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Microbiology Diagnosis of Intestinal and Urogenital Protozoa

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Overview of Diagnostic Approaches

1. Microscopy — wet mount, concentration techniques, permanent stained smears (trichrome, iron hematoxylin, modified acid-fast)
2. Culture — selected organisms only
3. Antigen detection — EIA/immunochromatographic assays
4. Molecular methods — NAATs/PCR (increasingly the gold standard)

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1. Entamoeba histolytica (Amebae)

Morphology

• Trophozoite: 12–60 µm; single nucleus with central karyosome and fine peripheral chromatin; pseudopods; motile. The hallmark diagnostic feature is ingested erythrocytes within the cytoplasm (distinguishes E. histolytica from the morphologically identical E. dispar).
• Cyst: 10–20 µm; round; 1–4 nuclei; chromatoid bars with rounded ends; glycogen vacuole.

Specimen Collection

• Stool (primary): fresh liquid stools (trophozoites) or formed stools (cysts). Fix immediately in PVA or SAF to preserve fragile trophozoites.
• Liquid diarrheal specimens or sigmoidoscopic aspirates/biopsies may yield trophozoites.
• Liver abscess aspirate: reddish-brown "anchovy paste" fluid; trophozoites found at the periphery, not in the necrotic center.

Diagnostic Methods

Key diagnostic point: Microscopy alone cannot distinguish E. histolytica from E. dispar. Only erythrophagocytosis (trophozoites with ingested RBCs), antigen testing, or PCR can confirm true E. histolytica. Serology is the test of choice for amebic liver abscess. — Sherris & Ryan's Medical Microbiology, 8th ed.

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2. Balantidium coli / Balantioides coli (Ciliates)

Morphology

• Trophozoite: the largest intestinal protozoan (~50–100 µm × 40–70 µm); entire surface covered with cilia; prominent kidney-bean-shaped macronucleus (for identification); smaller micronucleus beside it; cytostome (mouth) at anterior end.
• Cyst: 50–70 µm; spherical; enclosed in a thick double wall; the macronucleus and cilia may still be visible.

Specimen

• Stool (fresh or fixed). Trophozoites are found in liquid/dysenteric stools; cysts in formed stools.
• Intestinal biopsy in invasive cases (colonic ulcers).

Diagnostic Methods

No antigen or PCR tests are widely available; diagnosis rests on microscopic morphology. The organism is distinctive enough that experienced microscopists rarely miss it. — Henry's Clinical Diagnosis and Management by Laboratory Methods

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3. Giardia lamblia / duodenalis (Flagellates)

Morphology

• Trophozoite: 9–21 µm × 5–15 µm; stingray/pear-shaped; 2 nuclei resembling spectacled eyes; large ventral sucking disc; 4 pairs of flagella; "falling leaf" tumbling motility in fresh specimens.
• Cyst: 8–19 µm; oval; 4 nuclei (when mature); curved median bodies give a "smiley face" appearance; cytoplasm may be retracted from wall.

Specimen

• Stool (primary): cyst shedding is intermittent — examine 3 specimens collected on separate days for >90% sensitivity.
• Duodenal aspirate or small bowel biopsy (when stool exams negative but clinical suspicion high).
• Duodenal string test (Enterotest capsule): swallowed capsule retrieves bile-stained duodenal fluid.

Diagnostic Methods

Barium studies are not useful and impair subsequent stool examination. — Yamada's Textbook of Gastroenterology, 7th ed.

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4. Trichomonas vaginalis (Flagellates)

Morphology

• Trophozoite only (no cyst stage): 10–23 µm; pear-shaped; 4 anterior flagella + 1 posterior flagellum forming the outer edge of an undulating membrane that extends only halfway along the body (differentiates it from Pentatrichomonas hominis); prominent axostyle protrudes posteriorly; single nucleus.
• Motility: tumbling/rotary in fresh wet mounts.

Specimen

• Female: vaginal swab (highest yield), cervical swab, urine sediment.
• Male: urethral swab, prostatic fluid, urine sediment (first-catch).

Diagnostic Methods

NAAT is now the recommended method per CDC 2021 STI guidelines. Wet mount is acceptable only when NAATs are unavailable. — Henry's Clinical Diagnosis and Management by Laboratory Methods

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5. Cryptosporidium parvum (Sporozoa / Coccidia)

Morphology

• Oocyst (the diagnostic stage in stool): small, 4–6 µm; round/oval; contains 4 sporozoites (no sporocyst); passed fully sporulated and immediately infectious.
• Trophozoites, merozoites, and other stages are found within the brush-border vacuoles of intestinal epithelial cells on biopsy.

Specimen

• Stool (primary): formed or watery stool in 10% formalin or SAF. Multiple samples increase sensitivity.
• Intestinal biopsy in immunocompromised patients with heavy burden.
• Bile/respiratory specimens in disseminated immunocompromised infection.

Diagnostic Methods

Routine O&P exam often misses Cryptosporidium — laboratories must perform modified acid-fast or EIA specifically when requested. — Yamada's Textbook of Gastroenterology, 7th ed.

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6. Cystoisospora belli (formerly Isospora belli) (Sporozoa / Coccidia)

Morphology

• Oocyst: large, ~12 × 30 µm; elongated ellipse; when freshly passed, contains a single sporoblast (unsporulated); after 24–48 h at room temperature → 2 sporocysts, each containing 4 sporozoites (total: 8 sporozoites). This is the largest intestinal coccidian oocyst.
• Eosinophilia may be present (unique among intestinal protozoa).

Specimen

• Stool (primary): oocyst shedding is low-level and intermittent — examine multiple stool samples; oocyst concentration helps.
• Duodenal aspirate or small-bowel biopsy (with electron microscopy) when stool exams repeatedly negative.

Diagnostic Methods

The large size (~25–30 µm) distinguishes C. belli oocysts from those of Cryptosporidium (4–6 µm) and Cyclospora (8–10 µm). — Harrison's Principles of Internal Medicine, 22nd ed.

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Comparative Summary Table

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Key Diagnostic Pearls

• E. histolytica vs. E. dispar: Morphologically identical — erythrophagocytosis OR antigen EIA/PCR required for definitive species identification.
• Balantidium coli: Largest intestinal protozoan; the macronucleus and cilia make it unmistakable on wet mount.
• Giardia: Cyst shedding is intermittent; examine 3 stools on separate days. EIA/NAAT now routinely preferred over microscopy.
• Trichomonas vaginalis: No cyst stage — only trophozoites; wet mount is only 51–65% sensitive; NAAT is now the gold standard.
• Cryptosporidium: Must specifically request modified acid-fast stain — not detected on routine O&P; tiny oocysts (4–6 µm) may be overlooked.
• Cystoisospora belli: The largest coccidian oocyst in stool; eosinophilia is a distinguishing clue; low-level shedding mandates multiple specimens.

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