Cholera
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Direct answer
Painless, effortless, voluminous watery diarrhoea with flecks of mucus — the classic rice-water stool — plus vomiting and rapid dehydration with a history of contaminated water points to Vibrio cholerae. The toxin, encoded by a lysogenic CTX phage, ADP-ribosylates the Gs protein, constitutively activating adenylate cyclase; the resulting rise in cAMP drives chloride secretion through CFTR and blocks sodium absorption, so the stool is electrolyte-rich water with no blood, no pus and no fever. Treatment is aggressive rehydration — WHO Plan C with Ringer's lactate 100 ml/kg for severe dehydration — followed by a single dose of doxycycline to shorten shedding; antibiotics never replace fluids.
What you must remember
- Organism: Gram-negative comma-shaped, darting-motile bacillus; O1 (classical and El Tor biotypes, Ogawa and Inaba serotypes) and O139 Bengal; the current pandemic strain is derived from El Tor. Growth is alkaline — it thrives at pH 8-9 in alkaline peptone water and forms yellow colonies on TCBS agar (sucrose fermented).
- Rapid bedside clue: dark-field microscopy of fresh stool shows darting "shooting star" motility that is abolished by adding specific antisera (the immobilisation test).
- Toxin signature: no mucosal invasion — hence no fever, no tenesmus, no blood; the stool has a fishy odour and mucus flecks, with sodium, potassium and bicarbonate losses producing hypokalaemia and a non-anion-gap metabolic acidosis.
- Dehydration grading drives everything: no dehydration (Plan A, ORS at home), some dehydration (Plan B, ORS 75 ml/kg over four hours), severe dehydration (Plan C, IV Ringer's lactate 100 ml/kg — 30 ml/kg fast in adults within 30 minutes, then 70 ml/kg over 2.5 hours).
- Fluid rules: Ringer's lactate is preferred because it contains potassium and bicarbonate; normal saline is acceptable; plain dextrose or glucose-without-electrolyte fluids are useless. Reassess and upgrade the plan at each review.
- Antibiotics: doxycycline 300 mg single dose (azithromycin in pregnancy and children) shortens illness by about a day and stops shedding within 48 hours; tetracycline resistance circulates regionally.
- Special situations: cholera sicca — ileus with fluid pooling in the gut lumen and shock despite absent diarrhoea; hypokalaemia and hypoglycaemia appear after rehydration in children; pregnant women lose the fetus from volume depletion, not from the toxin.
- Vaccines: killed whole-cell oral vaccines (Shanchol, Euvichol) given as two doses protect for years; a simplified formulation, Euvichol-S, received WHO prequalification in 2025 to stretch the global stockpile — vaccines are outbreak tools, not substitutes for water safety.
A treatment pathway walked through
A 40-year-old man arrives from an area with a broken water supply with eight hours of profuse watery diarrhoea: sunken eyes, absent radial pulse, cold clammy skin, wrinkled "washerwoman" fingers, voice reduced to a whisper, and a systolic blood pressure barely recordable. This is severe dehydration, so he is treated under Plan C with Ringer's lactate 100 ml/kg — 30 ml/kg within the first half hour and the remaining 70 ml/kg over the next two and a half hours — and if a vein cannot be secured, a second peripheral line or intraosseous access is established, because in cholera the intravenous fluid is the treatment. Around this, the team checks the three traps: recheck pulse and mentation hourly; anticipate hypokalaemia once output resumes, adding potassium per protocol or ORS sips; and start ORS 800 ml per litre of ongoing stool loss once he can drink. Once rehydrated, give doxycycline 300 mg once, send stool for culture on TCBS with the dark-field clue available immediately, and switch surveillance on: a single confirmed case in a non-endemic area is an outbreak signal demanding notification, water testing, chlorine distribution and a ring assessment for other cases. Families are taught the three rules — give ORS continuously, keep feeding, and return for lethargy or absent urine — because most cholera deaths in India occur from delayed presentation, not from the microbe.
Where students slip
The recurring mistake is elevating antibiotics to primary therapy; the mark goes to "prompt rehydration with Ringer's lactate", and the antibiotic is worth a sentence, not a paragraph. Second is the fluid choice: choosing "normal saline with 5 per cent dextrose" or half-normal dextrose saline is a planted wrong option, because glucose solutions replace water but not the bicarbonate and potassium being lost. Third, mechanistic slips — writing that cholera toxin "kills enterocytes" or that the stool contains blood and pus; it is purely secretory, which is exactly why fever and leucocytosis argue for a different diagnosis such as shigellosis. A quiet viva favourite is the growth requirement: alkaline peptone water for enrichment and TCBS agar where V. cholerae forms yellow sucrose-fermenting colonies, while the O139 strain matters historically as the first non-O1 strain to cause epidemic cholera.
Frequently asked questions
What is the mechanism of cholera toxin?
The A subunit ADP-ribosylates the alpha subunit of Gs, locking adenylate cyclase in the on position; sustained cAMP rise opens CFTR-mediated chloride secretion and inhibits sodium chloride absorption, producing profuse isotonic secretory diarrhoea without mucosal invasion.
How is severe cholera dehydration managed?
WHO Plan C: Ringer's lactate 100 ml/kg intravenously — 30 ml/kg rapidly (within about 30 minutes in adults) followed by 70 ml/kg over the next 2.5-3 hours — with reassessment, oral ORS once tolerated, and potassium replacement as hypokalaemia emerges.
Which stool findings favour cholera over invasive bacterial dysentery?
Rice-water, odourless-to-fishy watery stool with mucus flecks but no blood or pus, absent fever and absent tenesmus, with dark-field darting motility abolished by antisera; blood and pus with fever point to shigella or salmonella instead.
What is the role of antibiotics in cholera?
Adjunctive only — a single dose of doxycycline 300 mg (azithromycin in pregnancy and children) shortens diarrhoea and terminates shedding within about 48 hours, reducing transmission but never replacing fluid therapy.
Which cholera vaccines are used in programmes?
Killed whole-cell oral cholera vaccines such as Shanchol and Euvichol, two doses for older children and adults, deployed in outbreaks and for travellers; the simplified Euvichol-S, prequalified by WHO in 2025, was designed to ease global vaccine shortages.