Gastrointestinal Infections

Introduction to GI Infections - Gut Invaders 101

• Overview: Infections targeting stomach, small & large intestines.
• Transmission: Primarily fecal-oral route; contaminated food/water.
• Host Defenses:
  • Gastric acid (pH <4)
  • Mucus barrier
  • Peristalsis
  • Normal gut flora (competitive inhibition)
  • Gut-Associated Lymphoid Tissue (GALT) - Secretory IgA.
• Pathogen Mechanisms:
  • Adherence to mucosa
  • Toxin production: Enterotoxins (watery diarrhea), Cytotoxins (cell damage, dysentery)
  • Mucosal invasion & inflammation (e.g., Shigella, invasive E. coli)
• Clinical Patterns:
  • Watery diarrhea: Small bowel; non-inflammatory; high volume.
  • Dysentery: Large bowel; inflammatory; blood, mucus, pus; lower volume.

⭐ Most GI infections are acquired via the fecal-oral route, often through contaminated food or water sources (e.g., Vibrio cholerae, Rotavirus).

Bacterial GI Infections - The Nasty Bugs

• Escherichia coli

  • ETEC: Traveler's diarrhea (watery). LT (↑cAMP), ST (↑cGMP) toxins.
  • EHEC (O157:H7): Hemorrhagic colitis, HUS. Shiga-like toxin. 📌 "Hamburger HUS"
  • EPEC: Infantile diarrhea. Adherence, microvilli effacement.
  • EIEC: Invasive, dysentery. Resembles Shigella.

• Salmonella

  • Non-typhoidal: Gastroenteritis (poultry, eggs).
  • Typhoidal (S. typhi): Typhoid fever (rose spots, step-ladder fever).

    ⭐ S. typhi gallbladder colonization causes chronic carrier state.

• Shigella spp. (S. dysenteriae)

  • Bacillary dysentery (bloody, mucoid). Shiga toxin. Low infective dose (10-100 organisms). Invades M cells.

• Vibrio cholerae

  • Profuse "rice-water" stools. Cholera toxin (↑cAMP). Severe dehydration. TCBS agar.

• Campylobacter jejuni

  • Common cause of bloody diarrhea. Guillain-Barré syndrome. Grows 42°C (microaerophilic). 📌 "Campy likes it hot (42°C)"

• Clostridioides difficile

  • Antibiotic-associated diarrhea, pseudomembranous colitis. Toxins A & B.

• Toxin-mediated (Preformed):

  • Staphylococcus aureus: Rapid vomiting (meats, custards).
  • Bacillus cereus: Emetic (fried rice) / diarrheal types.

• Yersinia enterocolitica

  • Pseudoappendicitis (pork, milk). Cold enrichment.

Viral & Parasitic GI Infections - Tiny Troublemakers

• Viral Agents:

  • Rotavirus: MCC severe childhood diarrhea (watery); villous blunting. Oral vaccine.
  • Norovirus: Highly contagious; epidemic non-bacterial gastroenteritis (cruise ships); abrupt V/D.
  • Adenovirus (enteric 40, 41): Pediatric diarrhea, can be prolonged.
  • Astrovirus: Star-shaped (EM); diarrhea in children, immunocompromised.

• Parasitic Agents:

  • Giardia lamblia: Flagellated protozoan; foul steatorrhea, malabsorption. Cysts in contaminated water.
    • 📌 Giardia: Greasy, Gassy, Giardiasis.
  • Entamoeba histolytica: Amebiasis: dysentery (bloody); liver abscess. Ingested RBCs in trophozoites.

    ⭐ Entamoeba histolytica causes flask-shaped colonic ulcers & "anchovy paste" liver abscesses.

  • Cryptosporidium parvum: Severe watery diarrhea in AIDS (CD4 < 100); oocysts acid-fast.
  • Cyclospora cayetanensis: Prolonged watery diarrhea; oocysts acid-fast (larger); contaminated produce.
  • Isospora belli (Cystoisospora): Chronic watery diarrhea in immunocompromised; oocysts oval, acid-fast.

Diagnosis & Complications - Spotting & Stopping Sickness

• Diagnosis (Spotting):
  • Stool analysis: Key for pathogens (microscopy, culture, antigen tests like C. diff toxin, PCR).
  • Endoscopy & Biopsy: For severe, persistent, or diagnostically challenging cases (e.g., CMV colitis, pseudomembranes).
  • Blood tests: Assess severity (CBC, electrolytes, renal function). Serology rarely for acute.
• Complications (Stopping Sickness):
  • Immediate: Dehydration & electrolyte imbalance (priority: ORS/IV fluids).
  • Severe GI: Hemorrhage, perforation (e.g., typhoid ulcers), toxic megacolon.
  • Extra-intestinal: HUS (EHEC O157:H7), GBS (Campylobacter), Reactive Arthritis (📌 CRYSY: Campylobacter, Reactive arthritis, Yersinia, Salmonella, Shigella).

⭐ Campylobacter jejuni infection is the most frequent antecedent of Guillain-Barré Syndrome (GBS).

High‑Yield Points - ⚡ Biggest Takeaways

• Vibrio cholerae: Rice-water stools, cholera toxin (↑cAMP).
• Salmonella typhi: Enteric fever, rose spots, pea soup diarrhea.
• Shigella: Bacillary dysentery (bloody, mucoid), Shiga toxin, invasive.
• EHEC (O157:H7): Hemorrhagic colitis, risk of HUS.
• Campylobacter jejuni: Common bacterial cause, linked to Guillain-Barré syndrome.
• Clostridium difficile: Pseudomembranous colitis, antibiotic-associated.
• Rotavirus: Major cause of severe childhood diarrhea.