Acute gastroenteritis: oral rehydration first, antibiotics rarely

What acute gastroenteritis actually is

Acute gastroenteritis is inflammation of the gastrointestinal tract causing diarrhoea ± vomiting ± abdominal cramps ± fever, lasting under 14 days. Around 17 million cases occur in Australia every year, resulting in approximately 1.2 million GP presentations. It is the second most common acute illness seen in Australian general practice after respiratory infections.

Approximately 70% of cases are viral — norovirus is the most common adult pathogen, followed by adenovirus, sapovirus, and astrovirus. Rotavirus has declined significantly since inclusion in the National Immunisation Program (NIP). Bacterial causes include Campylobacter (most commonly notified bacterial pathogen in Australia, poultry-associated), Salmonella (summer seasonal peak), Shigella, and Clostridioides difficile (post-antibiotic, increasingly community-associated). Giardia is the most common parasitic cause in Australia, particularly in returning travellers and those exposed to contaminated water.

The central clinical principle is the same as for viral URTI: most cases are self-limiting and the treatment is supportive care — principally oral rehydration. Antibiotics are for specific indications, not routine diarrhoea management.

A. Core clinical — the AU general-practice framework

History essentials

The history is the most important diagnostic tool. Key questions per eTG and RACGP:

• Onset, duration, and character — watery (non-inflammatory) vs bloody (inflammatory/invasive dysentery).
• Vomiting and fever — short incubation with prominent vomiting suggests toxin-mediated illness (Staphylococcus aureus or Bacillus cereus emetic toxin); fever with blood suggests invasive bacterial pathogen.
• Exposure history — sick contacts, childcare or aged care exposure, food history (undercooked poultry or eggs for Salmonella; unpasteurised dairy for Listeria and Salmonella; seafood for norovirus, Vibrio, and scombroid; reheated rice for B. cereus).
• Travel — destination, dates, water and food exposure (Giardia, typhoid, cholera, amoebiasis, hepatitis A/E).
• Recent antibiotic or hospitalisation — Clostridioides difficile risk.
• Immunocompromise, pregnancy, comorbidities.
• Group exposure — multiple people ill after a shared meal or event is a public health notification trigger.

Examination and assessment

The priority in examination is dehydration assessment:

• Mild: normal vital signs, moist mucous membranes.
• Moderate: tachycardia, dry mucous membranes, reduced skin turgor, capillary refill 2–3 seconds.
• Severe: hypotension, postural drop, altered mental state, capillary refill >3 seconds, oliguria — these patients need IV fluids.

Abdominal examination rules out surgical causes (appendicitis, mesenteric ischaemia). Peritonism, guarding, or localised tenderness warrant urgent surgical assessment. Skin findings of note: purpura suggests haemolytic uraemic syndrome (HUS); rose spots suggest typhoid; jaundice suggests hepatitis A/E.

Investigations

Most cases are clinical; no investigations are needed. Per eTG, stool testing is warranted when:

• Bloody diarrhoea (bacterial dysentery, EHEC, IBD differential).
• Severe illness.
• Symptoms persisting ≥7 days.
• Fever.
• Immunocompromised patient.
• Recent antibiotic use or hospitalisation (C. difficile).
• Travel-related illness.
• Public health indication (food handler, childcare worker, aged care resident, suspected outbreak).

Stool tests: MCS (MBS 69300), parasites and ova (MBS 69354), C. difficile toxin/GDH/PCR (MBS 69405 range), multiplex PCR for multiple pathogens.

Bloods: FBC (MBS 65070), UEC (MBS 66500), CRP (MBS 66509), LFTs (MBS 66512), blood cultures if septic features (MBS 69319). Blood film for schistocytes if HUS is suspected (bloody diarrhoea + AKI + thrombocytopenia). Hepatitis A/E serology if travel or food exposure history.

Management — supportive care

Per eTG and AMH:

Oral rehydration solution (ORS): Hydralyte or Gastrolyte — small frequent sips; 50–100 mL/kg over 4 hours for moderate dehydration. ORS is superior to plain water, sports drinks, or soft drink because glucose-sodium co-transport in the gut allows absorption even when active secretion is occurring.

IV fluids (Hartmann’s or normal saline): for severe dehydration, persistent vomiting, or altered mental state.

Antiemetics: ondansetron 4–8 mg for adults — helps reduce vomiting and facilitates oral rehydration. Use with caution in children (TGA guidance on QT and falls risk; generally reserved for children >12 years or >40 kg unless directed by a paediatrician).

Loperamide: appropriate in adults with non-bloody, non-febrile diarrhoea — reduces stool frequency; absolutely avoid in children, dysentery, EHEC, and C. difficile.

Paracetamol for fever and pain. Avoid NSAIDs in dehydration or renal impairment (AKI risk).

Continue eating and breastfeeding — early refeeding shortens recovery. Historical “BRAT diet” (bananas, rice, applesauce, toast) has no evidence advantage over normal diet.

B. Evidence — the C. difficile guideline update and rehydration-first principle

C. difficile: vancomycin now replaces metronidazole as first-line

The Australasian Society of Infectious Diseases (ASID) 2025 guidelines updated first-line treatment for initial non-severe CDI: oral vancomycin 125 mg four times daily for 10 days now replaces metronidazole. This aligns Australian guidance with the IDSA/SHEA 2021 focused update, which showed superior cure rates and lower recurrence with vancomycin over metronidazole.

For recurrent or severe CDI: fidaxomicin 200 mg twice daily for 10 days (PBS Authority Required — specific recurrence and severity criteria apply; check current PBS listing). Fidaxomicin has a narrower gut spectrum and lower recurrence rate than vancomycin.

Fulminant CDI (hypotension, ileus, toxic megacolon): vancomycin oral 500 mg four times daily plus IV metronidazole — ICU setting, surgical review, and gastroenterology input are required.

For patients with ≥2 recurrences: faecal microbiota transplantation (FMT) achieves cure rates exceeding 85% and is available through specialist gastroenterology centres in Australia.

Oral metronidazole is no longer recommended as initial CDI treatment. It remains on the general schedule but is no longer first-line per ASID 2025.

The EHEC exception — never antibiotics for E. coli O157

EHEC (enterohaemorrhagic E. coli, serotype O157:H7) produces Shiga toxin, which binds to renal endothelium and causes haemolytic uraemic syndrome (HUS). Antibiotic treatment — particularly with fluoroquinolones — dramatically increases HUS risk by triggering Shiga toxin release from lysing bacteria. This is an absolute contraindication.

EHEC should be suspected in children or adults with bloody diarrhoea without high fever, particularly after consumption of undercooked minced beef, raw dairy, or exposure to farm animals. Admission for monitoring of renal function, haematology (schistocytes, haemolysis, thrombocytopenia), and hydration is required. No antibiotic.

Antibiotics for Salmonella and Campylobacter

Most uncomplicated Salmonella and Campylobacter infections are self-limiting and do not require antibiotics. Per eTG, antibiotic treatment is warranted for: severe illness, extraintestinal infection, immunocompromised patients, or bacteraemia.

When treating: azithromycin 500 mg daily × 3–7 days is first-line for both Campylobacter and Salmonella gastroenteritis in Australia — fluoroquinolone resistance is rising and ciprofloxacin is a second-line option pending susceptibility results.

C. Pathogen-specific management: when and how antibiotics apply

Pathogen	Treatment	Key notes
C. difficile (non-severe, initial)	Oral vancomycin 125 mg QID × 10 days	ASID 2025 first-line; PBS Authority Required
C. difficile (recurrent/severe)	Fidaxomicin 200 mg BD × 10 days	PBS Authority; specialist referral
C. difficile (≥2 recurrences)	Faecal microbiota transplantation	Specialist gastroenterology
Giardia	Tinidazole 2 g single dose	Alternative: metronidazole 2 g daily × 3 days
Shigella	Azithromycin 500 mg daily × 3 days	Resistance rising; susceptibility-guided
Campylobacter (severe)	Azithromycin 500 mg daily × 3 days	Most uncomplicated cases: no antibiotic
Salmonella (severe/extraintestinal)	Azithromycin or ciprofloxacin	Most uncomplicated cases: no antibiotic
EHEC	NO antibiotic	Precipitates HUS; admit for monitoring
Amoebiasis (E. histolytica)	Metronidazole 800 mg TDS × 7–10 days + paromomycin	Specialist input if severe
Cholera	Doxycycline 300 mg single dose + ORS	Plus aggressive rehydration; notifiable
Typhoid (S. typhi)	Azithromycin or ceftriaxone	Notifiable; specialist management
Severe traveller’s diarrhoea	Azithromycin 1 g single dose	Resistance making ciprofloxacin second-line

D. Australian operations

Key MBS items:

• Standard GP consultations: items 23 (Level B), 36 (Level C), 44 (Level D).
• ATSI Health Assessment: MBS 715 (every 9 months; relevant for Salmonella and hepatitis A screening).
• Stool MCS: MBS 69300; stool parasites: MBS 69354; C. difficile PCR: MBS 69405 range.
• Bloods: FBC MBS 65070, UEC MBS 66500, LFTs MBS 66512, CRP MBS 66509, blood cultures MBS 69319.
• GPCCMP items 965/967 for post-infectious IBS chronicity (acute episode alone does not qualify for a chronic condition management plan).

PBS-listed treatments:

• Oral vancomycin — Authority Required for C. difficile.
• Fidaxomicin (Dificid) — Authority Required (recurrent/severe CDI criteria; specialist prescribing).
• Metronidazole — general schedule (no longer first-line for CDI per ASID 2025).
• Azithromycin — general schedule.
• Tinidazole, doxycycline — general schedule.
• Ciprofloxacin, ceftriaxone — Authority Required.
• Loperamide, ondansetron — general schedule (loperamide also OTC).
• ORS (Hydralyte, Gastrolyte) — OTC; some PBS formulations.

Notifiable diseases — the following must be reported to the state Department of Health: Salmonella, Shigella, Campylobacter, Cryptosporidium, hepatitis A, typhoid (S. typhi), cholera, listeriosis, EHEC, and Vibrio. Outbreak management: any cluster in childcare, aged care, school, food service, or healthcare facility must be reported to the local public health unit.

Food handler exclusion: typically 48 hours after symptom resolution for most organisms; S. typhi, Shigella, and EHEC require negative stool clearance before return. NHMRC Staying Healthy in Childcare sets exclusion periods for childcare settings.

E. Special populations

Infants and young children. Dehydration risk is higher and progresses faster. Use weight-based ORS volumes. Ondansetron is used cautiously and under paediatric guidance in young children. Loperamide is contraindicated in children. Continue breastfeeding throughout. Rotavirus vaccination (NIP, Rotarix) has markedly reduced severe infantile gastroenteritis in Australia.

Older adults and aged care residents. Higher risk of severe dehydration and electrolyte disturbance. CDI is more common post-hospitalisation or antibiotic use. Aged care facilities are required to notify public health units of outbreaks and implement cohorting, PPE, and isolation procedures for norovirus and CDI.

Immunocompromised patients. Infections are prolonged, more severe, and more likely to be caused by unusual or opportunistic pathogens (Cryptosporidium, CMV, Microsporidia). Lower threshold for investigation, specialist input, and hospitalisation. Fidaxomicin preferred over vancomycin for CDI given reduced disruption of gut microbiome.

Pregnancy. Listeriosis (Listeria monocytogenes) carries a high risk of intrauterine infection, preterm labour, and neonatal sepsis — it is a notifiable disease. Amoxicillin is first-line antibiotic for listeriosis in pregnancy. Severe hepatitis E in pregnancy carries high maternal mortality. Ondansetron is used with caution in the first trimester — specialist input for severe hyperemesis.

Travellers. Pre-travel consultation (via Smartraveller) includes hepatitis A and typhoid vaccination for at-risk destinations, advice on water and food precautions, and provision of a self-treatment kit (single-dose azithromycin 1 g) for severe traveller’s diarrhoea.

When to escalate

Emergency department immediately for:

• Severe dehydration — altered mental state, postural hypotension, oliguria, capillary refill >3 seconds.
• Sepsis — fever + tachycardia + hypotension + altered mental state.
• Suspected haemolytic uraemic syndrome — bloody diarrhoea with AKI, thrombocytopenia, or haemolytic anaemia, especially in children.
• Surgical abdomen — peritonism, guarding, suspected appendicitis or mesenteric ischaemia.
• Toxic megacolon — severe CDI or IBD complication with abdominal distension and systemic sepsis.
• Severely immunocompromised patients with significant illness.
• Severe CDI — hypotension, ileus, or deteriorating despite treatment.

Same-week: symptoms persisting ≥7–14 days; suspected typhoid or cholera; food handler awaiting stool clearance; complex traveller with unusual features.

What this article is and is not

This is general health information drawn from current Australian general practice guidelines — Therapeutic Guidelines, ASID 2025 C. difficile guidelines, RACGP, AMH, and NHMRC Staying Healthy. It does not constitute personal medical advice and does not create a doctor–patient relationship. Management decisions, including antibiotic selection and dehydration assessment, are made with your own GP based on your complete clinical situation.

For Australian consumer resources: HealthDirect — Gastroenteritis, Better Health Channel — Gastroenteritis, Smartraveller.

---

Sources cited

1. Therapeutic Guidelines — eTG Antibiotic
2. Longhitano et al. — ASID 2025 C. difficile guidelines, IMJ 2025
3. RACGP — Acute gastroenteritis resources
4. Australian Medicines Handbook
5. NHMRC — Staying Healthy in Childcare 5th edition
6. Department of Health — Notifiable diseases
7. IDSA/SHEA 2021 C. difficile focused update
8. HealthDirect — Gastroenteritis
9. Better Health Channel — Gastroenteritis
10. Smartraveller