Which parasite is associated with large, oval, ellipsoidal oocysts that require acid-fast staining for visualization in stool?

• A. Cyclospora cayetanensis
• B. Giardia lamblia
• C. Entamoeba histolytica
• D. Cystoisospora belli

Answer: (D)

Acid-fast staining is used to visualize certain coccidian oocysts that have a waxy, mycolic-acid–rich wall. Cystoisospora belli forms the largest, oval-to-ellipsoidal oocysts in stool and these oocysts are acid-fast positive when viewed with a modified acid-fast stain. Their distinctive size and shape set them apart from other intestinal parasites: Giardia cysts are smaller and not acid-fast; Entamoeba histolytica cysts are round and not acid-fast; Cyclospora cayetanensis oocysts are acid-fast variably staining and generally spherical rather than ellipsoidal. So the combination of large, oval oocysts visible only with acid-fast staining points to Cystoisospora belli.