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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Most acute infectious diarrhea is viral and self-limited; oral rehydration is the cornerstone
- Dysentery, fever, severe abdominal pain, sepsis, immunocompromise, outbreak, or persistent diarrhea warrants stool testing
- Avoid antibiotics and antimotility agents if Shiga toxin-producing E coli is suspected because of HUS risk
- Empiric azithromycin is preferred for severe travelers diarrhea or dysentery when treatment is needed
- Giardia causes foul-smelling greasy diarrhea, bloating, and flatulence after contaminated water; treat with tinidazole, metronidazole, or nitazoxanide
- Cryptosporidium causes watery diarrhea, severe in AIDS; treat with ART immune restoration plus nitazoxanide in immunocompetent patients
Overview
Infectious diarrhea is classified clinically as watery, inflammatory/dysenteric, persistent, or toxin-mediated. Viral gastroenteritis and preformed toxin illness usually need supportive care only. Bacterial invasive disease causes fever, tenesmus, severe pain, or blood. Parasitic infections should be considered with persistent diarrhea, travel, daycare exposure, untreated water, immunocompromise, or outbreaks. The key Step 2 CK decision is not the organism alone, but whether stool testing, antibiotics, avoidance of antibiotics, or rehydration is the next best step.
Epidemiology
Norovirus is the leading cause of acute gastroenteritis outbreaks in the United States. Foodborne bacterial causes include Campylobacter, Salmonella, Shigella, Shiga toxin-producing E coli, Vibrio, Yersinia, and toxin-mediated Staphylococcus aureus or Bacillus cereus. Giardia is common after untreated freshwater exposure; Cryptosporidium is associated with pools, daycare, and advanced HIV. Travel increases risk of enterotoxigenic E coli and invasive bacterial pathogens.
Clinical Features
Symptoms
Watery diarrhea with vomiting and low-grade fever suggests viral gastroenteritis or toxin-mediated disease
Bloody diarrhea, fever, tenesmus, or severe cramping suggests inflammatory bacterial diarrhea
Profuse rice-water diarrhea after travel or outbreak suggests cholera
Foul-smelling greasy stools, bloating, flatulence, and weight loss suggest Giardia
Persistent watery diarrhea in AIDS suggests Cryptosporidium, Cystoisospora, Microsporidia, or MAC
Post-diarrheal pallor, oliguria, bruising, or neurologic symptoms after bloody diarrhea suggests HUS
Signs
Signs of dehydration: dry mucosa, tachycardia, orthostasis, delayed capillary refill, poor skin turgor
Fever and abdominal tenderness in invasive disease
Peritoneal signs suggest toxic megacolon, perforation, ischemia, or surgical abdomen
Jaundice or hepatomegaly suggests viral hepatitis, amebic liver abscess, or systemic illness
Altered mental status or hypotension indicates severe dehydration or sepsis
Investigations
First-line
Assess hydration and electrolytesBMP if severe dehydration, older adult, renal disease, cholera-like stool, or need for IV fluids
Stool PCR/culture when indicatedIndications: bloody diarrhea, fever, severe pain, sepsis, immunocompromise, outbreak, persistent symptoms, or public health concern
Shiga toxin testingRequired when bloody diarrhea or suspected STEC; determines need to avoid antibiotics and antimotility drugs
Second-line
Ova and parasite testing / antigen testingPersistent diarrhea >7-14 days, travel, daycare, untreated water, immunocompromise, or eosinophilia
C difficile testingDiarrhea after antibiotics, hospitalization, long-term care, IBD flare, or healthcare exposure
CBCLeukocytosis in invasive disease; thrombocytopenia and anemia suggest HUS
Specialist
Blood culturesIf sepsis, enteric fever, severe immunocompromise, or suspected bacteremia
Colonoscopy or imagingIf diagnosis unclear, severe colitis, suspected IBD, ischemic colitis, toxic megacolon, or persistent symptoms despite evaluation
1
Supportive care
- Oral rehydration solution for mild/moderate dehydration; IV isotonic fluids for severe dehydration, shock, or inability to drink
- Continue feeding; avoid routine lactose restriction unless post-infectious intolerance develops
- Loperamide may be used for afebrile non-bloody diarrhea in adults, but avoid in dysentery, suspected C difficile, or STEC
2
When to use empiric antibiotics
- Usually avoid empiric antibiotics in immunocompetent adults with non-severe watery diarrhea
- Consider azithromycin for severe travelers diarrhea, dysentery, high fever, or suspected Campylobacter/Shigella when benefits exceed risks
- Severe sepsis or enteric fever concern: ceftriaxone or azithromycin pending cultures
- Avoid antibiotics when STEC is suspected or Shiga toxin positive
3
Pathogen-directed therapy
- Giardia: tinidazole single dose, metronidazole, or nitazoxanide
- Cryptosporidium: oral rehydration; nitazoxanide for immunocompetent; ART immune restoration in HIV
- Cyclospora/Cystoisospora: TMP-SMX
- Amebiasis: metronidazole or tinidazole followed by luminal agent such as paromomycin
- Cholera: aggressive rehydration; azithromycin or doxycycline can shorten severe disease
4
Public health and prevention
- Report outbreaks, cholera, typhoid, shiga toxin, and other reportable pathogens per local rules
- Hand hygiene, food safety, safe water, exclusion of symptomatic food handlers, and vaccination for rotavirus/typhoid/cholera when indicated
Complications
- Dehydration and AKI: Most immediate cause of morbidity
- HUS: STEC bloody diarrhea followed by microangiopathic hemolytic anemia, thrombocytopenia, AKI
- Reactive arthritis: After Campylobacter, Salmonella, Shigella, or Yersinia
- Guillain-Barre syndrome: Classically after Campylobacter jejuni
- Post-infectious IBS: Persistent altered bowel habit after acute gastroenteritis
USMLE Step 2 CK Exam Tips
- 1Bloody diarrhea after undercooked beef + no fever or low fever = STEC; avoid antibiotics and loperamide
- 2HUS triad = MAHA, thrombocytopenia, AKI after bloody diarrhea
- 3Rice-water stool = cholera; next best step is aggressive rehydration
- 4Foul-smelling greasy diarrhea after camping = Giardia; treat with tinidazole/metronidazole
- 5Poultry exposure + bloody diarrhea + later ascending weakness = Campylobacter followed by Guillain-Barre
- 6Left lower quadrant pain and pseudoappendicitis after pork/chitterlings = Yersinia
- 7Most acute watery diarrhea needs oral rehydration, not antibiotics
practicetest your knowledge on infectious diarrhea (bacterial, viral, parasitic)Apply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — infectious diseases and beyond.
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