Intestinal Protozoa
Microbiology · Parasitology · lean revision notes
Intestinal Protozoa
Intestinal protozoa are single-celled eukaryotic parasites that colonise the gut lumen or mucosa, producing diarrhoea, dysentery, malabsorption and (with Entamoeba histolytica) extraintestinal abscesses. For NEET PG, the trio of E. histolytica, Giardia lamblia and Cryptosporidium dominates, with cyst morphology, named lesions and drugs of choice being the recurring MCQ themes.
Classification of medically important intestinal protozoa
Intestinal protozoa are grouped by their mode of motility/organelles. A quick mental map prevents confusion in single-best-answer questions.
| Group | Locomotion organelle | Key intestinal species | Hallmark disease |
|---|---|---|---|
| Amoebae (Sarcodina) | Pseudopodia | Entamoeba histolytica, E. dispar (non-pathogenic), E. coli (commensal) | Amoebic dysentery, liver abscess |
| Flagellates | Flagella | Giardia lamblia (intestinalis/duodenalis), Dientamoeba fragilis | Steatorrhoea, malabsorption |
| Ciliates | Cilia | Balantidium coli (largest protozoan; pig reservoir) | Dysentery resembling amoebiasis |
| Apicomplexa (Coccidia) | None (gliding) | Cryptosporidium, Cyclospora cayetanensis, Cystoisospora belli | Watery diarrhoea, esp. in HIV/AIDS |
| Microsporidia (now fungi) | — | Enterocytozoon bieneusi | Chronic diarrhoea in AIDS |
High-yield: Only E. histolytica is the true pathogen among intestinal amoebae. E. dispar is morphologically identical but harmless; distinguishing them needs antigen detection or PCR, not microscopy.
Entamoeba histolytica
Morphology — the MCQ goldmine
Two stages exist: the trophozoite (invasive, motile, dies outside body) and the cyst (infective, resistant).
- Trophozoite: 15–30 µm, single nucleus with a central karyosome and fine, evenly distributed peripheral chromatin. Cytoplasm shows ingested RBCs (erythrophagocytosis) — the single most reliable feature distinguishing pathogenic E. histolytica from commensals. Pseudopodia give rapid, directional (unidirectional) motility.
- Cyst: 10–15 µm, mature cyst has 4 nuclei (quadrinucleate). Immature cysts contain chromatoid bars/bodies with rounded/blunt ends (aggregated ribosomes) and a glycogen vacuole that disappears on maturation.
| Feature | E. histolytica | E. coli (commensal) |
|---|---|---|
| Trophozoite size | 15–30 µm | 20–50 µm |
| Karyosome | Central | Eccentric |
| Peripheral chromatin | Fine, even | Coarse, clumped, irregular |
| RBCs ingested | Yes (diagnostic) | No (bacteria/debris) |
| Mature cyst nuclei | 4 | 8 |
| Chromatoid bar ends | Rounded/blunt | Splintered/pointed |
High-yield: Mature cyst nuclei — E. histolytica = 4, E. coli = 8, E. hartmanni = 4 (but smaller, <10 µm). Chromatoid bars with rounded ends = histolytica; splinter-like pointed ends = coli.
Life cycle & pathophysiology
Mode of infection is faeco-oral via mature quadrinucleate cyst in contaminated food/water. Excystation occurs in the small intestine → 8 trophozoites → colonise the caecum and ascending colon.
Invasion flow: Cyst ingested → excystation in ileum → trophozoites in colon → adhere via Gal/GalNAc lectin → kill epithelium by amoebapore (pore-forming peptide) + cysteine proteases → lateral spread under mucosa → flask-shaped ulcer → portal spread → liver abscess.
High-yield: The flask-shaped ulcer (narrow neck at mucosa, broad base in submucosa) is pathognomonic. Lesions are most common in caecum > sigmoidorectum.
Clinical features
- Intestinal amoebiasis: Insidious dysentery — loose stools with blood and mucus but little pus/few leucocytes (contrast bacillary dysentery, which is pus-rich). Tenesmus, colicky pain, low-grade or no fever. Amoeboma = annular granulomatous mass mimicking colonic carcinoma. Fulminant necrotising colitis and toxic megacolon are dreaded.
- Amoebic liver abscess (ALA): Commonest extraintestinal form. Right lobe of liver, single, postero-superior. Tender hepatomegaly, fever, point tenderness in intercostal space. Aspirate is reddish-brown "anchovy sauce" / chocolate-coloured pus that is odourless and sterile (trophozoites lie in the abscess wall, not centre — so aspirate is often negative on microscopy).
High-yield: Anchovy-sauce pus = amoebic liver abscess. It is odourless and sterile; foul-smelling pus suggests pyogenic abscess. Serology is strongly positive in ALA.
Diagnosis
- Stool microscopy: Fresh, warm "hot stool" exam for motile RBC-containing trophozoites (dysentery) or cysts (carriers). Three samples raise sensitivity.
- Stool antigen detection (ELISA for Gal/GalNAc lectin) and PCR — only methods that reliably separate E. histolytica from E. dispar.
- Serology (IHA/ELISA): Highly sensitive for invasive/extraintestinal disease; positive in >90% of liver abscess. Stays positive for years (cannot distinguish current vs past).
- Imaging: USG/CT for liver abscess. Sigmoidoscopy shows flask ulcers; scraping edge yields trophozoites.
High-yield: Investigation of choice to differentiate E. histolytica from E. dispar = faecal antigen detection / PCR. Stool microscopy alone cannot do it.
Management — drug of choice
Tissue + luminal coverage flow: Invasive disease (colitis/abscess) → Metronidazole (or tinidazole) → always follow with a luminal amoebicide (paromomycin / diloxanide furoate) to clear cysts and prevent relapse.
| Site | Drug class | Drugs |
|---|---|---|
| Tissue (colitis, liver abscess) | Tissue amoebicide | Metronidazole, tinidazole, ornidazole |
| Lumen (cyst carriers) | Luminal amoebicide | Diloxanide furoate, paromomycin, iodoquinol |
| Liver abscess (large/non-responding/left lobe/imminent rupture) | Drainage | Image-guided percutaneous aspiration |
High-yield: A patient treated with metronidazole alone may relapse because nitroimidazoles do not eradicate intraluminal cysts — add diloxanide furoate/paromomycin. Asymptomatic cyst passers are treated with a luminal agent only.
Complications
Intestinal perforation, toxic megacolon, amoeboma, peritonitis; liver abscess rupture into pleura (most common), peritoneum, or pericardium (most dangerous); lung abscess with expectoration of anchovy-sauce sputum (hepatobronchial fistula).
Giardia lamblia (G. intestinalis / G. duodenalis)
The commonest protozoal cause of diarrhoea worldwide and the prototypical cause of malabsorption from a non-invasive parasite.
Morphology
- Trophozoite: Pear/tear-drop (pyriform), bilaterally symmetrical, 2 nuclei, 4 pairs of flagella, a ventral sucking/adhesive disc, and 2 axostyles. Classic descriptions: "face-like / monkey-face" or "old-man-with-spectacles" appearance, and "falling-leaf" motility.
- Cyst: Oval, 4 nuclei (mature), retracted cytoplasm; the infective and diagnostic stage in stool.
High-yield: Giardia trophozoite = "falling-leaf" tumbling motility + monkey-face; cyst is the infective form. It is the classic non-invasive cause of steatorrhoea and fat-soluble vitamin malabsorption.
Pathophysiology & clinical features
Faeco-oral transmission of cysts; low infective dose (~10–25 cysts). Trophozoites attach to duodenal/jejunal mucosa by the sucking disc → villous atrophy, brush-border enzyme deficiency, mechanical coating of mucosa → malabsorption without tissue invasion.
Features: foul-smelling, greasy, bulky stools (steatorrhoea), bloating, flatulence, abdominal cramps, weight loss; chronic infection causes malabsorption and failure to thrive in children. Associated with hypogammaglobulinaemia (IgA deficiency), cystic fibrosis, and travellers ("backpacker's/beaver fever"). Person-to-person spread is notable in day-care centres and men who have sex with men.
High-yield: Giardia does not invade tissue and does not cause eosinophilia (true of most protozoa). Recurrent giardiasis should prompt evaluation for IgA deficiency / common variable immunodeficiency.
Diagnosis
- Stool microscopy: Cysts in formed stool, trophozoites in diarrhoeic stool. Excretion is intermittent → examine 3 samples.
- Stool antigen ELISA / DFA — high sensitivity, now first-line in many labs.
- String test (Entero-test) or duodenal aspirate/biopsy when stool repeatedly negative — duodenal biopsy shows trophozoites on the epithelial surface.
High-yield: If stool exam is negative thrice in a strongly suspected case → string test / duodenal aspirate ("examine duodenal contents"). Stool antigen detection is the most sensitive routine test.
Management
Drug of choice = Metronidazole (or tinidazole single dose). Alternatives: nitazoxanide (also good in children), albendazole, paromomycin (safe in pregnancy).
Cryptosporidium (C. parvum / C. hominis)
A coccidian parasite that is a leading cause of waterborne diarrhoea outbreaks and chronic life-threatening diarrhoea in AIDS.
Morphology & cycle
The infective and diagnostic stage is the thick-walled oocyst (~4–6 µm) containing 4 sporozoites; it is sporulated and immediately infective when passed (allowing autoinfection and person-to-person spread). It is chlorine-resistant — hence pool/municipal water outbreaks (e.g., Milwaukee 1993).
High-yield: Cryptosporidium oocysts are acid-fast and stain pink/red with modified Ziehl-Neelsen (modified Kinyoun) stain. They are chlorine-resistant — boiling/filtration is needed.
Clinical features
- Immunocompetent: Self-limited profuse watery (non-bloody, no leucocytes) diarrhoea lasting 1–2 weeks.
- HIV/AIDS (CD4 <100): Severe, chronic, cholera-like watery diarrhoea, dehydration, weight loss; may involve biliary tree (AIDS cholangiopathy/sclerosing cholangitis) and respiratory tract.
| Coccidian | Stain/Size | Key clue |
|---|---|---|
| Cryptosporidium | Acid-fast, 4–6 µm | Chlorine-resistant; AIDS watery diarrhoea |
| Cyclospora cayetanensis | Acid-fast (variable), 8–10 µm; autofluorescent (UV) | Imported berries/herbs; relapsing diarrhoea |
| Cystoisospora (Isospora) belli | Acid-fast; large ellipsoidal oocyst ~25 µm | AIDS; eosinophilia present (exception!) |
High-yield: Among coccidia, Cystoisospora belli is the protozoan that classically causes eosinophilia. Cyclospora oocysts autofluoresce under UV.
Diagnosis & management
- Modified acid-fast (Kinyoun) stain of stool; antigen ELISA/DFA and PCR also used.
- Management: Immunocompetent → supportive/rehydration ± nitazoxanide (drug of choice). In AIDS, the most effective therapy is immune reconstitution with ART (raising CD4). Cyclospora and Cystoisospora respond to cotrimoxazole (TMP-SMX) — note nitazoxanide does not reliably work for these.
High-yield: Drug of choice — Cryptosporidium → nitazoxanide + ART; Cyclospora & Cystoisospora → cotrimoxazole.
Other tested intestinal protozoa
- Balantidium coli: Largest human protozoan and only pathogenic ciliate; reservoir is pigs. Causes dysentery; trophozoite has cilia and a kidney-shaped macronucleus. Treat with tetracycline (DOC), alternative metronidazole.
- Dientamoeba fragilis: Flagellate (no cyst) with binucleate trophozoite; transmitted by Enterobius eggs; causes chronic diarrhoea. Treat with iodoquinol/metronidazole.
- Blastocystis hominis: Variable pathogenicity; central body form; metronidazole if symptomatic.
Key differentials
Bloody/mucoid dysentery (few leucocytes, insidious) → amoebiasis vs pus-rich, acute, febrile dysentery → bacillary (Shigella) dysentery. Greasy, foul, non-bloody malabsorptive stool → Giardia. Profuse watery diarrhoea in AIDS, acid-fast oocyst → Cryptosporidium. Right-lobe liver abscess + anchovy pus + positive serology → amoebic, whereas multiple/foul/left-sided/bacteria-positive → pyogenic.
| Parameter | Amoebic dysentery | Bacillary (Shigella) dysentery |
|---|---|---|
| Onset | Gradual | Acute |
| Fever | Low/absent | High |
| Stool | Blood + mucus, few cells, offensive | Scanty, many pus cells & RBCs, odourless |
| Cause | E. histolytica | Shigella |
| Microscopy | RBC-containing trophozoites | Pus cells |
Recently asked / exam angle
- Image-based: identify quadrinucleate cyst with rounded chromatoid bars → E. histolytica; 8-nucleated cyst → E. coli.
- "Trophozoite containing ingested RBCs" — single best clue for pathogenic E. histolytica.
- Anchovy-sauce pus, flask-shaped ulcer, Gal/GalNAc lectin adhesin.
- Differentiating histolytica from dispar → antigen/PCR (microscopy cannot).
- "Falling-leaf motility / monkey-face" + steatorrhoea + IgA deficiency → Giardia; investigation when stool negative → string test/duodenal aspirate.
- Acid-fast oocyst + chlorine-resistant + AIDS watery diarrhoea → Cryptosporidium; DOC nitazoxanide.
- Eosinophilia-causing intestinal protozoan = Cystoisospora belli; autofluorescent oocyst = Cyclospora; both treated with cotrimoxazole.
- Largest protozoan / only pathogenic ciliate / pig reservoir → Balantidium coli (DOC tetracycline).
- Mnemonic for amoebic liver abscess therapy: "Metronidazole then Diloxanide" — never stop at metronidazole.
High-yield: Most intestinal protozoa do not cause peripheral eosinophilia — the standout exception examiners exploit is Cystoisospora belli.
Rapid revision
- E. histolytica mature cyst = 4 nuclei, central karyosome, rounded chromatoid bars; E. coli = 8 nuclei, splintered bars.
- Erythrophagocytosis (ingested RBCs) = the diagnostic feature of pathogenic E. histolytica trophozoite.
- Adhesion via Gal/GalNAc lectin; killing by amoebapore + cysteine protease → flask-shaped ulcer (caecum, sigmoidorectum).
- Amoebic liver abscess = right lobe, single, anchovy-sauce odourless sterile pus; trophozoites in abscess wall; serology strongly positive.
- Histolytica vs dispar → only antigen ELISA / PCR; microscopy is inadequate.
- Invasive amoebiasis = metronidazole + luminal agent (diloxanide furoate/paromomycin); cyst carriers = luminal agent alone.
- Giardia = falling-leaf motility, monkey-face, 2 nuclei, 4 flagella pairs, ventral sucking disc; non-invasive steatorrhoea.
- Recurrent giardiasis → think IgA deficiency / CVID; string test if stool negative; DOC metronidazole/tinidazole.
- Cryptosporidium oocyst = acid-fast, 4–6 µm, chlorine-resistant, immediately infective; severe diarrhoea at CD4 <100; DOC nitazoxanide + ART.
- Cyclospora oocysts autofluoresce; Cystoisospora belli causes eosinophilia; both → cotrimoxazole.
- Balantidium coli = largest protozoan, only pathogenic ciliate, pig reservoir, DOC tetracycline.
- Amoebic vs bacillary dysentery: amoebic = gradual, blood+mucus, few cells, offensive; bacillary = acute, febrile, pus-cell-rich.