Parasite
Cystoisospora belli
Large elliptical oocyst coccidian (former Isospora)
sis-toh-eye-SOS-por-uh BELL-eye
High-yield clue
A large elliptical (~25-30 micron) oocyst containing two sporoblasts, acid-fast and autofluorescent, is the key study clue.
Overview
An intestinal coccidian (formerly Isospora belli) that causes chronic watery diarrhea, especially in immunocompromised and AIDS patients. It matters as the large-oocyst counterpart to Cyclospora and Cryptosporidium.
Classification
- Protozoa
- Apicomplexa
- Coccidian
- Oocyst-forming intestinal parasite
Lab & identification clues
- Large ellipsoidal oocyst ~25-30 microns
- Immature oocyst matures to two sporoblasts
- Modified acid-fast staining
- Oocyst autofluorescence vocabulary
Associations
- Chronic diarrhea in AIDS/immunocompromised hosts
- Fecal-oral via contaminated food/water
- Tropical/subtropical epidemiology framing
- Peripheral eosinophilia study association
Commonly confused with
- Cyclospora cayetanensis
- Cryptosporidium
Your notes
Original student-study summary. Sources checked: OpenStax Microbiology 2e, NCBI Bookshelf Medical Microbiology, and CDC topic pages where applicable; reviewed 2026-06. Educational only; no diagnosis, treatment, dosing, or specimen-handling guidance.