular cysts, number less than 100 and are of unknown origin. The asexual stages, including sarcocysts, can stimulate a strong inflammatory response. Livestock have suffered acute debilitating infections, resulting in abortion and death or chronic infections with failure to grow or thrive. This review provides a summary of Sarcocystis biology, including its morphology, life cycle, host specificity, prevalence, diagnosis, treatment, and prevention strategies, for human and food animal infections.

FIG. 1.

S. cruzi. Differential interference contrast microscopy shows intact oocysts containing two adjacent sporocysts, free sporocysts each containing four sporozoites, and free sporozoites. Magnification, ×1,000.

FIG. 2.

Asexual multinucleate stage (schizont) of S. cruzi in an endothelial cell protruding into the lumen of a small blood vessel in the lung. Hematoxylin and eosin stain. Magnification, ×1,000.

FIG. 3.

Merozoite of S. cruzi in a blood smear. Wright's stain. Magnification, ×2,400.

FIG. 4.

Immature sarcocyst of S. cruzi in skeletal muscle. The sarcocyst contains noninfectious metrocytes (mother cells) but no bradyzoites. Hematoxylin and eosin stain. Magnification, ×500.

FIG. 5.

Numerous sarcocysts in longitudinal and cross section in muscles of sheep tongue. Note the lack of direct inflammatory response to sarcocysts. Hematoxylin and eosin stain. Magnification, ×100.

FIG. 6.

Cross section of a sarcocyst in a skeletal muscle biopsy specimen from a human. Hematoxylin and eosin stain. Magnification, ×340.

FIG. 7.

S. cruzi. Differential interference contrast microscopy shows bradyzoites released from a sarcocyst. Magnification, ×1,000.