Infectious Colitis

Infectious colitis is inflammation of the colon due to bacterial, viral, or parasitic pathogens. Typical features are acute diarrhoea, abdominal pain, and systemic upset following food/water exposure or travel. Severe cases can mimic IBD or ischaemic colitis—history and stool testing are key.

🔬 Pathophysiology

• Toxin-mediated injury: Shiga toxin (E. coli O157), C. difficile toxins A/B → epithelial damage, secretory diarrhoea, risk of HUS or toxic megacolon.
• Invasive mucosal disease: Shigella, Campylobacter, Salmonella, Entamoeba → ulceration → dysentery (blood/mucus), fever.
• Inflammation → ↑ permeability, exudation; cytokine-driven systemic features (fever, CRP rise).
• Sequelae: post-infectious IBS, reactive arthritis, Guillain–Barré (Campylobacter).

🧾 Causes & Risk Factors

• 🦠 Bacterial: Campylobacter, Salmonella, Shigella, STEC/EHEC (E. coli O157), C. difficile, Yersinia.
• 🧬 Parasitic: Giardia, Entamoeba histolytica, Cryptosporidium.
• 🦠 Viral: Norovirus, adenovirus; CMV in immunosuppressed.
• Risks: undercooked poultry/meat 🍗, unpasteurised products, unsafe water 💧, travel ✈️, recent antibiotics 💊, PPIs, immunosuppression, extremes of age.

🩺 Clinical Features

• Watery diarrhoea 💦 or dysentery 🩸, urgency, tenesmus, lower abdo pain.
• Fever 🌡️, malaise; dehydration (thirst, oliguria, dizziness).
• Prolonged or recurrent symptoms → consider parasites (Giardia), HIV, or post-infectious IBS.

⚠️ Red Flags (admit/urgent)

• Severe dehydration, sepsis features (tachycardia, hypotension, confusion).
• Persistent bloody diarrhoea, severe abdominal pain/distension (toxic megacolon risk).
• Immunosuppression, pregnancy, frailty/older age, CKD.
• Suspected HUS (anaemia, low platelets, AKI) after bloody diarrhoea.

🔎 Investigations

• Bloods: FBC (WCC, Hb, platelets), CRP/ESR, U&E (renal function), LFTs.
• Stool tests:
  • C&S for Salmonella, Shigella, Campylobacter, and STEC (request E. coli O157/Shiga toxin if bloody diarrhoea).
  • C. difficile toxin/PCR if recent antibiotics or hospital exposure.
  • Ova, cysts, parasites for Giardia/Entamoeba (especially travel, prolonged course).
• Imaging: CT if severe illness, peritonism, toxic megacolon concern.
• Endoscopy: Rare in acute infective phase; consider if recurrent or to exclude IBD.

💊 Management (UK, initial)

• Supportive (cornerstone): Oral/IV rehydration 💧, electrolytes, simple diet; avoid alcohol and high-fat foods.
• Antimotility: Loperamide only if afebrile and non-bloody; avoid in dysentery/systemic illness.
• Antibiotics (case-by-case):
  • Not routinely for mild disease. Consider if severe/systemic, high-risk host, or proven susceptible pathogen.
  • Campylobacter/Salmonella/Shigella: consider azithromycin or a fluoroquinolone per local AMR guidance.
  • C. difficile: stop culprit antibiotic; oral vancomycin or fidaxomicin; isolate.
  • Giardia/Entamoeba: metronidazole/tinidazole; add luminal agent for amoebiasis if available.
  • STEC/EHEC (E. coli O157): avoid antibiotics (↑ HUS risk) and avoid loperamide.
• Infection control & public health: Hand hygiene 🧼, exclude food handlers/HCW until 48 h symptom-free; notify Health Protection Team for notifiable organisms (e.g., Salmonella, Shigella, STEC).
• Admission thresholds: red flags above, uncontrolled vomiting, AKI, frailty, or inability to maintain hydration.

🧪 Quick Pathogen Cheat Sheet

⚖️ Differentials

• IBD flare (UC/Crohn’s), ischaemic colitis, microscopic colitis, colorectal cancer.

🎯 High-Yield Exam Tips

• Start with hydration + infection control in any OSCE answer ✅.
• Bloody diarrhoea → send stool for Shiga toxin/STEC; avoid loperamide and antibiotics.
• Recent antibiotics or hospital → test for C. difficile and isolate.
• Notifiable infections: liaise with Health Protection Team; exclude food handlers/HCW until 48 h symptom-free.

🦠 Case 1 — Campylobacter Colitis

A 28-year-old man develops abdominal cramps, fever, and bloody diarrhoea three days after eating undercooked chicken at a barbecue. Stool culture grows Campylobacter jejuni. 💡 Campylobacter is the most common bacterial cause of foodborne colitis in the UK. It causes an acute, self-limiting colitis, but can be complicated by Guillain–Barré syndrome or reactive arthritis. Management is supportive with fluids; antibiotics (e.g. macrolides) are reserved for severe or high-risk cases.

🦠 Case 2 — Clostridioides difficile Colitis

A 72-year-old woman develops profuse watery diarrhoea, fever, and abdominal tenderness after a recent hospital admission and broad-spectrum antibiotics. Stool toxin assay is positive for C. difficile. 💡 C. difficile colitis occurs when disruption of gut flora allows toxin-producing bacteria to proliferate, causing colitis and potentially life-threatening toxic megacolon. Management includes stopping the offending antibiotic, initiating oral vancomycin or fidaxomicin, and strict infection control measures.

🦠 Case 3 — Amoebic Colitis

A 34-year-old man returning from India presents with gradual onset abdominal pain, tenesmus, and bloody mucoid diarrhoea. Stool microscopy reveals Entamoeba histolytica trophozoites. 💡 Amoebic colitis is a parasitic infection spread via contaminated food and water in endemic regions. It may mimic IBD clinically and can cause extraintestinal complications such as liver abscesses. Management involves metronidazole to eradicate tissue infection, followed by a luminal agent (e.g. paromomycin) to clear cysts.