GI Infections

• Audio chapter
  Single-voice audio, listen on the commute.
• ABIM-format MCQs
  5-option vignettes with full wrong-answer teaching.
• Study guide
  Tables, decision trees, primary sources.
• AI tutor
  Chapter-grounded, answers the question you're stuck on.

What this chapter covers

• Section 33.1: Clostridioides difficile diagnosis and severity

  Clostridioides difficile infection (CDI) causes approximately 500,000 infections and 30,000 deaths annually in the United States, with hospital-onset and community-onset cases roughly equal.

• Section 33.2: Clostridioides difficile first-episode and fulminant treatment

  The first-line agent for CDI shifted from vancomycin to fidaxomicin in the 2021 ACG and IDSA-SHEA updates, based on the OPT-80-003 trial result that fidaxomicin and vancomycin produced similar clinical cure rates (approximately 88 percent versus 86 percent) but fidaxomicin reduced sustained recurrence by approximately 14 percentage points.

• Section 33.3: Recurrent C. diff, bezlotoxumab, FMT, and live biotherapeutics

  A first recurrence of CDI affects 20 to 30 percent of treated patients, and the risk climbs above 60 percent after a third episode.

• Section 33.4: Travelers' diarrhea and bacterial enterocolitis

  Travelers' diarrhea affects 30 to 50 percent of travelers from high-income to low- and middle-income regions.

• Section 33.5: Parasitic GI infections

  Parasitic GI infection is suspected by exposure (untreated water, daycare, recent travel, immunocompromise), persistence beyond 7 to 14 days, or specific clinical patterns (steatorrhea with weight loss for Giardia, dysentery with hepatic abscess for Entamoeba histolytica, hyperinfection during steroid initiation for Strongyloides, peripheral eosinophilia for Strongyloides or Cystoisospora).

• Section 33.6: CMV colitis

  Cytomegalovirus (CMV) is a human herpesvirus that establishes lifelong latent infection after primary exposure and reactivates under immunosuppression.

• Section 33.7: MAC, microsporidia, and Cyclospora

  Mycobacterium avium complex, microsporidia, and Cyclospora are the opportunistic enteric pathogens whose pathogen-level diagnostic and treatment detail Ch 33 owns; the broader CD4 stratification framework, immune reconstitution inflammatory syndrome, and Kaposi sarcoma as a tissue diagnosis live in Ch 34.

• Section 33.8: Norovirus and viral gastroenteritis

  Norovirus is the leading cause of acute community-acquired and outbreak gastroenteritis in the United States and globally, year-round with a cold-weather peak.

Podcast episodes

1. 01

   Episode Treatment

   Episode one of the GI Infections chapter builds C. diff around two competing problems: the defaults moved off metronidazole and vancomycin, and asymptomatic colonization is common enough that the wrong test drives unnecessary treatment. The organizing idea is that diagnosis exists to separate active disease from colonization, while treatment exists to get the right drug to the colonic lumen at the right concentration for the right duration. Severity thresholds decide treatment intensity, and fulminant features bring surgery into the conversation at presentation rather than after medical failure. Everything reduces to those two problems.

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2. 02

   C. diff Recurrence Toolkit

   Recurrence is the dominant management problem in C. diff, and this episode organizes the entire toolkit around a single mechanism: the spore. Surviving spores germinate once the antibiotic stops, so every tool either keeps colonic drug above the killing threshold across successive germination waves or restores the commensal community that denies spores a niche. The drug you reach for depends on what was used first, the structural taper-and-pulse pattern matters more than the molecule, and after two or more recurrences the strategy shifts from antibiotics to microbiome restoration. Bezlotoxumab and prophylaxis close the loop in the right patients.

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3. 03

   Travelers' Diarrhea and Bacterial Toxins

   The community-acquired diarrheas separate cleanly once you group by acquisition pattern rather than symptoms, because the exposure history predicts the pathogen and the pathogen predicts the treatment. Travelers' diarrhea selects the empiric antibiotic by region and resistance, while rifaximin stays in the noninvasive niche because it is non-absorbed. Shiga-toxin-producing E. coli flips the rule entirely, since antibiotics release more toxin and loperamide prolongs exposure. Nontyphoidal Salmonella, Shigella, Yersinia, Vibrio, and Listeria each run on their own biology, and the foodborne toxin syndromes are recognized by time to onset that tells you cultures and antibiotics are unnecessary.

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4. 04

   Parasitic and Viral Diarrheas

   The parasitic diarrheas sort by exposure history, so the workup follows the suspected pathogen rather than a generic ova-and-parasite exam. Giardia comes from untreated water with a single-dose tinidazole answer, Entamoeba histolytica from endemic travel with two-stage tissue-then-luminal therapy, and Strongyloides is the pre-steroid serology question that prevents seventy-percent-mortality hyperinfection. Anisakis is the raw-fish larva cured by endoscopic removal. On the viral side, norovirus dominates on infectious dose and environmental persistence, which is why soap-and-water and bleach are the outbreak answers, while rotavirus fades under vaccination.

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5. 05

   Host Enteric Infections

   The immunocompromised gut expands the pathogen list, and the organizing move is that each organism lives behind a specific cue that names the therapy: name the host, the stain, and the histology, and you name the drug. CMV gives the owl-eye inclusion at the ulcer base treated with ganciclovir, with foscarnet as salvage when UL97 mutations break the prodrug pathway. MAC gives the PAS-positive, acid-fast-positive macrophage treated with a macrolide plus ethambutol, microsporidia turn on beta-tubulin pharmacology, and the coccidians answer to trimethoprim-sulfamethoxazole. The unifying thread is that antimicrobials hold the line while immune reconstitution provides durable clearance.

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Key topics

• ·Antibiotic exposure and C. diff risk factors
• ·Presentation and pseudomembranous colitis
• ·Two-step NAAT plus toxin EIA algorithm
• ·Who to test and why test of cure fails
• ·Severity stratification thresholds
• ·Fidaxomicin first-line and vancomycin alternative
• ·Metronidazole in third place and pregnancy
• ·Fulminant disease regimen and surgery
• ·The spore-germination cycle behind recurrence
• ·Fidaxomicin for a first recurrence
• ·Extended-pulse fidaxomicin dosing
• ·Vancomycin tapered-pulsed dosing
• ·Bezlotoxumab host-immunity layer
• ·Microbiome restoration after multiple recurrences
• ·Live biotherapeutics Rebyota and Vowst
• ·Oral vancomycin prophylaxis
• ·Acquisition pattern as the organizing principle
• ·Travelers' diarrhea and region-based empiric therapy
• ·Rifaximin in the noninvasive niche
• ·The no-antibiotics-for-EHEC rule and HUS
• ·Campylobacter and Salmonella treatment rules
• ·Shigella, Yersinia, and Vibrio biology
• ·Listeria in special hosts
• ·Foodborne toxin syndromes by time to onset
• ·Suspecting parasites by exposure history
• ·Giardia and single-dose tinidazole
• ·Cryptosporidium and immune status
• ·Entamoeba histolytica two-stage therapy
• ·Strongyloides pre-steroid screening and hyperinfection
• ·Anisakis and endoscopic removal