Acute & Chronic Diarrhoea — classification, red flags, mechanism, and the antibiotic-or-not decision viva

Interpretation

The examiner presents a 25-year-old man with 2 days of acute watery diarrhoea, vomiting and cramps, and a 68-year-old care-home resident 6 days after clindamycin with profuse watery diarrhoea, confusion and AKI. The examiner asks: "Tell me how you classify diarrhoea, how you decide who needs stool testing and antibiotics, and how your approach changes in the second patient."

• Classification by duration. Acute (under 14 days, usually infective and self-limiting), persistent (14 to 29 days) and chronic (over 4 weeks — demands a structured workup).^[1]
• Classification by mechanism. Watery non-inflammatory (viral, ETEC, cholera — small-bowel, enterotoxin, no blood, low inflammatory markers), inflammatory/invasive (Shigella, Salmonella, Campylobacter, EIEC, EHEC, amoebae — colonic, blood/mucus, fever, faecal leukocytes/calprotectin), and the four mechanisms of all diarrhoea — secretory (toxin drives CFTR-mediated Cl- and water secretion; persists on fasting), osmotic (unabsorbed solute; stops on fasting; osmotic gap over 50), inflammatory/exudative, and altered motility/dysbiosis.^[2]
• Bristol Stool Scale types 5 to 7 (especially type 7, watery) constitute diarrhoea; types 1 to 2 are constipation.

Key points

1. WHO/IMCI dehydration grading (must reproduce verbatim): no dehydration; some dehydration = two or more of restlessness/irritable, sunken eyes, drinks eagerly/thirsty, skin pinch goes back slowly (Plan B: ORS 75 mL/kg over 4 h); severe dehydration = two or more of lethargy/unconscious, sunken eyes, unable to drink, skin pinch goes back very slowly (Plan C: IV Ringer's lactate 100 mL/kg in divided boluses).^[1]
2. Most acute diarrhoea needs no testing and no antibiotics — supportive care with low-osmolarity ORS (Na 75, glucose 75, ~245 mOsm/L), exploiting the SGLT1 sodium-glucose co-transporter to absorb water across the intact enterocyte, plus early refeeding.^[1]
3. Test stool when: blood/mucus, severe or prolonged illness, fever, immunocompromise, recent antibiotics/healthcare exposure, recent travel, daycare/food-handler, suspected outbreak. Use stool culture (Salmonella/Shigella/Campylobacter/E. coli O157), ova/cysts/parasites (3 samples on alternate days; antigen/PCR more sensitive for Giardia/Crypto), C. difficile (GDH + toxin EIA, or NAAT + toxin; never formed stool), and multiplex PCR panels where available.^[3]
4. When to give antibiotics (ACG 2016/IDSA 2017): moderate-severe traveller's diarrhoea (azithromycin 500 mg daily x 3 d preferred, safe in pregnancy/children and in high fluoroquinolone-resistance regions; rifaximin 200 mg TDS x 3 d for non-invasive watery; fluoroquinolones where resistance is low); invasive dysentery (azithromycin); immunocompromise; fever with systemic illness. Specific agents: shigellosis, campylobacteriosis, cholera (doxycycline 300 mg single + aggressive ORS), Giardia (metronidazole 400 mg TDS x 5-7 d), Entamoeba (metronidazole 750 mg TDS x 10 d then a luminal agent), typhoid (ceftriaxone then azithromycin/fluoroquinolone).^[1][3]
5. When NOT to give antibiotics: suspected/confirmed Shiga-toxin E. coli (EHEC) — raises HUS risk; uncomplicated Salmonella — prolongs carriage; unrecognised C. difficile — worsens colitis. Suspected HUS (schistocytes, thrombocytopenia, AKI after bloody diarrhoea) is supportive only.^[3]
6. C. difficile end-to-end (SHEA/IDSA 2021): risk = antibiotics (clindamycin, fluoroquinolones, cephalosporins, penicillins), age over 65, healthcare exposure, PPI, chemotherapy. Diagnose with GDH + toxin EIA (or NAAT + toxin). Treat: fidaxomicin 200 mg BD x 10 d first-line; metronidazole 400 mg TDS x 10 d if unavailable; vancomycin 125 mg QDS (or 500 mg QDS oral + IV metronidazole if fulminant). Recurrence 15-30 percent — manage with vancomycin taper/pulse, fidaxomicin, or faecal microbiota transplant. Infection control: contact precautions, soap-and-water hand hygiene (alcohol gel does NOT kill spores), sporicidal environmental cleaning.^[4]
7. Antimotility cautions. Loperamide (4 mg load, then 2 mg per loose stool, max 16 mg/day) is safe for non-bloody, afebrile watery diarrhoea in adults; avoid in bloody/febrile/inflammatory diarrhoea, in young children, and in C. difficile (toxic megacolon). Zinc 20 mg daily x 10-14 d in children under 5 (WHO — shortens duration and prevents future episodes).^[1]
8. Chronic diarrhoea workup (over 4 weeks). IBS-D is the commonest cause (Rome IV, no red flags). Investigations: FBC, CRP, coeliac serology (tTG IgA + total IgA), TFTs, faecal calprotectin (under 50 excludes IBD), stool for ova/cysts/parasites and Giardia antigen, stool osmotic gap (over 50 osmotic; under 50 secretory), stool elastase (pancreatic insufficiency), bile-acid tests (SeHCAT, 7alpha-hydroxy-4-cholesten-3-one), breath tests (lactose, glucose for SIBO), colonoscopy + ileoscopy with biopsies (IBD, microscopic colitis), OGD with duodenal biopsies (coeliac), FIT and colonoscopy in over-50s or with red flags. Nocturnal diarrhoea, weight loss, blood, anaemia or onset over 50 exclude IBS.^[2]

Escalation questions

• "How would your management differ if the stool were bloody with fever and a recent hamburger meal?" — Suspect Shiga-toxin E. coli (O157:H7); send stool on sorbitol-MacConkey / Shiga-toxin PCR; avoid antibiotics and antimotility; monitor for HUS (FBC, film, U&E); supportive fluid and BP support; nephrology if HUS.^[3]
• "What post-infectious complications will you counsel the patient about?" — Guillain-Barre (Campylobacter), reactive arthritis/Reiter and HLA-B27 associations (Shigella, Salmonella, Campylobacter, Yersinia, Chlamydia), erythema nodosum (Yersinia), HUS (EHEC), and post-infectious IBS (~10 percent after a bacterial gastroenteritis).
• "A returning trekker has foul-smelling greasy floating stools and bloating — what is it and how do you treat it?" — Giardia lamblia (pear-shaped trophozoites with 'falling-leaf' motility, cysts); metronidazole 400 mg TDS x 5-7 days (or tinidazole 2 g single, or nitazoxanide); confirm with stool antigen/PCR if microscopy negative.
• "Why does ORS work even in cholera, where the toxin over-secretes?" — The cholera toxin leaves the enterocyte structurally intact and the SGLT1 sodium-glucose co-transporter functional, so oral sodium, glucose and water are still absorbed — the rationale for low-osmolarity ORS over IV fluids wherever the gut works.^[1]

References

1. Riddle MS, et al. ACG Guideline: Acute Diarrheal Infections in Adults. Am J Gastroenterol 2016.^[1]
2. Schiller LR. Evaluation of chronic diarrhea. Am J Gastroenterol 2018.^[2]
3. Shane AL, et al. IDSA Guideline: Infectious Diarrhea. Clin Infect Dis 2017.^[3]
4. Johnson S, et al. SHEA/IDSA C. difficile Focused Update 2021. Clin Infect Dis 2021.^[4]

References4ShowHide

1. [1]Riddle MS, DuPont HL, Connor BA, et al. ACG Clinical Guideline: Diagnosis, Treatment, and Prevention of Acute Diarrheal Infections in Adults. American Journal of Gastroenterology, 2016.PMID 27068718
2. [2]Schiller LR. Evaluation of chronic diarrhea and irritable bowel syndrome with diarrhea in adults in the era of precision medicine. American Journal of Gastroenterology, 2018.PMID 29713027
3. [3]Shane AL, Mody RR, Crump JA, et al. 2017 Infectious Diseases Society of America Clinical Practice Guidelines for the Diagnosis and Management of Infectious Diarrhea. Clinical Infectious Diseases, 2017.PMID 29194529
4. [4]Johnson S, Lavergne V, Skinner AM, et al. Clinical Practice Guideline by the Infectious Diseases Society of America (IDSA) and Society for Healthcare Epidemiology of America (SHEA): 2021 Focused Update Guidelines on Management of Clostridioides difficile Infection in Adults. Clinical Infectious Diseases, 2021.PMID 34492699