Question
A 40 years old male complains of effortless passage of loose watery stools for about 10 times in the past 24 hours. The stool is colorless, watery fluid with flecks of mucus resembling rice water. Answer the following:
- (a) What is the probable diagnosis. 1 mark(s)
- (b) What is the mode of infection for this disease. 1 mark(s)
- (c) Describe the pathogenesis of this disease. 3 mark(s)
- (d) Discuss the laboratory diagnosis of this disease. 3 mark(s)
- (e) Describe specific preventive measures to prevent this disease. 2 mark(s)
Answer
a) Probable diagnosis: Cholera — frequent, effortless, painless, profuse watery stools with the classical “rice-water” appearance (colourless, flecked with mucus, odourless) is pathognomonic.
b) Mode of infection Faecal-oral transmission — ingestion of water or food contaminated with Vibrio cholerae from the faeces of an infected patient or asymptomatic carrier; outbreaks are typically linked to contaminated water supplies, poor sanitation, and shellfish/seafood harvested from contaminated water.
c) Pathogenesis V. cholerae is non-invasive; it colonizes the small intestinal mucosa via flagellar motility and adhesins, without penetrating the epithelium. The organism elaborates cholera toxin, an AB-type exotoxin — the B subunit binds the GM1 ganglioside receptor on enterocytes, facilitating entry of the A subunit, which ADP-ribosylates the Gs protein, causing persistent activation of adenylate cyclase and a sustained rise in intracellular cAMP. This drives massive, unregulated active secretion of chloride (and passive water/sodium/bicarbonate) into the gut lumen, overwhelming absorptive capacity and producing the profuse, isotonic, rice-water diarrhoea characteristic of the disease, rapidly leading to severe dehydration and hypovolaemic shock if untreated.
d) Laboratory diagnosis
- Direct microscopy — hanging-drop preparation showing characteristic “darting/shooting-star” motility, immobilized by specific antiserum.
- Culture — enrichment in alkaline peptone water (pH 8.6), then plated on TCBS agar, giving characteristic yellow (sucrose-fermenting) colonies.
- Biochemical confirmation and slide agglutination with specific O1/O139 antisera.
- String test — a rapid presumptive test (mucoid string formation in 0.5% sodium deoxycholate).
e) Specific preventive measures Safe drinking water supply and proper sanitation/sewage disposal; food hygiene and thorough cooking of seafood; hand hygiene and health education; oral cholera vaccines (killed whole-cell, e.g., Dukoral, Shanchol) for high-risk populations/outbreak settings; chlorination of water sources and prompt outbreak response (case management, water testing) during epidemics.