Intestinal Protozoa

Intestinal Protozoa Overview - Gut Invaders Intro

• Microscopic, unicellular eukaryotic parasites colonizing human intestines, causing a spectrum of gastrointestinal diseases.
• A major global health burden, prevalent in regions with poor sanitation and contaminated water sources.
• Pathogenesis: mucosal adherence/invasion, toxin production, host inflammation, nutrient malabsorption.
• Key groups by motility:
  • Amoebae (e.g., Entamoeba): use pseudopods.
  • Flagellates (e.g., Giardia): possess flagella.
  • Ciliates (e.g., Balantidium): use cilia.
  • Apicomplexa (Sporozoa; e.g., Cryptosporidium): complex life cycles, apical complex.

⭐ Most intestinal protozoa transmit fecal-orally, typically via ingestion of resistant cysts or oocysts.

Entamoeba histolytica - Amoebic Menace

• Infective form: Cyst (quadrinucleate); Invasive form: Trophozoite.
• Transmission: Feco-oral route.
• Pathology: Flask-shaped ulcers in colon. 📌 Mnemonic: "Amoeba digs a FLASK to HIDE".
• Clinical Syndromes:
  • Intestinal: Asymptomatic carriage; Amoebic dysentery (blood & mucus).
  • Extra-intestinal: Amoebic liver abscess (RUQ pain, fever; "anchovy sauce" pus).
• Diagnosis:
  • Microscopy: Trophozoites with ingested RBCs in stool/biopsy; Cysts in stool.
  • Antigen detection in stool; Serology for invasive disease.
• Treatment: Metronidazole (tissue amoebicide) followed by a luminal amoebicide (e.g., Diloxanide furoate).

⭐ Ingested RBCs in the trophozoite stage is diagnostic for E. histolytica.

Giardia lamblia - Beaver Fever Blues

• Flagellated protozoan; causes giardiasis ("Beaver Fever"). Most common intestinal protozoan in US.
• Forms:
  • Trophozoite: Pear-shaped, binucleate (owl-eye), 4 pairs flagella. Motility: 📌 "Falling leaf".
  • Cyst: Oval, 4 nuclei. Infective, environmentally resistant. Transmitted via contaminated water.
• Pathogenesis: Attaches to duodenal/jejunal mucosa → villous blunting, malabsorption (fat, vitamins A, D, E, K, B12).
• Clinical: Non-bloody, foul-smelling diarrhea (steatorrhea), cramps, bloating, flatulence, weight loss.

  ⭐ Giardiasis is known for causing foul-smelling steatorrhea and malabsorption.

• Diagnosis: Stool O&P (cysts/trophozoites), ELISA (stool antigen), string test (Entero-test).
• Treatment: Metronidazole, Tinidazole, Nitazoxanide. Consider lactose intolerance post-infection.

Intestinal Coccidia - Tiny Gut Wreckers

• Opportunistic protozoa causing diarrhea, severe in immunocompromised (CD4 < 200 cells/µL).
• Diagnosis: Oocysts in stool via Modified Acid-Fast Stain (MAFS). 📌 "Crypto, Cyclo, Cysto are Acid-Fast".

⭐ Cryptosporidium parvum causes severe, chronic diarrhea in AIDS patients, especially with CD4 < 100 cells/µL.

Other Notable Gut Protozoa - Rare But Relevant

• Balantidium coli:
  • Largest protozoan, only pathogenic ciliate. Reservoir: Pigs.
  • Dysentery-like illness. Trophozoite: kidney-bean shaped macronucleus.
  • 
• Cystoisospora belli (Isospora belli):
  • Immunocompromised (AIDS, CD4 < 200): chronic watery diarrhea.
  • Oocysts in stool (modified Acid-Fast Stain positive, autofluorescent).
• Sarcocystis spp.:
  • Intestinal sarcocystosis (from undercooked beef/pork) or muscular sarcocystosis.
  • Intestinal form: often asymptomatic or mild GI symptoms.
• Microsporidia (e.g., Enterocytozoon bieneusi, Encephalitozoon intestinalis):
  • Obligate intracellular fungi (reclassified). Immunocompromised (AIDS): chronic diarrhea.
  • Small spores seen with special stains (e.g., Chromotrope R).
  • 

⭐ Balantidium coli is the largest intestinal protozoan and the only ciliate pathogenic to humans.

High‑Yield Points - ⚡ Biggest Takeaways

• Entamoeba histolytica: Flask-shaped ulcers, liver abscess (anchovy sauce), ingested RBCs in trophozoites.
• Giardia lamblia: Steatorrhea, malabsorption, falling leaf motility; commonest protozoal diarrhea.
• Cryptosporidium parvum: Severe watery diarrhea in AIDS (CD4 < 100); modified acid-fast stain for oocysts.
• Cyclospora cayetanensis: Prolonged watery diarrhea; oocysts are autofluorescent.
• Isospora belli: Chronic diarrhea in immunocompromised; oval oocysts on modified acid-fast stain.
• Balantidium coli: Largest protozoan, only pathogenic ciliate, pig association, causes dysentery.