Acute Gastroenteritis (AGE) — Final Year MBBS Review (Based on Harrison's Principles of Internal Medicine)
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"acute gastroenteritis" management
I now have all the content I need. Let me compile the comprehensive Final Year MBBS review on Acute Gastroenteritis based on Harrison's and supporting textbooks.
Acute Gastroenteritis (AGE) - Final Year MBBS Review
Based on Harrison's Principles of Internal Medicine 22nd Edition (2025)
1. DEFINITION
Acute Gastroenteritis (AGE) is an inflammation of the gastrointestinal tract (stomach and intestines) characterized by the acute onset of diarrhea (with or without nausea, vomiting, fever, and abdominal pain), typically lasting <14 days. Diarrhea is defined as passage of ≥3 unformed stools in 24 hours.
2. EPIDEMIOLOGY
- One of the most common illnesses worldwide; second leading cause of death in developing countries
- Predominantly affects children under 5 and the elderly
- Rotavirus: leading cause in infants/toddlers globally
- Norovirus: leading cause in adults and older children; responsible for most outbreaks in community settings (cruise ships, nursing homes, recreational camps)
- Seasonal variation: rotavirus peaks in winter (temperate climates)
- Fecal-oral route and contaminated food/water are primary transmission modes
3. CLASSIFICATION
By Duration
| Type | Duration |
|---|---|
| Acute | <14 days |
| Persistent | 14-30 days |
| Chronic | >30 days |
By Mechanism (Pathophysiology)
| Type | Mechanism | Pathogen Examples |
|---|---|---|
| Noninvasive / Secretory | Toxin or virus impairs absorption, stimulates secretion; no mucosal destruction | Norovirus, Rotavirus, ETEC, Vibrio cholerae |
| Invasive / Inflammatory | Organism invades mucosa; cytotoxin-mediated destruction; bloody/mucoid stools | Shigella, Salmonella, Campylobacter, EHEC, EIEC, Entamoeba |
| Osmotic | Non-absorbable solutes draw water into lumen | Lactose intolerance, laxative abuse, sorbitol ingestion |
4. ETIOLOGY
Viral (Most Common - ~70% of AGE)
| Virus | Key Features |
|---|---|
| Norovirus | Leading cause in adults; outbreaks on cruise ships/nursing homes; highly contagious (low inoculum); incubation 12-48 h; watery diarrhea, vomiting, fever, myalgia; lasts 1-2 days |
| Rotavirus | Leading cause in infants/toddlers; highly contagious (survives on surfaces); incubation 1-3 days; fever, vomiting, watery diarrhea; lasts 5-7 days (longer in immunocompromised); ~50% of household children infected when one member affected |
| Enteric Adenovirus | Common in infants/young children; incubation 8-10 days (longest); lasts 5-12 days; most adults asymptomatic |
| Astrovirus | Milder illness; children and elderly; similar to norovirus |
Bacterial
| Pathogen | Mechanism | Key Clue | Incubation |
|---|---|---|---|
| ETEC (Enterotoxigenic E. coli) | Heat-labile (LT) and heat-stable (ST) toxins → secretory diarrhea | Traveler's diarrhea | 14-50 h |
| EPEC | Attaches, effaces microvilli | Infantile diarrhea in developing world | 12-72 h |
| EHEC (O157:H7) | Shiga toxin → hemorrhagic colitis; HUS | Bloody diarrhea, no fever; rare/absent WBCs in stool | 3-4 days |
| EIEC | Invasive; similar to Shigella | Dysentery-like | 1-3 days |
| Shigella | Invasion of mucosal epithelium + Shiga toxin | Classic dysentery (fever, blood, mucus in stool); highly infectious (very low inoculum - 10-100 organisms) | 1-7 days |
| Salmonella (non-typhoidal) | Mucosal invasion, inflammatory diarrhea | Poultry, eggs, reptile contacts; bacteremia risk | 6-48 h |
| Salmonella Typhi | Penetrates Peyer patches → enteric fever | Relative bradycardia, Rose spots, splenomegaly, leukopenia | 7-14 days |
| Campylobacter | Invasive; ascending paralysis complication | Post-infectious Guillain-Barre syndrome risk | 1-7 days |
| V. cholerae | Cholera toxin → massive secretory diarrhea | "Rice-water stools", severe dehydration | Hours-5 days |
| Yersinia enterocolitica | Penetrates intact intestinal mucosa, multiplies in Peyer patches | Pseudoappendicitis (RLQ pain), reactive arthritis | 1-11 days |
| C. difficile | Toxin A (enterotoxin) + Toxin B (cytotoxin) | Post-antibiotic diarrhea, pseudomembranous colitis | During/after antibiotics |
| Staph. aureus | Preformed toxin (enterotoxin) | Rapid onset (1-6 h), no fever, vomiting predominant | 1-6 h |
| B. cereus | Preformed toxin (emetic type) or enterotoxin (diarrheal type) | Fried rice syndrome (emetic: 1-6 h); diarrheal: 8-16 h | 1-16 h |
| Clostridium perfringens | Enterotoxin | Meat/poultry; self-limited, 24 h | 8-24 h |
Protozoal
| Parasite | Key Features |
|---|---|
| Giardia lamblia | Watery/greasy, foul-smelling stools; bloating, flatulence; lasts >14 days; acquired from contaminated water; treat with metronidazole, tinidazole, or nitazoxanide |
| Cryptosporidium | Profuse watery diarrhea; self-limited in immunocompetent (5-10 days); chronic in HIV (CD4<50 → biliary involvement/sclerosing cholangitis); treat with nitazoxanide |
| Entamoeba histolytica | Amebic dysentery; bloody diarrhea; liver abscess complication; treat with metronidazole + luminal agent (diloxanide) |
| Cyclospora cayetanensis | Low-grade fever, fatigue, prolonged course (weeks-months if untreated); treat with TMP-SMX |
| Cystoisospora (Isospora) | Indistinguishable from Cryptosporidium clinically; responds promptly to TMP-SMX (unlike Cryptosporidium) |
5. PATHOPHYSIOLOGY
Host Defense Mechanisms (Harrison's)
-
Intestinal microbiota: Colonization resistance - geographic/nutritional exclusion of pathogens; >99% of normal colonic microbiota is anaerobic bacteria; acidic pH and volatile fatty acids are critical. Disruption by antibiotics increases susceptibility.
-
Gastric acid: Critical barrier - achlorhydria (PPI use, post-gastrectomy) increases susceptibility to Salmonella, Giardia, and helminths. Rotavirus and Shigella are notably acid-stable and can survive this barrier.
-
Intestinal motility: Peristalsis clears bacteria from the proximal small intestine. Impaired motility (opioids, antidiarrheal drugs, anatomic abnormalities) increases bacterial overgrowth and infection risk. Note: Lomotil (diphenoxylate + atropine) in Shigella infection prolongs fever and organism shedding; opioids in Salmonella gastroenteritis increase bacteremia risk.
-
Intestinal mucin: Complex mucus layer separates luminal microbiota from epithelium; turns over rapidly; acts as a physical and immunological barrier.
-
Secretory IgA: Prevents microbial adhesion to epithelium.
-
Innate immune responses: Toll-like receptors on enterocytes recognize pathogen-associated molecular patterns (PAMPs); trigger inflammatory cytokines.
Mechanisms of Diarrhea Production
Secretory (Non-inflammatory):
- Vibrio cholerae: Cholera toxin activates adenylyl cyclase → ↑cAMP → massive Cl⁻ secretion into intestinal lumen → osmotic water loss ("rice-water stools")
- ETEC: LT toxin (same mechanism as cholera - ↑cAMP); ST toxin (↑cGMP via guanylyl cyclase)
- Rotavirus/Norovirus: NSP4 (rotavirus enterotoxin) acts as viroporin; direct enterocyte damage and intestinal epithelium dysfunction
- Staphylococcal enterotoxin: Preformed; rapid onset; acts as superantigen
Invasive (Inflammatory/Dysenteric):
- Shigella/EIEC: Invade mucosal epithelial cells → intracellular multiplication → spread to adjacent cells → mucosal destruction → bloody mucoid stools (dysentery)
- Salmonella: Invades bowel mucosa without the full clinical dysentery syndrome; bacteremia risk
- S. Typhi and Yersinia: Penetrate intact mucosa → multiply in Peyer patches and lymph nodes → systemic dissemination → enteric fever
6. CLINICAL FEATURES
Approach to History
Key questions:
- Onset and duration of symptoms
- Character of stool: watery vs. bloody/mucoid (secretory vs. inflammatory)
- Associated symptoms: fever, vomiting, abdominal pain, tenesmus
- Epidemiologic clues:
- Recent travel (ETEC, Salmonella, Shigella, Giardia, Cryptosporidium)
- Food history (last meal, suspected contaminated food)
- Outbreak setting (multiple cases - norovirus, Staph. aureus, Salmonella)
- Contact with animals/reptiles (Salmonella, Campylobacter)
- HIV/immunocompromised status (Cryptosporidium, CMV, Microsporidium, Cyclospora)
- Antibiotic use (C. difficile)
- Hospitalization (C. difficile, nosocomial pathogens)
- Sexual history (MSM: Shigella, Neisseria, Entamoeba)
Clinical Syndromes
| Syndrome | Key Features | Common Causes |
|---|---|---|
| Watery (secretory) diarrhea | Large volume, no blood/mucus, minimal fever | Norovirus, Rotavirus, ETEC, V. cholerae |
| Dysentery | Small-volume bloody/mucoid stools, fever, tenesmus, abdominal pain | Shigella, EIEC, Entamoeba, Campylobacter |
| Enteric fever | Fever, relative bradycardia, rose spots, splenomegaly, leukopenia, abdominal pain | S. Typhi, S. Paratyphi, Yersinia |
| Food poisoning (preformed toxin) | Rapid onset (<6 h), vomiting predominant, brief | Staph. aureus, B. cereus (emetic) |
| Traveler's diarrhea | Self-limited watery diarrhea in traveler | ETEC, Campylobacter, Norovirus |
Dehydration Assessment (WHO Classification)
| Degree | Clinical Features |
|---|---|
| No dehydration | Alert, normal eyes/mouth, normal skin turgor, drinks normally |
| Some dehydration | Restless/irritable, sunken eyes, dry mouth, reduced skin turgor, drinks eagerly |
| Severe dehydration | Lethargic/unconscious, very sunken eyes, dry mucous membranes, absent skin turgor, unable to drink |
7. DIAGNOSIS
Approach (Harrison's)
Most AGE is self-limited and does NOT require investigations.
Indications for stool culture/workup:
- Bloody or inflammatory diarrhea (fecal leukocytes/lactoferrin positive)
- Severe dehydration
- Fever >38.5°C
- Duration >3-7 days
- Immunocompromised host
- Systemic manifestations
- Community outbreak investigation
- Recent antibiotic use (rule out C. difficile)
- Healthcare-associated diarrhea
Investigations
| Test | What It Detects | When to Use |
|---|---|---|
| Stool culture | Bacteria (Salmonella, Shigella, Campylobacter, Yersinia, E. coli O157:H7) | Inflammatory diarrhea, severity, outbreak |
| Fecal leukocytes | Inflammatory diarrhea | Screen for invasive etiology |
| Fecal calprotectin/lactoferrin | Intestinal inflammation marker | More sensitive than fecal leukocytes |
| C. difficile toxin A/B (EIA or NAAT) | C. difficile colitis | Post-antibiotic diarrhea |
| Stool O&P (ova and parasites) | Giardia, Entamoeba, Cryptosporidium | Persistent diarrhea, travel history |
| Stool ELISA/PCR | Viral antigens (Rotavirus, Norovirus), Giardia/Crypto antigen | Outbreak settings, immunocompromised |
| Multiplex PCR panels | Multiple pathogens simultaneously | Rapid, broad diagnosis |
| Blood culture | Bacteremia (Salmonella typhi) | Enteric fever, toxic patient |
| CBC, electrolytes, BUN/Cr | Assess severity, dehydration, HUS (low platelets, hemolytic anemia + renal failure) | Bloody diarrhea (EHEC), severe disease |
| Sigmoidoscopy/colonoscopy | C. difficile (pseudomembranes), CMV colitis, IBD | Persistent/severe colitis |
| D-xylose absorption test | Small intestine malabsorption | Chronic/osmotic diarrhea workup |
Note on EHEC (O157:H7): Antibiotics are contraindicated in EHEC as they increase the risk of Hemolytic Uremic Syndrome (HUS) by increasing toxin release. HUS = Microangiopathic hemolytic anemia + thrombocytopenia + acute renal failure.
8. MANAGEMENT
Oral Rehydration Therapy (ORT) - CORNERSTONE
The WHO low-osmolarity ORS (2002) is the standard:
- Na 75 mEq/L, Cl 65 mEq/L, Glucose 75 mmol/L, K 20 mEq/L, Citrate 10 mEq/L
- Osmolarity: 245 mOsm/L (reduced from original 311 mOsm/L)
- Mechanism: glucose-sodium co-transport (SGLT1) in enterocytes drives Na absorption, water follows
WHO ORS can be prepared at home: Dissolve in 1L of clean water: 3.5 g NaCl + 2.5 g NaHCO₃ + 1.5 g KCl + 20 g glucose.
IV Fluids: Indicated when:
- Severely dehydrated
- Unable to drink
- Vomiting precludes oral therapy
- Use Ringer's Lactate (preferred over normal saline)
Antiemetics
- Ondansetron (0.15 mg/kg/day): Serotonin (5-HT₃) antagonist; effective in reducing vomiting from gastroenteritis during oral rehydration phase. Supported by multiple studies.
Dietary Advice
- Continue normal feeding during treatment (do NOT fast)
- BRAT diet (Bananas, Rice, Applesauce, Toast) has limited evidence
- Breastfeeding should continue in infants
- Avoid hyperosmolar fluids (fruit juices, sports drinks) as they can worsen diarrhea
Antimotility Agents
- Loperamide: Safe in secretory (non-inflammatory) diarrhea; contraindicated in bloody diarrhea, high fever, suspected invasive infection, and children <2 years
- Codeine/opioids: Increase bacteremia risk in Salmonella gastroenteritis; avoid in invasive infections
- Diphenoxylate + atropine (Lomotil): Prolongs fever and organism shedding in Shigella; contraindicated
Specific Antibiotic Therapy
| Pathogen | First-Line | Alternative | Notes |
|---|---|---|---|
| Shigella | Ciprofloxacin 500 mg BD x 3 days | Azithromycin; TMP-SMX (if sensitive) | Resistance to TMP-SMX common |
| Salmonella (non-typhoidal) | Usually NO antibiotics | Ciprofloxacin if at risk for bacteremia | Antibiotics NOT recommended for uncomplicated disease (prolong carrier state); treat high-risk patients (extremes of age, immunocompromised, prosthetic devices, hemoglobinopathy) |
| Enteric fever (S. Typhi) | Ciprofloxacin / Ceftriaxone | Azithromycin (oral) | Increasing fluoroquinolone resistance from South Asia |
| Campylobacter | Azithromycin | Ciprofloxacin (increasing resistance) | Treat if severe/prolonged |
| C. difficile (mild-moderate) | Vancomycin 125 mg QID x 10 days OR Fidaxomicin | Metronidazole (alternative) | Stop precipitating antibiotics |
| C. difficile (severe/complicated) | Vancomycin PO ± IV metronidazole | Fecal microbiota transplant for recurrence | Colectomy for fulminant colitis |
| Giardia | Metronidazole 250 mg TID x 5-7 days | Tinidazole (single dose 2g); Nitazoxanide | |
| Entamoeba histolytica | Metronidazole x 7-10 days THEN luminal agent (diloxanide furoate or paromomycin) | Luminal agent essential to clear cysts | |
| Cryptosporidium | Nitazoxanide | Supportive in immunocompetent | Antiretroviral therapy is cornerstone in HIV |
| Cyclospora | TMP-SMX | ||
| Cystoisospora | TMP-SMX | Responds promptly (unlike Crypto) | |
| Cholera | Doxycycline 300 mg single dose | Azithromycin; TMP-SMX | Antibiotics adjunct to aggressive ORS |
| EHEC (O157:H7) | NO antibiotics | Supportive care | Antibiotics increase HUS risk |
| Norovirus/Rotavirus | Supportive (ORS) | Nitazoxanide (small trials for rotavirus) | Self-limited; vaccine available for Rotavirus |
| Traveler's diarrhea | Azithromycin OR Rifaximin | Ciprofloxacin (increasing resistance) |
Probiotics
- A 2025 meta-analysis (PMID: 40739406) confirmed efficacy of probiotics in reducing duration and severity of AGE in children (meta-analysis of RCTs). Lactobacillus rhamnosus GG and Saccharomyces boulardii have the most supporting evidence.
9. COMPLICATIONS
| Complication | Association |
|---|---|
| Dehydration & electrolyte imbalance | All causes, especially cholera and rotavirus |
| Hemolytic Uremic Syndrome (HUS) | EHEC O157:H7 (Shiga toxin) |
| Guillain-Barre Syndrome | Campylobacter jejuni (post-infectious) |
| Reactive arthritis (Reiter's syndrome) | Salmonella, Shigella, Campylobacter, Yersinia |
| Intestinal perforation | S. Typhi (3rd week), severe Shigella/Ameba |
| Liver abscess | Entamoeba histolytica |
| Toxic megacolon | C. difficile, severe Shigella |
| Chronic diarrhea | Post-infectious IBS, Giardia, Cryptosporidium (in HIV) |
| Intussusception | Natural rotavirus infection (especially in older children); original RotaShield vaccine (withdrawn) |
| Pseudomembranous colitis | C. difficile |
| Bacteremia/sepsis | Salmonella, Campylobacter (elderly, immunocompromised) |
10. PREVENTION
Vaccines
| Vaccine | Target | Notes |
|---|---|---|
| Rotavirus vaccine (RotaTeq, Rotarix) | Rotavirus | Oral live-attenuated; 2-3 doses in infancy; major reduction in rotavirus mortality globally; slight increased risk of intussusception (very small) |
| Typhoid vaccine (Vi polysaccharide / Ty21a) | S. Typhi | Used in endemic areas and travelers; not 100% protective |
| Cholera vaccine (Dukoral, Vaxchora) | V. cholerae | Oral; travelers to endemic areas |
Non-Vaccine Prevention
- Hand hygiene with soap and water (especially before food handling, after toilet use)
- Safe water and sanitation (WASH)
- Proper food handling and cooking temperatures
- Breastfeeding (protective against infantile diarrhea)
- Rotavirus vaccination (most significant public health intervention for childhood AGE)
11. SPECIAL CONSIDERATIONS
HIV/Immunocompromised Patients
- Greater risk of chronic/severe AGE from usual pathogens
- Cryptosporidium: biliary involvement when CD4 <50/μL (serum ALP elevation) → consider sclerosing cholangitis
- Cystoisospora, Cyclospora, CMV colitis, Microsporidium are important in AIDS
- Antiretroviral therapy to suppress viral load is the cornerstone of managing Cryptosporidiosis in HIV
Traveler's Diarrhea (TD)
- Most common illness in travelers
- Onset usually within first week of travel
- Common destinations: South/Southeast Asia, Sub-Saharan Africa, Latin America
- Etiology: ETEC (most common bacterial), Campylobacter, Shigella, Norovirus
- Treatment: Azithromycin (preferred, due to increasing fluoroquinolone resistance); rifaximin for non-invasive TD
- Prophylaxis: Bismuth subsalicylate; rifaximin (high-risk individuals)
Food-Borne Illness by Incubation Period (High-Yield)
| Onset | Organism | Mechanism |
|---|---|---|
| 1-6 hours | Staph. aureus, B. cereus (emetic) | Preformed toxin |
| 8-16 hours | C. perfringens, B. cereus (diarrheal) | Toxin produced in gut |
| 16-48 hours | Norovirus, ETEC | Infection |
| 1-3 days | Salmonella, Campylobacter | Invasion |
| 3-5 days | EHEC, Yersinia | Invasion/toxin |
| 7-14 days | Giardia | Cyst to trophozoite |
| 8-10 days | Adenovirus | Viral replication |
12. HIGH-YIELD EXAM MNEMONICS & PEARLS
"RICE-WATER stool" = Cholera (massive secretory diarrhea; V. cholerae)
"The 3 S's of low-inoculum organisms" = Shigella, Staph. aureus, Norovirus (few organisms cause disease)
"EHEC - No Antibiotics" = Hemolytic Uremic Syndrome risk (EHEC O157:H7)
"Campylobacter - GBS connection" = Most common antecedent infection in Guillain-Barre Syndrome
"cAMP up" = Cholera + ETEC (LT) | "cGMP up" = ETEC (ST)
"Rotavirus NSP4" = First described viral enterotoxin
Acid-stable pathogens (survive gastric acid): Rotavirus, Shigella (very low inoculum needed)
Antidiarrheals contraindicated in: Bloody diarrhea, suspected EHEC, Shigella (prolonged fever/shedding with Lomotil), children <2 years
ORS works because: Glucose-Na co-transport (SGLT1) is intact even during secretory diarrhea - this is the physiologic basis of oral rehydration therapy
Rotavirus vaccine - reduced osmolarity ORS (245 mOsm/L vs original 311 mOsm/L) - the reduced osmolarity version decreased unscheduled IV use by ~33%
Sources: Harrison's Principles of Internal Medicine 22nd Edition (2025), Frameworks for Internal Medicine, Goldman-Cecil Medicine, Sleisenger and Fordtran's Gastrointestinal and Liver Disease, Yamada's Textbook of Gastroenterology
Recent evidence: A 2025 meta-analysis (PMID: 40739406) confirms probiotics significantly reduce duration and severity of AGE in children.
Make a comparison table of viral vs bacterial vs parasitic gastroenteritis
Here's a comprehensive comparison table based on the content already synthesized from Harrison's and supporting textbooks:
Viral vs Bacterial vs Parasitic Gastroenteritis — Comparison Table
| Feature | Viral | Bacterial | Parasitic |
|---|---|---|---|
| Common Pathogens | Norovirus, Rotavirus, Enteric Adenovirus, Astrovirus | Salmonella, Shigella, Campylobacter, ETEC, EHEC, V. cholerae, C. difficile, Staph. aureus, C. perfringens | Giardia lamblia, Entamoeba histolytica, Cryptosporidium, Cyclospora, Cystoisospora |
| Onset | Acute (hours to 1-3 days) | Acute (hours to 5 days); preformed toxin = 1-6 h | Subacute to gradual (7-14 days) |
| Duration | Short: 1-7 days (Norovirus 1-2 days; Rotavirus 5-7 days; Adenovirus up to 12 days) | Short to medium: 1-7 days; C. difficile can be prolonged | Prolonged: >14 days; Giardia/Cyclospora can last weeks to months if untreated |
| Stool Character | Watery, large volume, no blood | Watery (toxigenic) OR bloody/mucoid (invasive) | Watery, greasy, foul-smelling (Giardia); bloody (Entamoeba); bulky, watery (Crypto) |
| Fever | Low-grade or absent (Rotavirus/Norovirus may have mild fever) | Often present; high fever in invasive infections (Shigella, Salmonella, Campylobacter) | Usually absent or low-grade; present in Entamoeba invasive disease |
| Vomiting | Prominent (especially Norovirus and Rotavirus) | Variable; prominent with preformed toxins (Staph. aureus) | Mild or absent |
| Abdominal Pain | Mild cramping | Moderate to severe; tenesmus in dysentery | Cramping, bloating, flatulence (Giardia); colicky pain (Entamoeba) |
| Tenesmus | Absent | Present in dysentery (Shigella, EIEC, Entamoeba) | Present in amebic dysentery |
| Blood/Mucus in Stool | Absent | Present in invasive types (Shigella, EHEC, Campylobacter, Salmonella) | Present in Entamoeba histolytica ("flask-shaped" ulcers) |
| Fecal Leukocytes | Absent | Present in invasive bacterial infections | Absent (most); present in Entamoeba |
| Inoculum Required | Very low (Norovirus, Rotavirus - highly contagious) | Variable: Shigella = 10-100 organisms; Salmonella = 10⁵-10⁸ organisms; V. cholerae = 10⁸ | Moderate to low: Giardia cysts = 10-25; Cryptosporidium = 10-100 oocysts |
| Transmission | Fecal-oral; vomitus aerosols (Norovirus); contaminated surfaces | Contaminated food/water; fecal-oral; zoonotic (Salmonella from poultry/reptiles) | Contaminated water (Giardia, Cryptosporidium); food, fecal-oral (Entamoeba) |
| Outbreak Setting | Cruise ships, nursing homes, schools (Norovirus); daycare (Rotavirus) | Restaurant/food handling outbreaks (Salmonella, Staph. aureus); hospital (C. difficile) | Travelers; waterborne outbreaks (Giardia, Crypto); immunocompromised clusters |
| Seasonality | Rotavirus peaks in winter (temperate); Norovirus year-round | Year-round; Campylobacter peaks in summer | Giardia peaks in late summer/fall; Cyclospora associated with produce outbreaks |
| Age Group | Rotavirus: infants/toddlers; Norovirus: all ages, predominant in adults | All ages; C. difficile in elderly/hospitalized; ETEC in travelers | All ages; Giardia peaks in children and late summer; Crypto severe in HIV |
| Mechanism | Direct enterocyte damage; viroporins (Rotavirus NSP4 - first viral enterotoxin described); impaired absorption | Secretory (↑cAMP/cGMP via toxins) OR invasive mucosal destruction | Trophozoite attachment and mucosal invasion (Entamoeba); brush-border damage (Giardia); intracellular parasitism (Crypto) |
| cAMP/cGMP | Not applicable | cAMP ↑: Cholera toxin, ETEC (LT); cGMP ↑: ETEC (ST) | Not applicable |
| Systemic Features | Myalgia, headache (Norovirus) | Bacteremia (Salmonella, Campylobacter); enteric fever (S. Typhi - rose spots, relative bradycardia, splenomegaly) | Liver abscess (Entamoeba); biliary involvement in Crypto + HIV |
| Extra-intestinal Complications | Post-infectious: rare | Guillain-Barre (Campylobacter); HUS (EHEC); reactive arthritis (Salmonella, Shigella, Campylobacter, Yersinia); toxic megacolon (C. difficile, Shigella) | Liver abscess (Entamoeba); sclerosing cholangitis (Crypto in HIV CD4 <50) |
| Diagnosis | Clinical; ELISA/PCR for rotavirus antigen, norovirus PCR; multiplex stool panels | Stool culture (gold standard); C. difficile toxin EIA/NAAT; stool NAAT panels | Stool O&P (microscopy); ELISA antigen detection (Giardia, Crypto); trichrome stain (Entamoeba) |
| Fecal Lactoferrin/Calprotectin | Negative | Positive in invasive infections | Negative (usually) |
| Blood Culture | Not indicated | Indicated in S. Typhi (enteric fever), severe Salmonella, toxic patients | Not indicated |
| Treatment | Supportive (ORS); Ondansetron for vomiting; self-limited | ORS + specific antibiotics (pathogen-dependent); NO antibiotics for EHEC; supportive for most food poisoning | Pathogen-specific antiparasitic agents (see below) |
| Specific Antibiotic/Antiparasitic | None routinely (Nitazoxanide trialed for Rotavirus) | Ciprofloxacin/Azithromycin (Shigella); Vancomycin/Fidaxomicin (C. difficile); Doxycycline (Cholera); Ceftriaxone (Typhi) | Metronidazole/Tinidazole (Giardia, Entamoeba); Nitazoxanide (Crypto, Giardia); TMP-SMX (Cyclospora, Cystoisospora) |
| Antibiotics Contraindicated | N/A | EHEC O157:H7 (↑ HUS risk); uncomplicated non-typhoidal Salmonella (prolongs carrier state) | N/A |
| Antidiarrheal Agents | Safe in uncomplicated cases | Contraindicated in invasive/bloody diarrhea (Shigella, EHEC) - prolongs shedding/increases bacteremia | Generally safe; treat underlying infection |
| Vaccine Available | Rotavirus (RotaTeq, Rotarix) - oral live-attenuated, 2-3 doses in infancy | Typhoid (Vi polysaccharide / Ty21a); Cholera (Dukoral, Vaxchora) | None routinely available |
| Hospitalization Rate | Rotavirus: high in infants (IV rehydration); Norovirus: low (most manage at home) | Variable: high in severe Salmonella, Shigella, Typhi; low in ETEC | Low in immunocompetent; high in HIV/immunocompromised |
| Immunocompromised Risk | Rotavirus prolonged shedding | Salmonella bacteremia greatly increased; C. difficile recurrence ↑ | Cryptosporidiosis: chronic, life-threatening; Cyclospora/Cystoisospora more severe |
| Key High-Yield Clue | Outbreak setting + no blood + short duration + vomiting prominent | Food history + incubation period + presence of blood/fever | Travel + prolonged course + greasy/foul stools + immunocompromised status |
Quick Memory Aid
| Viral | Bacterial | Parasitic | |
|---|---|---|---|
| Stool | Watery, no blood | Watery or bloody | Watery, greasy, or bloody |
| Fever | Low/absent | High (invasive) | Low/absent |
| Vomiting | Prominent | Variable | Mild |
| Duration | Days | Days-week | Weeks-months |
| Treatment | ORS only | ORS ± antibiotics | Antiparasitic |
| Fecal WBCs | Absent | Present (invasive) | Absent (mostly) |
Sources: Harrison's Principles of Internal Medicine 22nd Edition (2025), Frameworks for Internal Medicine, Goldman-Cecil Medicine
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