Gastroenteritis in adults
Most settle with fluids. Look for dehydration, blood, severe pain and high-risk patients, and remember typhoid, cholera and amoebic dysentery.
A 34-year-old man has had watery diarrhoea 10 times since yesterday with vomiting and cramps. He feels dizzy on standing. Pulse 108, BP 104/70, lying, dropping to 88/60 standing. No blood in the stool. Creatinine 1.6 mg/dL (141 µmol/L).
A 4-year-old girl has had diarrhoea for 2 days, is drinking eagerly and has slightly sunken eyes.
How dry is each one, who needs IV fluids, and does anyone need antibiotics?
- Assess dehydrationPulse, BP lying and standing, mucous membranes, urine output, mental state. In children: eyes, thirst, skin pinch, drinking.
- Mild to moderateOral rehydration solution (ORS), small frequent sips. Zinc in children.
- Severe or can't keep fluids downIV Ringer's lactate.
- Bloods if moderate or severeU&E, glucose, FBC. Stool culture if bloody, severe, prolonged, or outbreak.
- Red flagsBlood, high fever, severe pain, elderly, immunosuppressed, kidney injury, suspected cholera.
- Antibiotics only forDysentery (bloody diarrhoea with fever), cholera, severe traveller's diarrhoea, immunosuppressed.
- AdviceHand washing, safe water, return if worse.
- Severe dehydration
- Bloody diarrhoea with fever
- Kidney injury or electrolyte problems
- Rice-water stools (cholera)
- Child lethargic or unable to drink
- Diarrhoea lasting over 14 days
- Cholera
- Shigella dysentery
- Typhoid (fever with little diarrhoea)
- C. difficile after antibiotics
- Haemolytic uraemic syndrome
- Appendicitis or intussusception mimicking gastroenteritis
- DKA presenting with vomiting
| ORS (adult) | 200–400 mL after each loose stool, plus normal drinks |
| ORS (child, moderate dehydration) | 75 mL/kg over 4 hours |
| Zinc (child) | 10 mg daily under 6 months, 20 mg daily from 6 months, for 10–14 days |
| IV fluids (adult, severe) | Ringer's lactate 1–2 L quickly, then reassess |
| Severe dehydration (child) | Ringer's lactate 100 mL/kg (30 mL/kg then 70 mL/kg, timing by age per WHO plan C) |
| Avoid | Loperamide in bloody diarrhoea or in children |
Check every dose against a current drug reference and your hospital protocol.
On this page
Causes
| Cause | Clues |
|---|---|
| Viral (rotavirus, norovirus) | Vomiting, watery diarrhoea, outbreaks, children |
| E. coli (ETEC) | Traveller's diarrhoea, watery |
| Cholera | Large-volume "rice water" stools, rapid dehydration, outbreaks after floods |
| Shigella | Blood and mucus, fever, cramps |
| Campylobacter | Poultry, blood, abdominal pain |
| Salmonella (non-typhoidal) | Eggs, poultry, fever |
| Amoebiasis | Bloody diarrhoea, gradual, liver abscess |
| Giardia | Prolonged, greasy, bloating |
| C. difficile | Recent antibiotics, hospital |
How dry? (WHO for children)
Drinks normally, eyes normal, skin pinch goes back quickly: Plan A, ORS at home
Restless, sunken eyes, drinks eagerly, skin pinch goes back slowly: Plan B, ORS 75 mL/kg over 4 hours
Lethargic, very sunken eyes, drinks poorly, skin pinch very slow: Plan C, IV Ringer's lactate
Adults: moderate dehydration has thirst, dry mouth, tachycardia and a postural drop. Severe has low BP, confusion and very low urine output.
Investigations
| Test | When |
|---|---|
| U&E, glucose | Moderate or severe, elderly, comorbid |
| FBC, CRP | Fever, blood in stool |
| Stool culture and microscopy | Blood, severe, prolonged over 7 days, outbreak, immunosuppressed |
| C. difficile toxin | Recent antibiotics or hospital stay |
| Blood cultures | Fever, suspected typhoid |
| Stool for cholera (dark field, culture) | Rice-water stools |
Treatment
Pakistan: Shigella and Campylobacter often resist ciprofloxacin. Azithromycin is usually the first choice for bloody diarrhoea, guided by local data.
Special situations
- Children: continue breastfeeding and normal food. ORS and zinc. No anti-motility drugs.
- Elderly: dehydrate quickly; check kidney function and hold diuretics, ACE inhibitors, metformin and SGLT2 inhibitors (sick day rules).
- Pregnancy: fluids; avoid doxycycline and quinolones.
- Diabetes: check glucose and ketones; sick day rules.
Common mistakes
- Giving IV fluids when ORS would work.
- Loperamide for bloody diarrhoea.
- Antibiotics for all diarrhoea.
- Missing appendicitis or intussusception.
- Forgetting zinc in children.
- Not holding nephrotoxic drugs in a dehydrated patient.
Frequently asked questions
What is the first-line treatment for gastroenteritis?
Oral rehydration solution. IV fluids are for severe dehydration or when oral fluids fail.
When does gastroenteritis need antibiotics?
For dysentery with fever, cholera, severe traveller's diarrhoea, and in immunosuppressed patients.
How much ORS should a child with some dehydration get?
75 mL/kg over 4 hours, then reassess.
Why give zinc to children with diarrhoea?
It shortens the episode and reduces the chance of further episodes over the next few months.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A man has profuse rice-water diarrhoea, BP 88/56 and confusion during a local outbreak. What is the most important first treatment?
Answer: IV Ringer's lactate. Severe dehydration from cholera needs rapid IV Ringer's lactate (30 mL/kg over 30 minutes, then 70 mL/kg over 2.5 hours), then ORS and a single-dose antibiotic.
- Loperamide: Not for cholera.
- IV metronidazole: Wrong drug.
- ORS in small sips only: Too slow with shock and confusion.
- Oral doxycycline only: Fluids save lives first.
2. A 30-year-old has bloody diarrhoea with mucus, fever and cramps. Which antibiotic is preferred in Pakistan?
Answer: Azithromycin. Bacillary dysentery is usually shigella. Azithromycin 500 mg daily for 3 days is preferred.
- Amoxicillin: Resistance.
- Nitrofurantoin: Urinary only.
- Loperamide: Not an antibiotic; avoid in dysentery.
- Ciprofloxacin: Shigella resistance is common.
3. A patient on metformin, ramipril and furosemide has severe diarrhoea and vomiting. What advice applies?
Answer: Pause them until recovered. Sick-day rules: pause metformin, ACE inhibitors, ARBs, diuretics, NSAIDs and SGLT2 inhibitors during dehydrating illness.
- Stop them permanently: Restart once well.
- Double the furosemide: Worsens dehydration.
- Take them with ORS: Not enough.
- Continue all as usual: Risk of AKI and lactic acidosis.
4. An elderly inpatient on co-amoxiclav for pneumonia develops 6 watery stools a day, fever and WBC 18 × 10⁹/L. C. difficile toxin is positive. What is first-line therapy?
Answer: Oral vancomycin. C. difficile infection follows broad-spectrum antibiotics (cephalosporins, clindamycin, co-amoxiclav, quinolones) and PPIs. Stop the precipitating antibiotic if possible, isolate with contact precautions (alcohol gel doesn’t kill spores — wash with soap and water). First-line: oral vancomycin or fidaxomicin (fewer recurrences); metronidazole only if these unavailable for mild disease. Fulminant disease: oral/NG vancomycin plus IV metronidazole, surgical review. Recurrence: fidaxomicin, bezlotoxumab or faecal microbiota transplant.
- IV vancomycin infusion: IV vancomycin does not reach the colon.
- Continue co-amoxiclav: The causative antibiotic should be stopped if possible.
- Loperamide and fluids: Loperamide risks toxic megacolon.
5. A 30-year-old in a flood relief camp has profuse rice-water stools, sunken eyes, no radial pulse and is lethargic. What is the first treatment?
Answer: IV Ringer’s lactate 100 mL/kg. Severe dehydration (lethargy, absent/weak pulse, very sunken eyes) needs rapid IV Ringer’s lactate (or normal saline): 100 mL/kg — in adults 30 mL/kg in 30 minutes, then 70 mL/kg in 2½ hours. ORS is started as soon as the patient can drink. A single dose of doxycycline (or azithromycin in children/pregnancy) reduces volume and duration. Cholera toxin activates adenylate cyclase via Gs, increasing cAMP and chloride secretion.
- Oral ORS sips: Oral sips are too slow for severe dehydration.
- IV 5% dextrose: 5% dextrose contains no sodium.
- Ciprofloxacin before fluids: Fluids come before antibiotics.
6. Bloody diarrhoea follows eating undercooked chicken; weeks later the patient develops ascending paralysis. Which organism was responsible?
Answer: Campylobacter jejuni. Campylobacter jejuni (curved Gram-negative, grows at 42 °C, microaerophilic) is a commonest cause of bacterial gastroenteritis from poultry and unpasteurised milk. Complications: Guillain–Barré syndrome (molecular mimicry of gangliosides) and reactive arthritis (HLA-B27). Shigella causes dysentery with a very low infectious dose; Salmonella from eggs and poultry; Vibrio parahaemolyticus from seafood.
- Salmonella Enteritidis: Possible cause of diarrhoea, but GBS link is Campylobacter.
- Shigella dysenteriae: Dysentery, HUS.
- Vibrio parahaemolyticus: Seafood.
- Clostridium perfringens: Food poisoning, cramps.
7. After a wedding meal, several guests develop fever, cramping lower abdominal pain, tenesmus and frequent small stools containing blood and mucus. Stool culture grows a non-motile, non-lactose-fermenting Gram-negative rod. Very few organisms are needed to cause infection. Which organism is this?
Answer: Shigella. Shigella species (S. dysenteriae, flexneri, boydii, sonnei) cause bacillary dysentery. They are non-motile, non-lactose-fermenting Gram-negative rods that are not killed easily by gastric acid, so an infective dose as low as 10–100 organisms is enough; spread is faecal–oral, person to person, and via food and flies. They invade colonic M cells and epithelial cells, spread cell to cell, and cause mucosal ulceration: bloody mucoid diarrhoea with tenesmus. S. dysenteriae type 1 produces Shiga toxin (inhibits the 60S ribosomal subunit), which can cause haemolytic uraemic syndrome. Most cases are self-limiting; antibiotics (ciprofloxacin, azithromycin) shorten severe illness. Antimotility drugs are avoided.
- Salmonella Typhi: Salmonella is motile and causes enteric fever rather than dysentery with a tiny infective dose.
- Vibrio cholerae: Cholera causes painless watery diarrhoea; Vibrio is motile.
- Enterotoxigenic Escherichia coli: ETEC causes watery traveller's diarrhoea, not dysentery.
- Campylobacter jejuni: Campylobacter is a curved, motile rod; it can cause bloody diarrhoea but is not described here.
References and dose sources
- WHO fact sheet: Cholera
- ACG guideline: acute diarrhoeal infections in adults (Am J Gastroenterol 2016)
- NICE CKS — Gastroenteritis
- UKHSA — Infection control in the community
- Oral rehydration salts (ORS) dosing: MSF Essential Drugs: Oral rehydration salts (see also our oral rehydration salts (ors) dose page)
- Ringer's lactate (Hartmann's solution) dosing: Drugs.com: Lactated Ringer's dosage (see also our ringer's lactate (hartmann's solution) dose page)
- Ondansetron dosing: Drugs.com: Ondansetron dosage (see also our ondansetron dose page)
- Azithromycin dosing: Drugs.com: Azithromycin dosage (see also our azithromycin dose page)
- Doxycycline dosing: Drugs.com: Doxycycline dosage (see also our doxycycline dose page)
- Metronidazole dosing: Drugs.com: Metronidazole dosage (see also our metronidazole dose page)
- Zinc dosing: WHO: The treatment of diarrhoea, a manual for physicians (see also our zinc dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.