GI Pathogens Overview - Rogues' Gallery Intro
- Pathogen Classification:
- Bacteria: e.g., E. coli, Salmonella, Shigella, Campylobacter, V. cholerae. Cause diverse illnesses.
- Viruses: e.g., Rotavirus (children), Norovirus (all ages), Adenovirus. Often self-limiting.
- Parasites: Protozoa (Giardia, Entamoeba, Cryptosporidium) and helminths. Chronic infections possible.
- Common Transmission Modes:
- Fecal-oral route (direct/indirect).
- Ingestion of contaminated food or water.
- General Host Defenses:
- Stomach acidity (HCl).
- Intestinal peristalsis.
- Protective normal microbiota.
- Mucosal immunity (Secretory IgA).
⭐ Most common cause of traveler's diarrhea is Enterotoxigenic E. coli (ETEC).
Bacterial Diarrheas - Tummy Bug Battle
- E. coli variants:
- ETEC: Traveler's diarrhea (watery).
- EPEC: Pediatric diarrhea (watery).
- EAEC: Persistent diarrhea (biofilm).
- EHEC (O157:H7): Bloody diarrhea, HUS; Shiga-like toxin.
- Salmonella (Non-typhoidal): Poultry/eggs; inflammatory diarrhea; can cause reactive arthritis.
- Shigella: Dysentery (bloody, mucoid stools); Shiga toxin.
⭐ Shigella requires a very low infective dose (10-100 organisms).
- Vibrio cholerae: Profuse "rice-water" stool; dehydration; TCBS agar.
- Campylobacter jejuni: Poultry; bloody diarrhea, fever. 📌 Guillain-Barré Syndrome (GBS) association.
- Clostridioides difficile: Antibiotic-associated; pseudomembranous colitis; toxins A (enterotoxin) & B (cytotoxin).
Viral Gastroenteritis - Viral Gut Busters
- Rotavirus: Common in infants/young children; oral live-attenuated vaccine available; distinct winter seasonality. 📌 ROTA = Right Out The Anus.
- Norovirus: Affects all ages; notorious for outbreaks in closed settings (e.g., cruise ships, hospitals); highly contagious.
- Adenovirus: Enteric serotypes 40/41; can cause prolonged diarrhea, especially in young children.
| Feature | Rotavirus | Norovirus |
|---|---|---|
| Target Age | Infants/Young Children | All ages |
| Seasonality | Winter | Year-round |
| Outbreak Setting | Daycares | Cruise ships/Hospitals |
| Vaccine | Yes (Live attenuated) | No |
Parasitic GI Infections - Unwelcome Guests
| Organism | Unique Symptom | Diagnostic Form | Key Association |
|---|---|---|---|
| Giardia lamblia | Foul-smelling steatorrhea; malabsorption | Trophozoite/Cyst (string test) | Campers/hikers; 📌 'Giardia gives Greasy, Foul-smelling stools' |
| Entamoeba histolytica | Dysentery; liver abscess ('anchovy paste') | Trophozoite (ingested RBCs)/Cyst | Flask-shaped ulcers |
| Cryptosporidium parvum | Severe watery diarrhea (immunocompromised) | Oocysts (acid-fast stain) | HIV (CD4 < 100 cells/µL) |
GI Dx & Rx - Unravel & Resolve
- Diarrhea Approach: Acute (<2wks) vs Chronic (>4wks); Inflammatory (fever, blood, pus) vs Non-inflammatory.
- Stool Exam: Leukocytes, occult blood, ova & parasites, culture, C. diff toxin/PCR, Ag tests.
- Management:
- ORS: WHO ($Na^+/K^+/Cl^-/Citrate/Glucose$), 245 mOsm/L. Diet.
- Antimotility: Loperamide (⚠️ No bloody diarrhea, high fever, C. diff).
- Antibiotics:
- Empirical (Traveler's): Fluoroquinolones/Azithromycin.
- Specific: Cholera, Shigellosis; C. diff (Metronidazole/Vancomycin/Fidaxomicin); Giardiasis (Metronidazole).
⭐ Oral Rehydration Solution (ORS) composition is critical for effective fluid and electrolyte replacement.
High‑Yield Points - ⚡ Biggest Takeaways
- Traveler's diarrhea: Most commonly ETEC.
- Bloody diarrhea (dysentery): Key causes include Shigella, EIEC, Campylobacter jejuni, E. histolytica.
- Rice-water stools: Hallmark of Vibrio cholerae.
- Antibiotic-associated diarrhea: Primarily C. difficile.
- Campylobacter jejuni enteritis: Can precede Guillain-Barré syndrome.
- Typhoid fever (S. Typhi): Step-ladder fever, rose spots, relative bradycardia.
- Rotavirus: Leading cause of severe childhood dehydrating diarrhea_
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