A to Z of cholera

Cholera is an acute, diarrhoeal illness caused by infection of the intestine with the bacterium Vibrio cholerae. The infection is often mild or without symptoms, but sometimes it can be severe. Between approximately one in 10 and one in 20 infected persons has severe disease characterised by profuse watery diarrhoea, vomiting, and leg cramps. In these persons, rapid loss of body fluids leads to dehydration and shock. Without treatment, death can occur within hours, especially in the poorly nourished, children and elderly. Person-to-person transmission is rare.

The vibrio responsible for the current pandemic in KwaZulu-Natal is known as V. cholerae O1, biotype El Tor.
The El Tor strain can survive in fresh water for long periods. Persons with asymptomatic infections play an important role in carrying V. cholerae from place to place, causing epidemics to spread. The pandemic began in 1961 when the vibrio first appeared as a cause of epidemic cholera in Indonesia. The disease then spread rapidly to other countries of eastern Asia and reached Bangladesh in 1963, India in 1964, and the USSR, Iran and Iraq in 1965-1966. In 1970 cholera invaded West Africa, which had not experienced the disease for more than 100 years. The disease quickly spread to a number of countries and eventually became endemic in most of the continent. In 1991 cholera struck Latin America, where it had also been absent for more than a century. Within the year it spread to 11 countries, and subsequently throughout the continent.

Until 1992, only a very specific subtype (called serogroups) of cholera, V. cholerae serogroup O1 caused epidemic cholera. Some other serogroups could cause sporadic cases of diarrhoea, but not epidemic cholera. Late that year, however, large outbreaks of cholera began in India and Bangladesh that were caused by a previously unrecognised serogroup of V. cholerae, designated O139, synonym Bengal. Isolation of this vibrio has now been reported from 10 countries in South Asia. It is still unclear whether V. cholerae O139 will extend to other regions, and careful epidemiological monitoring of the situation is being maintained.

When cholera occurs in an unprepared community, case-fatality rates may be as high as 50% — usually because there are no facilities for treatment, or because treatment is given too late.

In contrast, a well organised response in a country with a well established diarrhoeal disease control programme can limit the case-fatality rate to less than 1%.
Most cases of diarrhoea caused by V. cholerae can be treated adequately by giving a solution of oral rehydration salts. During an epidemic, 80-90% of diarrhoea patients can be treated by oral rehydration alone, but patients who become severely dehydrated must be given intravenous fluids. In severe cases, an effective antibiotic can reduce the volume and duration of diarrhoea and the period of vibrio excretion. Cholera bacteria sampled during the current Kwa-Zulu-Natal epidemic are multi-resistant, and sensitive to only one commonly prescribed antibiotic.

Epidemic Control and Preventive Measures
When cholera appears in a community it is essential to ensure three things:
i. hygienic disposal of human faeces,
ii. an adequate supply of safe drinking water, and
iii. good food hygiene.
Effective food hygiene measures include cooking food thoroughly and eating it while still hot; preventing cooked foods from being contaminated by contact with raw foods, contaminated surfaces or flies; and avoiding raw fruits or vegetables unless they are first peeled.

Routine treatment of a community with antibiotics, or “mass chemoprophylaxis”, has no effect on the spread of cholera, nor does restricting travel and trade between countries or between different regions of a country. Setting up a cordon sanitaire at frontiers uses personnel and resources that should be devoted to effective control measures, consumes manpower and hampers collaboration between institutions that should unite their efforts to combat cholera.

The older injectable cholera vaccine that was previously available confers only partial protection (50% or less) and for a limited period of time (3-6 months maximum). Use of this vaccine to prevent or control cholera outbreaks is not recommended because it may give a false sense of security to vaccinated subjects and to health authorities, who may then neglect more effective measures. Certain high risk individuals might benefit from use of the newer oral cholera vaccine which is more effective. North American challenge studies in volunteers given live cholera after immunisation with the oral vaccine have shown the vaccine to offer significant protection.

The bacteria that cause cholera are very sensitive to the acids normally present in the stomach and digestive tract. If small numbers of bacteria are swallowed they are usually killed by stomach acid before they can establish themselves in the body. Only when large numbers of bacteria overwhelm the body?s natural defences, or when the body?s defences have been weakened by something else, does cholera pose a significant risk.

(Source: British Airways Travel Clinic)


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