Gastroenteritis for USMLE Step 3 Exam

• Gastroenteritis: Inflammation of the gastrointestinal tract primarily caused by infectious agents (viruses, bacteria, parasites), leading to diarrhea, vomiting, and abdominal discomfort.
• Non-infectious causes include toxins, medications, and food allergies.

• Viral Gastroenteritis:
• Norovirus: Most common in adults, spread via contaminated food, water, or person-to-person contact; frequently seen in closed environments (e.g., cruise ships).
• Rotavirus: Leading cause of severe diarrhea in children, though vaccination has reduced incidence in many countries.
• Adenovirus and Astrovirus: Typically affect young children and immunocompromised patients.

• Bacterial Gastroenteritis:
• Escherichia coli (E. coli):
• ETEC: Common cause of traveler’s diarrhea, resulting in watery diarrhea.
• STEC: Produces Shiga toxin, causing bloody diarrhea and potential hemolytic uremic syndrome (HUS).
• Shigella: Causes dysentery, characterized by bloody diarrhea, and is highly contagious.
• Salmonella: Linked to contaminated poultry, eggs, and dairy, causing either gastroenteritis or enteric fever (Salmonella Typhi).
• Campylobacter jejuni: Typically acquired from undercooked poultry; associated with post-infectious complications such as Guillain-Barré syndrome.
• Clostridioides difficile: A cause of diarrhea in patients after recent antibiotic use, leading to pseudomembranous colitis.

• Parasitic Gastroenteritis:
• Giardia lamblia: Acquired from contaminated water, causing prolonged diarrhea and malabsorption.
• Entamoeba histolytica: Causes amebic dysentery and liver abscesses, especially in regions with poor sanitation.

• Viral Gastroenteritis:
• Primarily affects the small intestine, impairing water and electrolyte absorption.
• Rotavirus produces NSP4 toxin, leading to increased chloride secretion and watery diarrhea.

• Bacterial Gastroenteritis:
• Toxin-mediated: ETEC and Vibrio cholerae increase intracellular cAMP or cGMP, promoting chloride secretion and water loss.
• Invasive organisms: Shigella, Salmonella, and Campylobacter invade intestinal mucosa, leading to bloody diarrhea and systemic symptoms.

• Parasitic Gastroenteritis:
• Giardia causes villous atrophy and malabsorption.
• Entamoeba histolytica invades the colon, causing flask-shaped ulcers and bloody diarrhea.

• General Symptoms:
• Diarrhea: Watery (viral or toxin-mediated) or bloody (invasive bacterial infections).
• Vomiting: Common in viral infections like Norovirus.
• Abdominal pain: Cramping, typically associated with diarrhea.
• Fever: Common with bacterial gastroenteritis (e.g., Shigella, Salmonella).
• Dehydration: Particularly concerning in infants, elderly, and immunocompromised patients.

• Specific Presentations:
• Rotavirus: Severe watery diarrhea and vomiting in infants.
• STEC: Bloody diarrhea with increased risk for hemolytic uremic syndrome (HUS).
• C. difficile: Watery diarrhea following antibiotic use, with potential for pseudomembranous colitis and toxic megacolon.

• Clinical History: Exposure history, recent travel, antibiotic use, and contact with sick individuals.
• Stool Testing:
• Stool Culture: Recommended for severe or bloody diarrhea.
• PCR: Rapid identification of viral and bacterial pathogens.
• C. difficile Toxin Assay: Used when there is a history of recent antibiotic use and watery diarrhea.
• Stool Ova and Parasites: Useful in diagnosing parasitic infections, especially after travel to endemic areas.

• Rehydration:
• Oral Rehydration Therapy (ORT): First-line treatment for mild to moderate dehydration.
• Intravenous Fluids: Required for severe dehydration or when ORT is not tolerated.

• Antibiotics:
• Reserved for specific bacterial infections:
• Shigella: Treated with fluoroquinolones or azithromycin.
• C. difficile: Managed with oral vancomycin or fidaxomicin.
• Traveler’s Diarrhea: Azithromycin or fluoroquinolones in moderate to severe cases.

• Antimotility Agents: Can be used cautiously in non-bloody diarrhea but contraindicated in suspected C. difficile or STEC infections.

• Probiotics: May reduce symptom duration in viral gastroenteritis and prevent antibiotic-associated diarrhea.

• Dehydration: The most common complication, particularly dangerous in children and elderly patients.
• Hemolytic Uremic Syndrome (HUS): Complication of STEC infection, characterized by acute kidney injury, hemolytic anemia, and thrombocytopenia.
• Guillain-Barré Syndrome: Associated with Campylobacter infection.
• Toxic Megacolon: A complication of severe C. difficile infection, which may require surgical intervention.

• Vaccination: Routine rotavirus vaccination in infants.
• Hygiene: Handwashing, safe food preparation, and clean water are crucial in preventing gastroenteritis.
• Traveler’s Precautions: Avoiding unsafe water and food in endemic areas; vaccinations for cholera and typhoid for high-risk travelers.

• Gastroenteritis is commonly viral (Norovirus, Rotavirus), bacterial (E. coli, Shigella, Campylobacter), or parasitic (Giardia, Entamoeba).
• Symptoms include diarrhea (watery or bloody), vomiting, abdominal pain, and dehydration.
• Diagnosis is clinical, supported by stool tests for severe or prolonged cases.
• Treatment focuses on rehydration, with antibiotics reserved for specific bacterial infections.
• Complications include dehydration, hemolytic uremic syndrome (STEC), and toxic megacolon (C. difficile).