Acute Diarrhoea 

1. Definition

Acute diarrhoea = Passage of ≥3 loose/watery stools per day lasting <14 days.

• Persistent: 14–28 days
• Chronic: >4 weeks

Always clarify duration, volume, blood, travel, antibiotics, immunocompromise, and systemic toxicity.

Acute Diarrhoea vs Dysentery 

2. Pathophysiology 

Diarrhoea occurs when:

• ↓ Absorption
• ↑ Secretion
• ↑ Osmotic load
• ↑ Motility
• Mucosal inflammation

Types of Acute Diarrhoea

3. Causes of Acute Diarrhoea

A. Infectious (Most Common)

Viral (Most common overall)

• Rotavirus – common in children
• Norovirus – outbreaks, cruise ships
• Adenovirus
• Astrovirus

Features:

• Watery diarrhoea
• Vomiting prominent
• Low-grade fever
• Self-limiting

 Bacterial

• Salmonella typhi (Reptiles)
• Shigella dysenteriae(Blood/mucus)
• Campylobacter jejuni( Poultry)
• Vibrio cholerae(Severe dehydration)

• Clostridioides difficile(Recent antibiotics)
• Enterotoxigenic E. coli (ETEC)(travellers Diarrhea)

 Protozoal

• Giardia lamblia – greasy stool
• Entamoeba histolytica – blood + liver abscess risk
• Cryptosporidium – immunocompromised

B. Non-Infectious Causes

• Drugs (antibiotics, metformin, magnesium)
• Inflammatory bowel disease
• Ischaemic colitis
• Thyrotoxicosis
• Early presentation of coeliac disease

4. Clinical Assessment 

History

Incubation Period 

Red Flags 

• Hypotension
• Tachycardia
• Oliguria
• Bloody diarrhoea
• Severe abdominal pain
• Age >65
• Immunocompromised

5. Investigations

When NOT required

• Mild watery diarrhoea <3 days
• No systemic toxicity

When required

• Bloody diarrhoea
• Severe dehydration
• Hypotension
• Elderly (>65)
• Immunocompromised
• Persistent >7 days
• Recent antibiotics
• Recent hospital admission
• Suspected outbreak
• Severe abdominal pain

1. Full Blood Count (FBC)

Thrombocytopenia + AKI + haemolysis → Think EHEC → HUS.

2. Urea & Electrolytes (U&E)

Essential in moderate–severe cases

Look for:

• ↑ Urea → dehydration
• Hypokalaemia → diarrhoeal losses
• Metabolic acidosis → bicarbonate loss
• AKI

Always correct potassium.

3. CRP

• High CRP → inflammatory diarrhoea
• Normal CRP → viral more likely

4. Blood Cultures

Indicated if:

• High fever
• Systemic toxicity
• Immunocompromised
• Suspected typhoid

Important in:

• Salmonella typhi
• Severe Salmonella enterica

5. Stool Microscopy, Culture & Sensitivity (MCS)

Indications:

• Bloody diarrhoea
• Severe illness
• Travel history
• Persistent >7 days
• Immunocompromised

Detects:

• Campylobacter jejuni
• Shigella dysenteriae
• Salmonella enterica
• Yersinia enterocolitica

 6. Stool C. difficile Testing

Indicated if:

• Recent antibiotics
• Hospitalised
• Age >65

Tests:

1. GDH antigen
2. Toxin assay
3. PCR

Target organism:

• Clostridioides difficile

 Do NOT send formed stool for C. diff testing.

 7. Stool Ova, Cysts & Parasites (OCP)

Indications:

• Travel to tropics
• Persistent diarrhoea
• Immunocompromised
• Greasy stool

Detects:

• Giardia lamblia
• Entamoeba histolytica
• Cryptosporidium

 8. Stool PCR Panels

Advantages:

• Faster
• Detects multiple pathogens
• More sensitive

Disadvantage—>May detect colonisation

 9. Stool for Shiga Toxin

Important if:

• Bloody diarrhoea
• Minimal fever
• Children
• Suspected EHEC

Organism:

• Escherichia coli O157:H7

 Avoid antibiotics in suspected EHEC (risk of HUS).

10. Imaging

Usually NOT required.

CT Abdomen Indicated If:

• Severe abdominal pain
• Suspected:
• Toxic megacolon
• Ischaemic colitis
• Perforation

Common in:

• Severe Clostridioides difficile
• Elderly with vascular disease

11. Endoscopy

Rarely needed acutely.

Indications:

• Suspected IBD
• Persistent diarrhoea
• Severe C. diff with complications

6. Management 

STEP 1: Assess Severity (Always First)

Assess:

• BP, HR
• Capillary refill
• Urine output
• Mental state
• Degree of dehydration

Signs of Severe Dehydration

• Hypotension
• Tachycardia
• Oliguria
• Sunken eyes
• Dry mucosa
• Confusion

If unstable → Resuscitate first (ABCDE).

 Admission Criteria

• Severe dehydration
• Electrolyte imbalance
• AKI
• Elderly/frail
• Immunocompromised
• Suspected surgical abdomen

 FLUID MANAGEMENT 

 A. Mild–Moderate Dehydration

Oral Rehydration Solution (ORS)

• First-line in almost all patients
• WHO-type balanced glucose-electrolyte solution

Mechanism:

• Glucose enhances sodium absorption via SGLT1 → water follows

Encourage:

• Small frequent sips
• Continue feeding

 B. Severe Dehydration

IV Fluids

• 0.9% normal saline or balanced crystalloid
• 1–2 L bolus if hypotensive
• Monitor electrolytes

Correct:

• Hypokalaemia
• Metabolic acidosis

 NUTRITION

• Continue normal diet
• Avoid prolonged fasting
• Avoid excessive fruit juice (osmotic diarrhoea)

No need for “BRAT diet” routinely in adults.

 ANTIDIARRHOEAL DRUGS

 Loperamide

Indications:

• Mild watery diarrhoea
• No blood
• No fever

Mechanism:

• μ-opioid receptor → ↓ gut motility

 Avoid if:

• Bloody diarrhoea
• Suspected invasive bacterial infection
• Suspected C. diff

 ANTIBIOTICS – SELECTIVE USE ONLY

Most acute diarrhoea = viral → NO antibiotics.

 When NOT to Give Antibiotics

• Mild viral gastroenteritis
• Suspected EHEC (risk of HUS)
• Escherichia coli O157:H7

7. Complications

• Dehydration
• Electrolyte imbalance
• Acute kidney injury
• Haemolytic uraemic syndrome (EHEC)
• Reactive arthritis (Campylobacter)
• Guillain–Barré syndrome