Intestinal infections

Intestinal Infections - Gut Feelings Gone Wrong

• Watery Diarrhea (Non-inflammatory):
  • Vibrio cholerae, ETEC, Rotavirus.
  • Mechanism: Non-invasive, toxin-mediated secretion in the small bowel.
• Bloody Diarrhea (Inflammatory/Dysentery):
  • Campylobacter, Shigella, Salmonella, EHEC, Entamoeba histolytica.
  • Mechanism: Invasion or cytotoxin damaging the large bowel mucosa.
• Key Associations:
  • Campylobacter: Guillain-Barré syndrome.
  • Yersinia enterocolitica: Pseudoappendicitis.
  • C. difficile: Pseudomembranous colitis after antibiotics.

⭐ EHEC O157:H7 can cause Hemolytic Uremic Syndrome (HUS); unlike other E. coli, it does not ferment sorbitol.

Watery Diarrhea - The Toxin Tap Dance

• Pathophysiology: Non-invasive, toxin-mediated intestinal fluid secretion. No inflammation or mucosal destruction.
• Key Organisms & Toxins:
  • Vibrio cholerae: Cholera toxin activates Gs → ↑ adenylate cyclase → ↑ cAMP → massive Cl⁻ secretion. Classic "rice-water" stools.
  • ETEC: Traveler's diarrhea.
    • Heat-Labile Toxin (LT): ↑ cAMP (like choLerA).
    • Heat-Stable Toxin (ST): ↑ cGMP.
  • Bacillus cereus: Diarrheal type produces watery diarrhea via a heat-labile enterotoxin.
  • Staphylococcus aureus: Rapid-onset from preformed heat-stable enterotoxin.

⭐ 📌 Mnemonic (ETEC): "Labile in the Air, Stable on the Ground." (LT → cAMP, ST → cGMP)

Inflammatory Diarrhea - Bloody Battle in Bowels

• General: Invasion of intestinal mucosa → fever, tenesmus, fecal leukocytes, blood/pus in stool.
• Key Pathogens & Features:
  • Campylobacter jejuni: Most common bacterial cause. Guillain-Barré syndrome (GBS) risk. S-shaped organism, grows at 42°C. From undercooked poultry.
  • Shigella: Low infectious dose. Invades M cells. Produces Shiga toxin → HUS risk. Fecal-oral spread (daycares).
  • Salmonella (non-typhoidal): From poultry, eggs. Risk of osteomyelitis in sickle cell disease.
  • EHEC (O157:H7): Shiga-like toxin (SLT). HUS risk. From undercooked beef. Does not ferment sorbitol.
  • Yersinia enterocolitica: Mimics appendicitis (pseudoappendicitis). From pork, unpasteurized milk.
  • Entamoeba histolytica: Protozoa. "Flask-shaped" ulcers. Liver abscess (anchovy paste). Trophozoites with ingested RBCs.

📌 Mnemonic (Bloody Stool): "Calmly Eat Some Salty Yogurt" - Campylobacter, Entamoeba, Shigella, Salmonella, Yersinia.

⭐ EHEC/STEC Infection: Avoid antibiotics! They can ↑ release of Shiga-like toxin, increasing the risk of developing Hemolytic Uremic Syndrome (HUS).

Protozoal Pathogens - Uninvited Microscopic Guests

• Giardia lamblia
  • Transmission: Cysts from contaminated water (camping/hiking).
  • Symptoms: Fatty, foul-smelling diarrhea (steatorrhea), bloating, malabsorption.
  • Dx: Trophozoites or cysts in stool (O&P exam).
• Entamoeba histolytica
  • Symptoms: Bloody diarrhea (dysentery), RUQ pain.
  • Complications: Liver abscess (classic "anchovy paste" consistency).
  • Dx: Trophozoites with engulfed RBCs in stool.
• Cryptosporidium spp.
  • Symptoms: Severe, chronic watery diarrhea in immunocompromised.
  • High-risk: AIDS patients with CD4 count < 100 cells/mm³.
  • Dx: Oocysts on modified acid-fast stain.

⭐ High-Yield: E. histolytica invades the colonic submucosa, creating characteristic "flask-shaped" ulcers visible on histology.

High‑Yield Points - ⚡ Biggest Takeaways

• Rotavirus is the leading cause of severe diarrhea in young children, causing villous atrophy.
• Norovirus is the most common cause of diarrhea in adults, often in outbreaks on cruise ships.
• Campylobacter jejuni is the most frequent cause of bacterial enterocolitis and a key trigger for Guillain-Barré syndrome.
• EHEC (O157:H7), from undercooked beef, causes bloody diarrhea and HUS; it does not ferment sorbitol.
• Clostridioides difficile infection, often following antibiotic use, results in pseudomembranous colitis.