Red flags
- Bloody diarrhoea + fever + abdominal pain -> invasive bacterial pathogen (Shigella, Campylobacter, EHEC) - avoid antimotility agents
- Bloody diarrhoea + AKI + thrombocytopenia -> HUS (E. coli O157:H7) - do not give antibiotics, may increase toxin release
- Recent antibiotic use + diarrhoea -> C. difficile until excluded
- Severe dehydration, haemodynamic instability
- Immunosuppression - broadens differential (parasitic, viral, atypical) and raises invasive disease risk
- Diarrhoea >2 weeks -> reconsider parasitic, IBD, or non-infective cause
Differential by mechanism
By mechanism (organises the work-up)
- Secretory/toxin-mediated (watery): Vibrio cholerae, enterotoxigenic E. coli (traveller's diarrhoea), Staph aureus/Bacillus cereus food poisoning (rapid onset, preformed toxin), norovirus/rotavirus
- Inflammatory/invasive (bloody, +/- fever): Campylobacter (commonest bacterial cause in Australia), Salmonella, Shigella, enterohaemorrhagic E. coli (EHEC/O157:H7), Entamoeba histolytica
- C. difficile - post-antibiotic, healthcare-associated, toxin-mediated colitis
- Parasitic (subacute/chronic, often travel-related): Giardia (bloating, foul-smelling, no blood), Cryptosporidium (severe in immunosuppressed)
Non-infective mimics
- IBD flare, ischaemic colitis, IBS, coeliac disease, medication effect, overflow diarrhoea
Focused history
- Onset, duration, stool character (watery vs bloody vs mucoid), frequency
- Recent antibiotic use (any class, any recency within ~8 weeks) - C. difficile
- Travel history and food exposures - undercooked meat/poultry (Campylobacter, Salmonella), seafood, unpasteurised dairy, untreated water
- Sick contacts, institutional outbreaks (aged care, childcare), recent cruise/camp
- Immunosuppression, HIV status
- Systemic symptoms - fever, weight loss (chronicity marker)
- Hydration status indicators (thirst, urine output, dizziness)
Focused examination
- Hydration assessment - skin turgor, mucous membranes, postural BP, capillary refill, mental state
- Abdominal exam - tenderness, peritonism (concerning for toxic megacolon/perforation in severe colitis)
- Fever, tachycardia
- Signs of chronic disease if prolonged course (weight loss, clubbing, extraintestinal IBD signs)
Investigation strategy
- Most acute, self-limiting watery diarrhoea in a well patient needs no stool testing
- Stool culture/PCR panel indicated for: bloody diarrhoea, severe illness, immunosuppression, recent hospitalisation/antibiotics, outbreak setting, prolonged symptoms (>1 week), or before public health notification requirement
- C. difficile toxin/PCR - specifically requested if recent antibiotic exposure
- Stool microscopy for ova/cysts/parasites if travel history, prolonged symptoms, or eosinophilia
- Bloods: UEC (dehydration/AKI), FBE (eosinophilia, thrombocytopenia if HUS suspected), blood cultures if systemically unwell
- Notify public health for notifiable pathogens (Salmonella, Shigella, EHEC, and others per state requirements) - legal obligation, not optional
Management
A. Supportive care - the mainstay for most cases
- Oral rehydration first-line; IV fluids if severe dehydration/unable to tolerate oral intake
- Avoid antimotility agents (loperamide) in bloody/inflammatory diarrhoea or suspected EHEC - risk of toxic megacolon or worsened toxin-mediated disease; reasonable for uncomplicated watery traveller's diarrhoea
B. Antibiotics - targeted, not routine
- Most bacterial gastroenteritis in an otherwise well host is self-limiting and does not need antibiotics (including most Campylobacter and Salmonella)
- Consider antibiotics for: severe illness, immunosuppression, extremes of age with systemic symptoms, or specific high-risk pathogens (typhoidal Salmonella, Shigella in severe disease, Giardia)
- Empirical fluoroquinolone or azithromycin if treatment indicated for presumed bacterial dysentery pending culture, adjusted to local resistance patterns
- EHEC/suspected HUS - avoid antibiotics (may increase Shiga toxin release and HUS risk)
C. C. difficile infection
- Stop the precipitating antibiotic if possible
- Mild-moderate: oral vancomycin or fidaxomicin first-line (metronidazole now reserved for when these are unavailable, per updated guidance); avoid antimotility agents
- Severe/fulminant (hypotension, ileus, megacolon): oral vancomycin +/- IV metronidazole, surgical review
- Recurrent CDI: fidaxomicin preferred, consider faecal microbiota transplant for multiple recurrences
D. Giardia
- Tinidazole or metronidazole
E. Public health
- Exclusion from work/school (especially food handlers, childcare) until asymptomatic per local health department guidance
- Notify as required
Traps
- Giving loperamide in bloody diarrhoea or suspected EHEC
- Giving antibiotics for suspected EHEC/HUS, risking toxin release
- Treating routine uncomplicated bacterial gastroenteritis with antibiotics when supportive care alone is appropriate
- Missing C. difficile because the antibiotic exposure was "a while ago" - risk persists for weeks after cessation
- Failing to notify public health for a notifiable enteric pathogen
Talk track
Mechanism (secretory/watery vs inflammatory/bloody) and exposure history (antibiotics, travel, food, sick contacts) drive the differential and the decision to test. Most acute watery diarrhoea in a well patient needs oral rehydration alone, no stool testing, no antibiotics. Bloody diarrhoea changes the approach - avoid antimotility agents, consider EHEC/HUS and withhold antibiotics if suspected, and test/notify for the invasive bacterial pathogens. Recent antibiotic exposure means test for C. difficile and treat with oral vancomycin or fidaxomicin, not metronidazole first-line.
8 of 8 sections written · drafted 2026-09-13