Describe Vibrio cholerae with its morphology, toxin, clinical features of cholera and management.

The Organism

• Vibrio cholerae — a gram-negative, comma-shaped (curved) bacillus with a single polar flagellum giving rapid darting or "shooting star" motility, seen in a hanging drop and abolished by specific antiserum
• Oxidase positive — which immediately separates it from the Enterobacteriaceae
• Alkaliphilic — it tolerates pH up to 9.2 but is killed rapidly by acid, which is exploited in both enrichment and epidemiology
• Classification — over 200 O serogroups, but only O1 and O139 cause epidemic cholera. O1 has two biotypes, classical and EL TOR (the current pandemic strain, which is hardier, produces more asymptomatic carriers and survives longer in the environment), and three serotypes: OGAWA, INABA and Hikojima

Cholera Toxin

The toxin has one A subunit and five B subunits ("A-5B") → The B subunits bind the GM1 ganglioside receptor on the enterocyte → The A subunit enters the cell → It catalyses ADP-ribosylation of the Gs alpha subunit of the regulatory G protein → This locks Gs in its active state — the GTPase that would switch it off is disabled → adenylate cyclase is permanently ON and intracellular cAMP rises enormously → Active secretion of chloride from crypt cells and blocked absorption of sodium → Water follows osmotically — massive isotonic fluid loss into the lumen

Clinical Features

• Incubation a few hours to 5 days; most infections are asymptomatic or mild, particularly with El Tor, and these people spread it
• Abrupt painless profuse watery diarrhoea with effortless vomiting; the stool becomes "rice-water" — colourless with flecks of mucus, with a characteristic inoffensive fishy odour
• NO fever, NO abdominal pain, NO blood or pus in the stool; the patient remains afebrile and mentally clear until shock supervenes
• Fluid loss may reach 1 litre per hour, giving severe dehydration within hours — sunken eyes, "washerwoman's hands", loss of skin turgor, feeble pulse, hypotension
• Consequences — hypovolaemic shock, metabolic acidosis from bicarbonate loss, hypokalaemia (paralytic ileus, arrhythmia), acute kidney injury, and in children hypoglycaemia
• Death may occur within hours, and untreated mortality reaches 50%; with adequate rehydration it falls below 1%

Treatment and Prevention

• Rehydration IS the treatment — everything else is secondary. Ringer lactate intravenously for severe dehydration, replacing the estimated deficit within 3 to 4 hours and then matching ongoing losses; oral rehydration solution for mild and moderate cases and to continue after resuscitation
• ORS composition depends on glucose-coupled sodium transport; the reduced-osmolarity formulation is now standard and lessens vomiting and stool output
• Monitor and replace potassium, which is lost heavily and is often overlooked while attention is on volume
• Antibiotics are adjunctive — a single dose of doxycycline, or azithromycin in children and pregnancy; they shorten the illness, reduce stool volume by about half and shorten excretion, but they do not save the life — fluid does
• Zinc in children reduces duration and severity
• Prevention — safe water and sanitation above all; food hygiene; chlorination; case isolation and disinfection of excreta; and oral killed vaccines (Shanchol, Dukoral), used for outbreak control and in endemic areas, though they do not replace clean water

Assessment of Dehydration and Fluid Replacement

Applied Aspects

• Assess dehydration and start fluid immediately; no investigation should delay resuscitation, and the diagnosis in an outbreak is clinical
• A single case of cholera constitutes an outbreak and must be notified; it indicates faecal contamination of a water supply serving many people
• Cholera remains endemic in parts of India with seasonal outbreaks during and after the monsoon, and in flooded and displaced populations where sanitation has failed
• Oral rehydration therapy was developed largely in the Indian subcontinent and is among the most important therapeutic advances of the twentieth century, having saved tens of millions of lives at negligible cost
• Send stool in Cary-Blair and tell the laboratory cholera is suspected, so that alkaline peptone water and TCBS are set up; ordinary stool culture will miss it
• Treat the water supply, not just the patient; chlorination, safe storage and sanitation end an outbreak, whereas treating cases one by one does not
• Weigh the patient on admission and again after rehydration; the change in weight is the most reliable measure of the deficit replaced, particularly in children
• Do not use plain glucose saline or dextrose alone; the losses are isotonic with bicarbonate and potassium, and Ringer lactate replaces them far better
• Continue feeding and breastfeeding throughout; withholding food prolongs recovery and worsens the nutritional consequences, particularly in children
• Set up a cholera treatment centre with cholera cots in an outbreak, allowing continuous measurement of stool losses and matched replacement — simple organisation that reduces mortality more than any drug
• Give a single dose of doxycycline to household contacts in an outbreak, alongside the measures directed at water and sanitation
• El Tor produces many mild and asymptomatic infections, so the cases presenting to hospital represent a small fraction of those infected and spreading it
• Distinguish cholera from other watery diarrhoeas by its sheer volume and by the absence of fever and abdominal pain; the clinical picture is characteristic once seen
• Vibrio dies rapidly in ordinary transport, so use Cary-Blair or alkaline peptone water and inform the laboratory, or the culture will be falsely negative
• Hypoglycaemia is a cause of death in children with cholera and must be anticipated and treated, since it is easily missed behind the dehydration
• Oral cholera vaccine is a supplement, not a substitute, and is most useful in outbreak control and in displaced populations where sanitation cannot be provided quickly
• Cholera follows the breakdown of infrastructure — floods, displacement, conflict — which is why its appearance is as much a signal about water supply as about microbiology
• Chlorinate the water and disinfect excreta around a case; the organism is killed readily by chlorine, drying and heat, so control measures are simple where they can be applied
• O139 emerged as a genuinely new epidemic strain in 1992 from the Bay of Bengal, infecting people already immune to O1 — a reminder that immunity to cholera is serogroup-specific