Algorithm 1: Acute Diarrhea After Well Water Exposure

Patient with diarrhea + well water exposure
                |
                ↓
        Is diarrhea bloody?
                |
        ┌───────┴───────┐
        |               |
       YES             NO
        |               |
        ↓               ↓
    STEC likely    Watery diarrhea
        |               |
        ↓               ↓
    Order:          Mild/self-limited?
    - Stool culture     |
    - Shiga toxin   ┌───┴───┐
                    |       |
    While pending: YES     NO
    - NO antibiotics |       |
    - NO antimotility ↓       ↓
    - IV hydration  Supportive  Moderate-severe?
    - Monitor HUS   care only   |
                                ↓
                            Consider antibiotics:
                            - Azithromycin 500mg x 3d
                            - IF STEC RULED OUT

Algorithm 2: Antibiotic Decision for Confirmed E. coli Pathotype

Confirmed E. coli pathotype identified
                |
                ↓
        Which pathotype?
                |
    ┌───────────┼───────────┐
    |           |           |
    ↓           ↓           ↓
  STEC/      ETEC/EAEC/   UPEC/
  EHEC        EIEC/EPEC    NMEC
    |           |           |
    ↓           ↓           ↓
   NO       Risk stratify  YES
Antibiotics      |       Antibiotics
    |            ↓       (Always)
    |     Severe/persistent/
    |     high-risk patient?
    |            |
    |      ┌─────┴─────┐
    |     YES         NO
    |      |           |
    |      ↓           ↓
    |   Antibiotics  Supportive
    |   indicated    care only
    |      |
    └──────┴──────────────────→ Treatment complete

1. The STEC Exception - NEVER FORGET

Why STEC is different: - ONLY E. coli pathotype where antibiotics are CONTRAINDICATED - Antibiotics increase HUS risk from baseline ~6% to ~56% (14-fold increase) - Mechanism: Antibiotics cause bacterial lysis → massive Shiga toxin release → endothelial damage - Even bacteriostatic antibiotics at sub-MIC concentrations increase toxin production

Mnemonic: “STEC = STOP antibiotics”

What DOES work for STEC: - Early IV hydration (within 4 days of symptom onset) - ONLY proven intervention - Close monitoring for HUS (daily CBC, BMP up to 14 days) - Avoid: Antibiotics, antimotility agents, NSAIDs

High-risk STEC features predicting HUS: - Age <5 years or >65 years - High initial WBC count (>15,000) - Stool culture positive early in illness - Stx2 (more virulent than Stx1) - Use of antibiotics

2. Geographic & Travel Considerations

ETEC - “Traveler’s Diarrhea” - Most common cause of traveler’s diarrhea (30-40% of cases) - Endemic in: Latin America, Africa, South Asia, Middle East - Classic scenario: Tourist develops watery diarrhea 2 days into Mexico trip - Prevention: “Boil it, cook it, peel it, or forget it” - Prophylaxis: Generally NOT recommended (promote resistance)

EPEC - “Infantile Diarrhea in Developing Countries” - After rotavirus, leading cause of death in children <2 in developing countries - Rare in developed countries - Important consideration in adopted children from endemic areas - Associated with malnutrition and formula feeding (lack of protective maternal antibodies)

STEC - “The Burger Bug” - Primarily developed countries (USA, Canada, UK, Japan, Germany) - Peak incidence: Summer months (BBQ season) - Outbreaks linked to: Ground beef, unpasteurized milk/juice, swimming pools, petting zoos - Cattle are asymptomatic reservoirs

3. Antibiotic Resistance - Emerging Threats

ESBL-Producing E. coli (20-40% of isolates in some areas): - Confers resistance to most β-lactams (including 3rd generation cephalosporins) - Most common gene: blaCTX-M (CTX-M-15 predominates) - Risk factors: Recent hospitalization, nursing home residence, recent antibiotic use, international travel - Treatment: Carbapenems (ertapenem, meropenem) for severe infections

Fluoroquinolone Resistance (Rising globally): - Campylobacter: 40% in some US regions, >80% in Southeast Asia - ETEC: 10-25% in travelers returning from Asia - Treatment failure rates: 15-30% with ciprofloxacin for traveler’s diarrhea - Alternative: Azithromycin (maintains low resistance <5%)

Why Azithromycin is increasingly preferred: - Low resistance across all diarrheagenic E. coli (<5%) - Safe in pregnancy and children - Once-daily dosing - Does NOT increase STEC toxin production (unlike fluoroquinolones) - Reduces fecal shedding (institutional benefit)

4. Not All E. coli Are Equal

Commensalism vs. Pathogenicity: - ~10⁸-10⁹ E. coli CFU/gram in normal stool - >99% of gut E. coli are BENEFICIAL or neutral - Produce vitamin K2, compete with pathogens, maintain gut homeostasis - Pathogenic E. coli require SPECIFIC virulence factors (toxins, adhesins, invasins)

Cannot distinguish by routine culture: - All E. coli grow on MacConkey agar as pink/red (lactose-fermenting) colonies - Exception: EHEC O157:H7 (sorbitol-negative) and EIEC (lactose-negative) - Requires molecular testing (PCR) or immunoassays to identify pathotypes - Many labs only test for STEC; other pathotypes require specific multiplex panels

Clinical implication: - Stool culture showing “normal flora including E. coli” does NOT rule out pathogenic E. coli - Must specifically order: STEC testing, GI pathogen panel, or send to reference lab

5. Special Populations - Different Rules Apply

Immunocompromised Patients (HIV, chemotherapy, transplant): - EAEC → persistent diarrhea (weeks to months), wasting - Higher risk of bacteremia with any InPEC pathotype - More likely to need antibiotics even for typically self-limited infections - Consider empiric treatment while awaiting cultures

Neonates (<28 days): - NMEC → life-threatening meningitis/sepsis - ANY E. coli bacteremia in neonate warrants LP (20-30% have concurrent meningitis) - Empiric coverage must include E. coli (ampicillin + gentamicin OR cefotaxime) - High mortality (15-40%) even with appropriate treatment

Pregnant Women: - UTIs (UPEC) progress to pyelonephritis more rapidly - Asymptomatic bacteriuria requires treatment (unlike non-pregnant women) - Increased risk of preterm labor with pyelonephritis - Antibiotic choices: Cephalexin, nitrofurantoin (avoid in 3rd trimester), fosfomycin - Avoid: Fluoroquinolones, TMP-SMX in 1st/3rd trimester

Elderly with Atherosclerosis: - Non-typhoidal Salmonella (and E. coli) can seed atherosclerotic plaques - Risk of mycotic aneurysm, endovascular infection - Lower threshold to treat Salmonella bacteremia in elderly - May require prolonged antibiotics (4-6 weeks)

6. Post-Infectious Complications - Think Beyond Acute Illness

Hemolytic Uremic Syndrome (HUS) - STEC: - Triad: Hemolytic anemia + Thrombocytopenia + Acute kidney injury - Occurs 5-10 days after diarrhea onset (as diarrhea resolving) - Most common cause of acute renal failure in children - 3-5% mortality, 12% develop ESRD, 25% have long-term renal sequelae - No specific treatment - supportive care, dialysis if needed - Avoid platelet transfusions unless life-threatening bleeding (may worsen thrombotic microangiopathy)

Reactive Arthritis (Reiter’s Syndrome): - Follows EIEC, Shigella, Salmonella, Campylobacter infections - Classic triad: Arthritis + Urethritis + Conjunctivitis (“Can’t see, can’t pee, can’t climb a tree”) - Strongly associated with HLA-B27 - Onset: 1-3 weeks after GI infection resolves - Treatment: NSAIDs, sometimes DMARDs if persistent

Guillain-Barré Syndrome (GBS): - Most commonly follows Campylobacter (but can follow other GI infections) - Molecular mimicry: Bacterial lipooligosaccharides cross-react with gangliosides in peripheral nerves - Onset: 1-3 weeks after infection - Ascending paralysis, areflexia, potential respiratory failure - Treatment: IVIG or plasmapheresis

Post-Infectious IBS (PI-IBS): - Develops in 10-15% of patients after acute bacterial gastroenteritis - Risk factors: Female sex, younger age, severity of acute illness, psychological stress - Symptoms persist >6 months after infection clears - Treatment: Standard IBS management (dietary, probiotics, antispasmodics)

7. Laboratory Pearls - Optimizing Diagnosis

When to order what:

Multiplex GI Panels (BioFire, Luminex, BD MAX): - Pros: Rapid (1-2 hours), detects multiple pathogens simultaneously, high sensitivity - Cons: Expensive ($200-400), may detect colonization vs. true infection, not all labs have - What they detect: STEC, ETEC, EPEC, EAEC, EIEC + Salmonella, Shigella, Campylobacter, viruses, parasites - Clinical use: Consider for severe illness, immunocompromised, outbreak, negative routine culture

8. Supportive Care - Never Forget the Basics

Hydration is the cornerstone: - Prevents dehydration-related mortality (especially children, elderly) - Oral rehydration solution (ORS) preferred if patient can tolerate PO - WHO ORS formula: Na+ 75 mEq/L, glucose 75 mmol/L (promotes Na-glucose cotransport) - IV fluids if: Severe dehydration, hemodynamic instability, intractable vomiting, altered mental status

What NOT to do: - Antimotility agents (loperamide, diphenoxylate) with bloody diarrhea or fever - Can precipitate toxic megacolon - Increase risk of HUS with STEC - Prolong illness with invasive pathogens (Shigella, EIEC) - Bismuth subsalicylate in children <12 years (Reye’s syndrome risk) - NSAIDs in suspected STEC (may worsen renal injury)

Nutrition: - Resume normal diet as soon as tolerated (including regular dairy if tolerated) - BRAT diet (bananas, rice, applesauce, toast) is outdated - insufficient calories/protein - Continue breastfeeding in infants - Small, frequent meals if nausea/cramping prominent

Probiotics: - Evidence mixed; generally safe - Possible benefit: Lactobacillus GG, Saccharomyces boulardii - Do NOT give if immunocompromised (risk of fungemia with S. boulardii)

9. Public Health & Reporting Requirements

Reportable Conditions (varies by state, but commonly include): - STEC/EHEC infection (ALL cases) - Hemolytic Uremic Syndrome - Shigellosis - Salmonellosis - Campylobacteriosis - Outbreaks (≥2 linked cases)

When to report: - Suspicion is sufficient - definitive diagnosis NOT required - Report to local/state health department within 24-48 hours - Helps identify outbreaks, contaminated food sources, common exposures - Health department may require follow-up stool cultures to clear food handlers

High-risk occupations requiring clearance: - Food handlers - Healthcare workers (direct patient care) - Daycare workers - Usually require 2 negative stool cultures ≥24 hours apart, ≥48 hours after antibiotic completion

10. Prevention Strategies - Patient Counseling

For patients with well water: - Annual testing for coliforms (minimum) - Test more frequently if: Nearby contamination, heavy rain/flooding, pregnant/infant in home - Shock chlorination after positive result - Consider UV treatment or filtration system - Safe distance: Well ≥50 feet from septic system, livestock

Food safety: - Cook ground beef to 160°F (71°C) - use meat thermometer - Avoid raw milk and raw milk products - Wash produce thoroughly, especially leafy greens - Separate cutting boards for raw meat and produce - Refrigerate promptly (<2 hours, <1 hour if >90°F)

Travel advice (ETEC prevention): - “Boil it, cook it, peel it, or forget it” - Avoid: Ice, tap water, raw vegetables, street food, buffets - Safe: Bottled water (sealed), hot coffee/tea, beer, wine, freshly cooked food - Consider taking azithromycin or rifaximin for self-treatment (not prophylaxis) - Prophylactic antibiotics NOT routinely recommended (promotes resistance)

When Patient Has Diarrhea + Well Water Exposure:

1. Is it bloody diarrhea? - YES → Suspect STEC → DO NOT give antibiotics → Order Shiga toxin test + culture - NO → Continue evaluation

2. Is there fever or severe illness? - YES → Consider bacterial pathogen → Order stool culture - NO → Likely viral or mild bacterial → Supportive care

3. Is STEC ruled out (negative Shiga toxin + culture)? - YES → Can consider antibiotics if indicated by pathogen/severity - NO → Continue to withhold antibiotics

4. Which pathogen is identified? - Shigella → Give antibiotics (azithromycin) - Campylobacter → Risk stratify → Give if high-risk - Salmonella → Risk stratify → Usually no antibiotics unless high-risk - ETEC/EAEC/EIEC/EPEC → Assess severity → Consider antibiotics if moderate-severe

5. Patient is asymptomatic but worried about exposure? - NO prophylactic antibiotics - Education about symptoms to watch for - Reassurance that most exposures don’t cause illness