Acute Gastroenteritis

Acute inflammation of the GI tract, typically infectious, causing diarrhea with or without vomiting: usually self-limiting but capable of causing severe dehydration, particularly in children and the elderly.

Common etiologies: viral (rotavirus, leading cause in children; norovirus, leading cause across all ages and outbreaks; adenovirus), bacterial (Salmonella, Campylobacter, Shigella, E. coli including toxin-producing strains, Vibrio cholerae), parasitic (Giardia, Cryptosporidium, Entamoeba histolytica), and toxin-mediated (S. aureus, B. cereus: rapid onset within hours of ingestion).

• Diarrhea; watery suggests a viral, toxin or small bowel process; bloody or mucoid suggests an invasive bacterial or inflammatory process (Shigella, Campylobacter, EHEC, amoebiasis); with vomiting, abdominal cramping and fever

• Onset timing is a clue to cause: <6 hours (preformed toxin — S. aureus, B. cereus), 8-16 hours (C. perfringens), >16 hours (viral, invasive/toxigenic bacterial, parasitic)

• Dehydration assessment (critical, especially in children and the elderly):

  • Mild: normal examination, thirsty

  • Moderate: dry mucous membranes, reduced skin turgor, sunken eyes or fontanelle, tachycardia

  • Severe: lethargy, very sunken eyes, minimal or no urine output, cold extremities, weak pulse, hypotension

• Red flags for an invasive bacterial cause: bloody diarrhea, high fever, severe abdominal pain, recent antibiotic use (C. difficile), immunocompromise, recent travel, systemic toxicity

• EHEC (E. coli O157:H7): hemolytic uremic syndrome risk — microangiopathic hemolytic anemia, thrombocytopenia and acute kidney injury, typically 5-10 days after diarrhea onset, more common in children. AVOID antibiotics if EHEC is suspected, as they may increase HUS risk through increased toxin release

Most cases are a clinical diagnosis needing no investigation.

Investigate if: bloody diarrhea, severe illness, immunocompromise, recent hospitalization or antibiotic use, symptoms >7 days, systemic toxicity, outbreak setting, or dehydration requiring IV therapy:

• Stool culture, microscopy and sensitivity
• Stool ova and parasites (travel history, prolonged symptoms, immunocompromise)
• C. difficile toxin/PCR if recent antibiotic exposure or healthcare-associated diarrhea
• Stool viral PCR or antigen where relevant
• FBC, U&E/creatinine (dehydration, AKI), blood cultures if systemically unwell

Differentials: appendicitis (if pain localizes or worsens with minimal diarrhea), inflammatory bowel disease flare, ischemic colitis, medication-induced diarrhea, other surgical abdomen.

Rehydration — the cornerstone:

• Mild-moderate dehydration: oral rehydration solution (WHO low-osmolarity formula preferred); continue age-appropriate diet and breastfeeding in infants

• Severe dehydration: IV fluids

• Antiemetics: ondansetron (oral or IV) can reduce vomiting and improve oral rehydration success, particularly useful in children to avoid the need for IV fluids

Antibiotics: NOT routine. Most cases are viral or self-limiting bacterial infections. Consider only for:

• Severe Shigella infection, or Shigella in vulnerable patients: ciprofloxacin or azithromycin

• Campylobacter: azithromycin if severe or prolonged (>1 week) or immunocompromised, most cases self-limit

• Confirmed C. difficile: oral vancomycin or fidaxomicin first-line; metronidazole reserved for milder cases or where alternatives are unavailable

• Giardiasis: metronidazole or tinidazole

• Amoebic dysentery: metronidazole followed by a luminal agent (paromomycin) to eradicate cyst carriage

• AVOID antibiotics if EHEC/STEC is suspected: HUS risk

Antimotility agents (loperamide): AVOID in bloody diarrhea, suspected invasive bacterial infection, or in children (toxic megacolon, prolonged illness, HUS risk). May be considered cautiously in adults with non-bloody, non-severe traveler's diarrhea.

Zinc supplementation: children <5 years per WHO protocol: reduces duration and severity.

Nutrition: continue age-appropriate diet as tolerated once rehydrated: prolonged fasting is not recommended, early refeeding supports mucosal recovery.

Public health: notifiable in outbreak settings, exclude from food handling or childcare until symptom-free per local guidance (typically 48 hours post-resolution), hand hygiene education.

Admission criteria: severe dehydration, inability to tolerate oral rehydration despite antiemetics, significant comorbidity, extremes of age with moderate-severe illness, or signs of systemic sepsis.