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Cyclospora cayetanensis is a coccidian parasite of humans of known and growing importance. However, we are surprisingly naïve as to our understanding of how to diagnose it and how it develops inside the human body. Here we provide details of the developmental stages of C. cayetanensis in the gallbladder of a 33-yr-old male with human immunodeficiency virus. The gallbladder was removed surgically in 2001 because of severe abdominal pain. For the present study, the archived paraffin block of gallbladder was processed for light microscopy and transmission electron microscopy (TEM). Histological sections were examined after staining with hematoxylin and eosin (HE) or using the periodic acid Schiff (PAS) reaction. Immature and mature asexual stages, gamonts, and oocysts were seen in epithelial cells, both in the superficial epithelium and in glands. The merozoites were present singly, in pairs, and 3 or more in a single parasitophorous vacuole in the host cytoplasm. Up to 6 nuclei were seen in immature schizonts without evidence of merozoite formation. Mature schizonts were 7.6 × 5.1 μm and contained up to 10, 3-4 μm long merozoites. Merozoites were 0.6 to 2.0 μm wide, and their shape varied from pear-shaped to slender. Merozoites were generally PAS-positive; however, some were intensely positive, some had only minute granules, while others were PAS-negative. The microgamonts (male) were 6.6 × 5.2 μm and contained fewer than 20 microgametes around a residual body. The microgametes were up to 2 μm long and were flagellated. Macrogamonts (female) contained distinctive eosinophilic wall-forming bodies that varied in size and were less than 1 μm in HE-stained sections. Macrogamonts were 5.8-6.5 × 5.3-6.5 μm. Oocysts in sections were unsporulated and had a diameter of 5.7-7.5 μm. The TEM examination confirmed the histologic findings. The DNA extracted from paraffin sections was confirmed as C. cayetanensis with real-time PCR. The detailed description of the life cycle stages of C. cayetanensis reported here in an immunosuppressed patient could facilitate histopathologic diagnosis of this parasite. We have shown that the parasite's development more closely resembles that of Cystoisospora than Eimeria and that the parasite has multiple nuclei per immature meront indicating schizogony, and we have undermined evidence for a Type II meront.

Endogenous Developmental Cycle of the Human Coccidian Cyclospora cayetanensis