Vibrio cholerae: How it spreads, kills, and can be eradicated

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CHOLERA is caused by poisonous bacteria which multiply rapidly in the small intestine. The microbe, Vibrio cholerae, produces a toxin which causes diarrhoea, massive dehydration, vomiting and eventual death if left untreated.

The bacteria are passed on in contaminated water or food, and thrive in dirty, unsanitary conditions, especially those which cause raw sewage to become mixed with drinking water. After an incubation period of 12 to 28 hours, the disease usually starts with painless, watery diarrhoea, which can result in the loss of three to four gallons of fluid within a 24-hour period.

Severe dehydration, a fall in blood pressure and muscular cramps soon follow. Without prompt rehydration with salts and water, the patient can fall into a coma and die of shock within two to seven days. With good medical treatment and antibiotics, recovery can be complete and suprisingly quick.

The disease is endemic in parts of South-east Asia, particularly India and Pakistan, and in Africa. In 1991, it spread across South America from the Pacific coast of Peru, claiming thousands of lives. By the summer of that year, Peru alone had registered 231,600 cases, nearly 2,300 of them fatal.

Although there is a vaccine, made from killed cholera bacteria, it is neither very effective nor does it provide immunity for long. Because of its unreliability, the World Health Organisation does not recommend it. Scientists are attempting to design better vaccines; by genetic engineering, they hope to rid the cholera microbes of their toxin so that reliable, safe live vaccines can be made.

By far the best form of defence against the disease, however, is prevention. Improved sanitation and better public health considerably reduce the risks of cholera outbreaks. The disease first came to Britain in 1831 after the opening of the trades routes between Europe and the East. In the following few decades, regular outbreaks resulted in many thousands of deaths.

At that time, many people suspected it was a disease transmitted through the air. However, it was John Snow, Queen Victoria's obstetrician, who was the first to show that it was a water-borne infection. He noticed that the highest casualty rates in London at that time were in people who had drawn water from a hand pump in Broad Street, Soho. Snow asked the authorites to remove the handle from the pump, and the incidence of the disease quickly dropped.

He also noticed that the worst affected areas in London were Southwark and Vauxhall, which drew their water from a part of the Thames contaminated by sewage from ships that arrived from the Baltic. The findings ultimately led to the improvement of London's sanitation and water supply, and the consequent eradication of cholera in the city.