Severe Diarrhea / Gastroenteritis

Acute diarrheal illness — most commonly viral; evaluate for dehydration, AKI, and electrolyte disturbances

Symptoms / Associated Sx

  • Watery non-bloody (viral) or bloody diarrhea (bacterial/inflammatory)

  • Nausea, vomiting, crampy abdominal pain, fever

  • Dehydration signs: dry mucous membranes, decreased UO, tachycardia, orthostatic hypotension

Denies

  • Rectal bleeding (rules out invasive/inflammatory etiology if absent)

  • Recent antibiotics (rules out C. diff if truly absent)

  • Chronic GI disease (rules out IBD flare or malabsorption)

  • Immunosuppression (reduces atypical organism concern)

Social History (SHx)

Recent travel (ETEC, Salmonella, Shigella, Campylobacter), sick contacts, dietary history (shellfish — norovirus/Vibrio), food handlers, daycare/institutional exposure, immunosuppression.

Main Etiology

  • Viral (most common): norovirus, rotavirus, adenovirus

  • Bacterial: Campylobacter, Salmonella, Shigella, STEC, ETEC, Yersinia

  • Foodborne toxin: Staph aureus (<6h onset, vomiting dominant), B. cereus, C. perfringens

  • Parasitic: Giardia, Cryptosporidium, Entamoeba; Traveler's: ETEC most common

Most Common DDx

  • C. difficile colitis (recent antibiotics; WBC >15k; C. diff PCR positive; watery diarrhea ≥3/day — always test if antibiotic exposure or hospitalized)

  • IBD first presentation (younger patient; persistent >2 weeks; bloody mucoid stool; fecal calprotectin very elevated; colonoscopy confirms)

  • Ischemic colitis (elderly + vascular disease; sudden crampy pain then bloody diarrhea; watershed zone thickening on CT)

  • Bowel obstruction (nausea + vomiting + crampy pain but obstipation, not diarrhea; dilated loops on imaging)

  • Appendicitis (periumbilical to RLQ pain migration; nausea/vomiting; rebound; CT confirms; no profuse diarrhea)

  • Staph aureus food poisoning (vomiting dominant <6h from ingestion; pre-formed toxin; self-limited; no fever; no antibiotic needed)

  • Giardiasis (prolonged watery diarrhea >7 days + travel; bloating + sulfurous belching; stool O&P or fecal antigen positive)

DATA

  • BMP (AKI — prerenal; electrolytes); CBC (leukocytosis → bacterial; lymphocytosis → viral)

  • Stool cultures, C. diff PCR, O&P (travel), norovirus PCR if outbreak; lactate (if septic)

Home Meds

  • Loperamide (hold if bloody, febrile, or suspected STEC); ACE inhibitors/ARBs (hold if AKI)

  • Diuretics (hold if dehydrated); metformin (hold if AKI); NSAIDs (hold)

Plan

  • Rehydration (primary treatment): ORS mild-moderate; IV LR/NS 1–2 L bolus if severe/AKI; goal UO >0.5 mL/kg/hr

  • Antiemetics: Ondansetron 4–8 mg IV/PO q6h PRN; Promethazine 12.5 mg IV q6h PRN

  • Antidiarrheals: Loperamide 4 mg PO × 1, then 2 mg after each loose stool (max 16 mg/day) — ONLY non-bloody, non-febrile, non-STEC

  • Antibiotics by organism:

    • Traveler's diarrhea: Azithromycin 1 g PO × 1 OR Cipro 500 mg BID × 3 days (if non-STEC)

    • Shigella: Azithromycin 500 mg daily × 3 days; Salmonella bacteremia: Cipro 500 mg BID × 5–7 days

    • Giardia: Metronidazole 250 mg TID × 5–7 days OR Tinidazole 2 g PO × 1

    • Cryptosporidium (immunocompetent, persistent): Nitazoxanide 500 mg BID × 3 days

  • Electrolyte repletion; BRAT diet advancing to regular; hold ACE/ARB/metformin/diuretics if AKI

  • Daily BMP until AKI resolving; trend Cr; trend fever curve; follow stool culture results; PT/OT if elderly

  • Discharge: Continue ORS; advance diet; complete antibiotics if prescribed; food safety counseling; PCP follow-up 1 week; return for bloody diarrhea/high fever/dehydration recurrence

Red Flags

  • Bloody diarrhea + fever → invasive bacterial; AVOID antibiotics/antidiarrheals if STEC suspected

  • HUS features (hemolysis + thrombocytopenia + AKI) → STEC → supportive only; nephrology

  • Severe dehydration + AKI + elderly → admit for IV hydration; monitor electrolytes for arrhythmias

  • Immunocompromised + prolonged diarrhea → atypical organisms (CMV, Cryptosporidium) — ID consult

Senior IM Resident Pearls

  • Antibiotics in STEC increase HUS risk ~10× — supportive hydration only; most important rule

  • Norovirus: Alcohol-based hand sanitizer ineffective against spores; bleach cleaning + soap and water required

  • Staph aureus food poisoning: Onset <6h; vomiting dominant; pre-formed toxin; no antibiotics; self-limited in 24h

  • Common mistake: Loperamide in febrile or bloody diarrhea — toxic megacolon risk

ORDERS

Labs

Admission Labs

  • CBC with diff

  • BMP

  • CMP

  • Magnesium

  • Phosphorus

  • Lactate (if septic, hypotensive, severe illness)

Stool Studies

  • GI pathogen PCR panel

  • Stool culture (Salmonella, Shigella, Campylobacter)

  • C. difficile PCR

  • Shiga toxin assay (STEC)

  • Stool O&P (travel, prolonged symptoms)

  • Giardia antigen

  • Cryptosporidium antigen

Additional Labs

  • Blood cultures ×2 (fever, sepsis)

  • HIV test (immunocompromised)

  • CMV PCR (immunocompromised)

  • UA (dehydration)

Trending Labs

  • BMP daily

  • CBC daily

  • Cr daily

  • Electrolytes daily

  • Lactate q4–6 hr if septic

Imaging

Usually Not Needed

If Severe Pain / Diagnostic Uncertainty

CT Abdomen/Pelvis

Evaluate:

  • Colitis

  • Appendicitis

  • Ischemic bowel

  • Obstruction

If Toxic Megacolon Concern

KUB

Evaluate:

  • Colonic dilation (>6 cm)

Procedures

Usually None

If Severe Colitis

Colonoscopy / Sigmoidoscopy

  • IBD

  • CMV colitis

  • Unclear diagnosis

Medications

Rehydration (Most Important)

Mild–Moderate

Oral Rehydration Solution (ORS)

Goal:

  • Maintain urine output

  • Replace stool losses

Severe Dehydration / AKI

LR Preferred

  • 1–2 L IV bolus

Then:

  • 100–150 mL/hr maintenance

OR

NS

  • If LR contraindicated

Goal:

  • UO >0.5 mL/kg/hr

Antiemetics

Ondansetron

  • 4–8 mg IV/PO q6h PRN

Alternative

  • Promethazine 12.5 mg IV q6h PRN

Alternative

  • Metoclopramide 10 mg IV q6h PRN

Antidiarrheals

ONLY IF:

  • Non-bloody diarrhea

  • No fever

  • No STEC concern

Loperamide

  • 4 mg PO x1

  • Then 2 mg after each loose stool

  • Max 16 mg/day

Avoid

  • Bloody diarrhea

  • Fever

  • STEC

  • Severe colitis

Organism-Specific Treatment

Traveler's Diarrhea

Azithromycin

  • 1 g PO x1

OR

Ciprofloxacin

  • 500 mg PO BID x3 days

Shigella

Azithromycin

  • 500 mg PO daily x3 days

OR

Ciprofloxacin

  • 500 mg PO BID x3 days

Salmonella

Severe Disease/Bacteremia

Ciprofloxacin

  • 500 mg PO BID
    OR

  • 400 mg IV q12h
    x5–7 days

Campylobacter

Azithromycin

  • 500 mg PO daily x3 days

Giardia

Metronidazole

  • 250 mg PO TID x5–7 days

OR

Tinidazole

  • 2 g PO x1

Cryptosporidium

Nitazoxanide

  • 500 mg PO BID x3 days

STEC (E. coli O157:H7)

Supportive Care Only

DO NOT GIVE:

  • Antibiotics

  • Loperamide

Monitor:

  • CBC

  • Platelets

  • Creatinine

(HUS surveillance)

Electrolyte Repletion

Potassium

  • Replete K >4

Magnesium

  • Replete Mg >2

Phosphorus

  • Replete as needed

Hold Medications

Hold If AKI

  • ACE inhibitors

  • ARBs

  • Diuretics

  • Metformin

  • NSAIDs

Hold

  • Loperamide if bloody/febrile

  • Antibiotics if STEC suspected

Consults

Infectious Disease

  • Severe diarrhea

  • Immunocompromised

  • Persistent symptoms

  • CMV concern

Gastroenterology

  • Persistent >2 weeks

  • Bloody diarrhea

  • Suspected IBD

Nephrology

  • HUS

  • Severe AKI

  • Electrolyte abnormalities

PT/OT

  • Elderly

  • Weakness

  • Deconditioning

Nutrition

  • Malnutrition

  • Prolonged illness

Nursing Orders

Monitoring

  • Strict I&O

  • Stool count

  • Daily weights

  • Orthostatic vitals

Diet

  • BRAT diet initially

  • Advance as tolerated

Notify Physician Immediately

  • Bloody diarrhea

  • Fever

  • Hypotension

  • Decreased urine output

  • Worsening abdominal pain