Infectious Disease: Question 2

Worked solution

Part (a): Pathogen and mode of action of the toxin

Cholera is caused by Vibrio cholerae, a bacterium. Once established in the small intestine, it releases a toxin that acts on the epithelial cells lining the gut.

The toxin causes chloride ion channels in the membranes of these epithelial cells to open. Chloride ions move out of the cells into the lumen of the gut, and sodium ions follow. This lowers the water potential of the fluid in the gut lumen relative to the cells and the blood, so water moves out of the cells and blood into the gut lumen by osmosis. The result is a huge volume of watery fluid passing into the intestine, which is lost from the body as profuse watery diarrhoea, quickly causing severe dehydration and loss of ions.

Part (b): Transmission between people

Cholera is transmitted by the faecal-oral route. Infected people shed enormous numbers of Vibrio cholerae bacteria in their faeces. If sanitation is inadequate, this contaminates a shared water supply, or food that is washed or prepared using contaminated water. Another person becomes infected by drinking this water or eating this food, taking the bacteria into their own gut.

Part (c): Biological, social and economic factors in the outbreak

Biological factors: Vibrio cholerae survives well in water outside a human host. Because the bacterium is sensitive to acid, most cells swallowed in a typical mouthful of contaminated water are killed by stomach acid, so normally a comparatively large dose must be ingested for infection to occur. After the sewage system failed, however, the drinking-water supply became so heavily contaminated that many people took in enough bacteria to reach this infective dose. Once infected, each person then sheds enormous numbers of the bacterium in their watery diarrhoea, which re-contaminates the same water supply and drives further infections, a positive-feedback effect that helps explain why the outbreak grew so large and quickly.

Social factors: Flooding often damages homes and forces people into crowded temporary shelters, where clean water and working toilets may be scarce and people are in closer contact with each other and with contaminated water. Disruption to normal services also makes it harder to communicate safe-water advice (such as boiling water) to everyone affected.

Economic factors: Repairing a damaged sewage system, providing an alternative clean water supply, and funding enough oral rehydration salts, intravenous fluids and vaccination doses for a large outbreak all cost money. A district without the funds or infrastructure to do this quickly will see the outbreak continue for longer and be harder to bring under control than a wealthier region able to respond immediately.

Taken together, these three types of factor explain both why the outbreak was so large and why it proved difficult to control.

Final answers

• (a) Vibrio cholerae (a bacterium) releases a toxin that opens chloride ion channels in gut epithelial cells; chloride and sodium ions move into the gut lumen and water follows by osmosis, causing watery diarrhoea.
• (b) Transmission is by the faecal-oral route, via water or food contaminated with the faeces of an infected person.
• (c) Biological (bacterium survives in water; heavy contamination lets enough bacteria reach the acid-sensitive infective dose; shedding by infected people re-contaminates the supply), social (crowded shelters, disrupted communication) and economic (lack of funds for repairs, clean water and treatment) factors together explain the scale of the outbreak and the difficulty controlling it.