Diarrheal Diseases and GI Infections

Preformed toxin (onset in hours, vomiting predominates)

• Staphylococcus aureus: heat-stable enterotoxin acts as a superantigen and stimulates vagal afferents; mayonnaise/custard, symptoms within ~1–6 hours
• Bacillus cereus: emetic cereulide toxin from reheated fried rice; the diarrheal form (enterotoxin) begins later
• Clostridium perfringens: toxin elaborated in the gut after ingestion of contaminated meat/gravy; watery diarrhea roughly 8–16 hours later

Toxigenic/secretory (no mucosal invasion, no fecal leukocytes)

• Vibrio cholerae and ETEC: cholera toxin and heat-labile toxin ADP-ribosylate Gs → sustained cAMP → CFTR-mediated chloride secretion; heat-stable toxin acts via cGMP
• Viruses: norovirus (cruise ships, closed populations) and rotavirus (unvaccinated infants) cause villous blunting with osmotic plus secretory loss

Invasive/inflammatory

• Shigella, Campylobacter, nontyphoidal Salmonella, EIEC, Yersinia, Entamoeba histolytica: epithelial invasion with neutrophilic colitis; Shiga toxin (S. dysenteriae type 1, STEC O157:H7) inactivates the 60S ribosome and injures endothelium

Toxin-mediated colitis after microbiome disruption

• Clostridioides difficile: toxins A/B glucosylate Rho GTPases → cytoskeletal collapse, tight-junction loss, pseudomembranes

Modifiable risk factors (the ones planted in stems)

• Recent antibiotics: clindamycin, fluoroquinolones, cephalosporins, and broad-spectrum penicillins are the classic C. difficile precipitants
• Acid suppression: PPIs remove the gastric acid barrier, lowering the infectious dose for Salmonella, Vibrio, and C. difficile
• Food and water exposures: undercooked ground beef and unpasteurized milk/cider (STEC), poultry and eggs (Salmonella, Campylobacter), raw shellfish (Vibrio, norovirus), untreated fresh water (Giardia), pet reptiles and turtles (Salmonella)
• Travel to low-sanitation regions, daycare attendance, and healthcare exposure

Non-modifiable / host factors

• Age extremes: infants and adults >65 have the highest mortality and C. difficile recurrence risk
• Immunosuppression: HIV with low CD4 (Cryptosporidium, CMV, MAC), transplant, chemotherapy
• Sickle cell disease and asplenia: functional asplenia predisposes to Salmonella bacteremia and osteomyelitis
• Achlorhydria, prior gastrectomy, IBD, and HLA-B27 positivity (predisposing to post-infectious reactive arthritis)

• Osmotic diarrhea: Non-absorbed solutes (lactose in lactase deficiency, sorbitol, magnesium sulfate) retain water in the intestinal lumen; osmotic gradient overwhelms intestinal reabsorptive capacity; typically decreases with fasting and has high stool osmolar gap (>125 mOsm/kg)

• Secretory diarrhea: Pathogenic enterotoxins (cholera toxin, heat-labile/heat-stable E. coli toxins) permanently activate adenylyl cyclase or guanylyl cyclase, increasing cAMP/cGMP; results in active chloride secretion and secondary water loss; persists despite fasting and has low osmolar gap (<50 mOsm/kg)

• Inflammatory diarrhea: Bacterial invasion and toxin-mediated mucosal damage (Shigella, Campylobacter, invasive E. coli, Salmonella) cause epithelial ulceration, neutrophil infiltration, and bloody stools; increased intestinal permeability allows bacterial translocation; fever and systemic symptoms predominate

• Motility-related diarrhea: Altered intestinal peristalsis (viral infections affecting myenteric plexus, post-infectious IBS) disrupts normal absorption kinetics; stool frequency increases without necessarily increasing daily volume

• Malabsorption mechanisms: Villous atrophy (viral gastroenteritis, tropical sprue), brush border enzyme deficiency, or impaired fat absorption increases intraluminal osmotically active particles and stool volume

• Host factors: Intestinal barrier integrity (tight junctions, mucus layer) is compromised by pathogenic lipopolysaccharides and zonula occludens toxins; immune dysregulation in HIV/AIDS, malnutrition, or hypogammaglobulinemia increases susceptibility and severity

• Acute watery diarrhea (viral, toxigenic bacteria): Sudden onset of frequent liquid stools without blood or mucus; often preceded by prodromal symptoms (myalgias, malaise); crampy periumbilical pain; typically self-limited within 3-7 days; cholera presents with "rice-water stools" (painless, voluminous, up to 1 liter/hour) and rapid severe dehydration within hours

• Bloody diarrhea (dysentery) (invasive bacteria, some viruses): Frank blood and mucus in stools; fever, tenesmus, and urgency; crampy lower abdominal pain; suggests mucosal invasion (Shigella, Campylobacter, invasive E. coli, Entamoeba); may be accompanied by systemic toxicity and complications (perforation, toxic megacolon)

• Persistent diarrhea (lasting >14 days): Associated with malnutrition, small bowel pathology, immunodeficiency; may progress to tropical sprue or post-infectious IBS; more common in developing countries and HIV patients; requires investigation for chronic causes

• Traveler's diarrhea: Explosive onset 3-5 days after travel to developing countries; typically enterotoxigenic E. coli (ETEC) or Campylobacter; watery stools without blood; self-limited in immunocompetent hosts (3-5 days); risk higher with consumption of contaminated food/water

• Dehydration severity correlates with diarrhea type: Secretory diarrhea causes rapid, severe dehydration (cholera); watery viral diarrhea moderate dehydration; inflammatory diarrhea variable depending on volume losses; assess using clinical signs (sunken fontanelle in children, skin turgor, mucous membrane dryness, orthostatic vital signs) and percentage body weight loss

• Constitutional symptoms: Fever (especially invasive pathogens), vomiting (proximal GI involvement), myalgias, headache; severity indicates systemic infection rather than isolated enteritis

• Important clinical pearl: Absence of fever in acute diarrhea suggests viral or toxigenic bacterial origin; high fever with dysentery indicates invasive pathogen requiring specific antimicrobials

• Stool osmolarity and electrolytes: Calculate stool osmolar gap [290 - 2(Na + K)] to differentiate secretory (<50 mOsm/kg, persists with fasting) from osmotic (>125 mOsm/kg, resolves with fasting) diarrhea; guides treatment strategy

• Stool microscopy and culture: Examine for leukocytes and red blood cells (suggests inflammatory/invasive etiology); Wright-Giemsa or methylene blue stain detects fecal leukocytes (sensitivity ~70% for bacterial dysentery); culture identifies specific bacterial pathogens; consider Clostridium difficile toxin (PCR or enzyme immunoassay) in hospitalized patients or antibiotic users, or Entamoeba histolytica serology if dysentery with risk factors

• Stool antigen and molecular testing: Rotavirus antigen detection (immunoassay) in children; multiplex PCR panels increasingly available for simultaneous detection of enteroviruses, norovirus, adenovirus, and bacterial pathogens; rapid turnaround enables targeted therapy

• Serologic testing: Consider for Vibrio cholerae (IgM, IgG) in endemic areas or documented outbreaks; E. histolytica serology (>90% sensitivity in invasive disease); less useful acutely but guides epidemiologic follow-up

• Colonoscopy and biopsy: Reserved for persistent diarrhea (>2 weeks), severe colitis, or suspected inflammatory bowel disease; may reveal mucosal ulceration, edema, or pseudomembranes (C. difficile); allows targeted pathogen detection and excludes surgical complications

• Abdominal imaging: CT abdomen/pelvis indicated if toxic megacolon or perforation suspected (pneumoperitoneum, dilated colon >6 cm); plain films assess for obstruction or free air in acute presentations with severe symptoms

• Important diagnostic consideration: Most acute diarrhea is self-limited and viral; extensive testing rarely changes management in immunocompetent adults with mild-to-moderate disease; reserve testing for severe dehydration, bloody stools, fever >38.5°C, immunocompromised hosts, or symptoms >7 days

• Oral rehydration therapy (ORT) - FIRST-LINE for all diarrheal diseases: WHO/UNICEF standard solution contains sodium 75 mEq/L, chloride 65 mEq/L, potassium 20 mEq/L, glucose 75 mmol/L; promotes coupled sodium-glucose absorption in small bowel preserving absorptive capacity even during active diarrhea; administer 50-100 mL/kg over 4 hours for mild-moderate dehydration, then maintenance fluids 10 mL/kg per stool; continue feeding during rehydration; more effective than IV therapy in uncomplicated cases and prevents relapse

• Intravenous fluids: Reserved for severe dehydration (>10% body weight loss, shock, unable to tolerate oral intake, persistent vomiting); use lactated Ringer's solution (resembles plasma) or normal saline; bolus 20-40 mL/kg over 1-2 hours if hypotensive, then reassess; add potassium (20-40 mEq/

Volume and electrolyte emergencies

• Hypovolemic shock: massive secretory losses (cholera can exceed a liter per hour); signaled by tachycardia, thready pulse, sunken eyes/fontanelle, anuria — resuscitate before any diagnostic workup
• Non-anion-gap metabolic acidosis with hypokalemia: stool bicarbonate and potassium loss; look for a low serum bicarbonate with normal anion gap, flattened T waves, and U waves. Correcting with large-volume normal saline can add a hyperchloremic acidosis

Hemolytic uremic syndrome

• Shiga toxin–mediated endothelial injury in STEC O157:H7 and S. dysenteriae type 1 produces the triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury; the signal is schistocytes on smear with rising creatinine 5–10 days into bloody diarrhea. This is an emergency; management is supportive, and IDSA advises against antibiotics in suspected STEC because bacterial lysis may increase toxin release

Colonic catastrophes (surgical emergencies)

• Toxic megacolon: transmural inflammation paralyzes smooth muscle; colonic dilation beyond ~6 cm with systemic toxicity, and paradoxically decreasing stool output as ileus develops
• Perforation and peritonitis: free air on imaging, rigid abdomen — obtain urgent surgical consultation
• Fulminant C. difficile colitis: hypotension, ileus, markedly elevated leukocytosis and lactate; per IDSA/SHEA, treat with high-dose oral (or per-rectum) vancomycin plus IV metronidazole and involve surgery for colectomy consideration

Extraintestinal and post-infectious sequelae

• Salmonella bacteremia with endovascular seeding (mycotic aneurysm) or osteomyelitis in sickle cell disease
• Guillain–Barré syndrome after Campylobacter via molecular mimicry between lipooligosaccharide and gangliosides — ascending weakness with areflexia weeks later
• Reactive arthritis (can't see, can't pee, can't climb a tree) after Shigella, Salmonella, Campylobacter, or Yersinia in HLA-B27 carriers
• Post-infectious lactase deficiency and IBS; in children, repeated episodes cause stunting

Treatment-related harms

• Antimotility agents (loperamide) in invasive or C. difficile colitis prolong toxin contact and can precipitate toxic megacolon
• Antibiotic exposure itself precipitates C. difficile
• FMT carries a documented risk of transmitting multidrug-resistant organisms, prompting FDA safety alerts and donor screening requirements

• Bloody diarrhea in a child after a barbecue, then oliguria and schistocytes: STEC O157:H7 with HUS. Best next step is supportive care and fluids — not antibiotics and not loperamide, both of which are the intended distractors (IDSA 2017)
• C. difficile drug choice has changed: IDSA/SHEA's 2021 focused update favors oral fidaxomicin, with oral vancomycin as the acceptable alternative; metronidazole is reserved for when neither is available. The classic trap is IV vancomycin, which does not reach the colonic lumen — only oral/rectal vancomycin works, whereas metronidazole is effective IV because it is biliarily excreted
• Recurrent C. difficile: after appropriate courses for multiple recurrences, fecal microbiota transplantation restores colonization resistance and is endorsed by ACG and AGA; bezlotoxumab (anti–toxin B monoclonal) is an adjunct for recurrence prevention, not primary treatment
• Don't test formed stool and don't test for cure: NAAT alone detects asymptomatic colonization, so a two-step algorithm (GDH plus toxin immunoassay, with NAAT arbitration) is preferred
• Campylobacter is the classic antecedent of Guillain–Barré syndrome; on the plate it is a comma/seagull-shaped, oxidase-positive gram-negative growing microaerophilically at 42°C
• Nontyphoidal Salmonella in an otherwise healthy adult should generally not be treated — antibiotics prolong fecal carriage. Treat infants, adults over 50 with atherosclerotic disease, the immunocompromised, and those with sickle cell disease (Salmonella osteomyelitis)
• Raw oysters plus cirrhosis equals Vibrio vulnificus: hemorrhagic bullae and fulminant sepsis; iron overload feeds the organism
• Typhoid fever: fever with rose spots and relative bradycardia (Faget sign); the diagnostic yield is highest from blood culture, not stool — a favorite distractor
• Rice-water stools point to cholera, and the single best next step in nearly every diarrhea vignette without shock remains oral rehydration solution, which exploits intact SGLT1-mediated sodium–glucose cotransport