Sarcocystis infection of human muscle.

A 31-year-old man with fatigue and muscle aches was found to have protozoan cysts within the most painful muscle upon microscopic examination of biopsy material. Electron microscopic studies revealed the ultrastructural characteristics of the cysts to conform to those of the genus Sarcocystis. This report represents the first ultrastructural study of human tissue infection with Sarcocystis, which may be a rare cause of muscle aching in humans.