Subtitle: How to tell Cryptosporidium, Cyclospora, Cystoisospora, microsporidia, Giardia and Entamoeba apart on GI biopsies: size, position in the cell, stains, clinical clues, treatment, a virtual slide and 10 MCQs.

Author: PathologyMCQ Editorial Board

Category: GI Pathology / Infectious Pathology

Last Updated: October 2026

Read Time: 9 minutes

At-a-Glance

• Where it sits: on the brush border (Cryptosporidium), inside a supranuclear vacuole (Cystoisospora, Cyclospora, microsporidia), or in the lumen (Giardia, Entamoeba).
• Size: Cystoisospora is largest; Cyclospora is intermediate; Cryptosporidium is small; microsporidia are tiniest.
• Stains: modified acid-fast for the coccidia (Cyclospora variably); modified trichrome for microsporidia; PAS for Entamoeba.
• Note: microsporidia are now classified with the fungi, not the protozoa, but they are still taught alongside them.

The Four Intracellular Organisms at a Glance

Small bowel biopsySize, site and stain

CryptosporidiumOn the brush border2 to 5 µm in tissueBasophilic beads on the surface; acid-fast in stool

CyclosporaSupranuclear vacuole8 to 10 µm oocystsVariably acid-fast; autofluorescent; fresh produce

CystoisosporaSupranuclear vacuoleLargest oocystsBanana-shaped merozoites; eosinophils in tissue

MicrosporidiaSupranuclear clusters1 to 2.5 µm sporesModified trichrome pink; Gram-positive; now fungi

Quick rule: on the surface means Cryptosporidium; inside the cell above the nucleus means one of the other three, and size plus stains decide.

Cryptosporidium

• Histology: small round basophilic bodies, 2 to 5 µm, on the microvillous surface of small bowel or colon. Villi may be blunted with crypt hyperplasia and mild inflammation.
• Location: intracellular but extracytoplasmic. The organism lies in a parasitophorous vacuole covered by host membrane at the brush border.
• Stains: visible on H&E; stool oocysts (4 to 6 µm) are modified acid-fast positive. Stool antigen tests and PCR are more sensitive.
• Clinical: self-limited watery diarrhoea in healthy people (often waterborne, including swimming pools); chronic, severe diarrhoea in AIDS. Can involve the biliary tree.
• Treatment: nitazoxanide; in AIDS, antiretroviral therapy to restore immunity matters most.

Virtual Slide: Cryptosporidium

Cystoisospora belli

• Histology: intracytoplasmic parasitophorous vacuoles, usually above the nucleus, containing merozoites (banana-shaped), schizonts or gametocytes. Villous blunting and many eosinophils in the lamina propria are common.
• Stains: stool oocysts are large and ellipsoidal (about 20 to 30 µm long) and modified acid-fast positive.
• Clinical: watery diarrhoea, mainly in immunocompromised patients; peripheral eosinophilia is a useful clue.
• Treatment: trimethoprim-sulfamethoxazole.

Cyclospora cayetanensis

• Histology: intracytoplasmic parasitophorous vacuoles similar to Cystoisospora but smaller, with mild villous blunting and inflammation.
• Stains: stool oocysts are 8 to 10 µm, stain variably with modified acid-fast (from unstained to deep red), and glow blue-green under UV light (autofluorescence).
• Clinical: prolonged, relapsing watery diarrhoea and fatigue. It affects healthy people, often travellers or outbreaks linked to imported fresh produce.
• Treatment: trimethoprim-sulfamethoxazole.

Microsporidia

• Organisms: Enterocytozoon bieneusi (most common in the gut) and Encephalitozoon intestinalis.
• Histology: clusters of tiny spores (1 to 2.5 µm) above the nucleus of enterocytes, mainly at villous tips. Easy to miss on H&E.
• Stains: modified trichrome (pink spores), Gram-positive, Giemsa; electron microscopy shows the coiled polar tube.
• Classification: now placed with the fungi rather than the protozoa.
• Treatment: albendazole for Encephalitozoon; fumagillin for E. bieneusi; immune restoration in AIDS.

Giardia and Entamoeba

• Giardia lamblia: pear- or sickle-shaped binucleate trophozoites in the mucus over duodenal villi. The mucosa is often normal. If plasma cells are absent or there is nodular lymphoid hyperplasia, think of common variable immunodeficiency.
• Entamoeba histolytica: colonic flask-shaped ulcers with trophozoites in the necrotic debris. Trophozoites have a round nucleus with a central karyosome, are PAS-positive and contain ingested red cells. Do not confuse them with macrophages.
• Treatment: metronidazole or tinidazole, with a luminal agent for amoebiasis.

Comparison Table

Test Yourself: 10 MCQs

Exam Pearls

• Cryptosporidium sits on the brush border: intracellular but extracytoplasmic.
• Cystoisospora: supranuclear vacuole, banana-shaped merozoites, eosinophilia.
• Cyclospora: variably acid-fast, autofluorescent, fresh produce outbreaks.
• TMP-SMX treats both Cystoisospora and Cyclospora.
• Microsporidia: tiniest spores, modified trichrome pink, now classified with fungi.
• Giardia with absent plasma cells: think of common variable immunodeficiency.

Video lesson

GI protozoa, explained

How to tell the intestinal protozoa apart on biopsy, slide by slide.

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References

• Odze RD, Goldblum JR, eds. Surgical Pathology of the GI Tract, Liver, Biliary Tract and Pancreas. 4th ed. Elsevier; 2023.
• Lamps LW. Infectious disease of the gastrointestinal tract. In: Diagnostic Pathology: Gastrointestinal. Elsevier.
• Centers for Disease Control and Prevention. DPDx: laboratory identification of parasites of public health concern (Cryptosporidiosis, Cyclosporiasis, Cystoisosporiasis, Microsporidiosis).
• Kumar V, Abbas AK, Aster JC. Robbins and Cotran Pathologic Basis of Disease. 10th ed. Elsevier; 2020.