Cambridge International A Level Biology Revision Guide

Chapter 10: Infectious diseases
As the disease is water-borne, cholera occurs where
people do not have access to proper sanitation (clean
water supply) and uncontaminated food. Infected people,
three-quarters of whom may be symptomless carriers,
pass out large numbers of bacteria in their faeces. If these
contaminate the water supply, or if infected people handle
Pathogen
Methods of transmission
Global distribution
Incubation period
Site of action of pathogen
Clinical features
Method of diagnosis
Annual incidence worldwide
Vibrio cholerae
food-borne, water-borne
Asia, Africa, Latin America
two hours to five days
wall of small intestine
severe diarrhoea (‘rice
water’), loss of water
and salts, dehydration,
weakness
microscopical analysis of
faeces
3–5 million
Annual mortality worldwide 100 000–120 000
Table 10.2 The features of cholera.
food or cooking utensils without washing their hands,
then bacteria are transmitted to uninfected people.
To reach their site of action in the small intestine, the
bacteria have to pass through the stomach. If the contents
are sufficiently acidic (pH less than 4.5), the bacteria are
unlikely to survive. However, if the bacteria do reach
the small intestine, they multiply and secrete a toxin,
choleragen, which disrupts the functions of the epithelium
lining the intestine, so that salts and water leave the blood.
This causes severe diarrhoea and the loss of fluid can be
fatal if not treated within 24 hours.
Treating cholera
Almost all people with cholera who are treated make a
quick recovery. A death from cholera is an avoidable death.
The disease can be controlled quite cheaply by a solution
of salts and glucose given intravenously to rehydrate the
body (Figure 10.3). If people can drink, they are given
oral rehydration therapy. Glucose is effective, because it
is absorbed into the blood and takes ions (for example,
sodium and potassium ions) with it. It is important to
make sure that a patient’s fluid intake equals fluid losses in
urine and faeces, and to maintain the osmotic balance of
the blood and tissue fluids (Chapter 4).
Preventing cholera
In developing countries, large cities that have grown
considerably in recent years, but as yet have no sewage
treatment or clean water, create perfect conditions for the
spread of the disease. Increasing quantities of untreated
faeces from a growing population favour cholera’s
survival. Countries that have huge debts do not have the
financial resources to tackle large municipal projects such
as providing drainage and a clean water supply to large
areas of substandard housing. In many countries, the use
of raw human sewage to irrigate vegetables is a common
cause of the disease, as are inadequate cooking, or washing
in contaminated water. Areas of the world where cholera is
endemic are West and East Africa and Afghanistan.
Cholera is now almost unknown in the developed
world, as a result of sewage treatment and the provision of
clean piped water, which is chlorinated to kill bacteria. The
transmission cycle has been broken.
Health authorities always fear outbreaks of cholera and
other diarrhoeal diseases following natural disasters.
In Haiti in 2010, a cholera epidemic broke out several
months after the earthquake that destroyed large parts of
the country.
Travellers from areas free of cholera to those where
cholera is endemic used to be advised to be vaccinated,
although the vaccine only provided short-term protection.
This recommendation has now been dropped.
Figure 10.3 People being treated in Port-au-Prince in Haiti
during the 2010 cholera epidemic. The drips contain a solution
of salts to replace those lost through severe diarrhoea.
Cholera causes many deaths when normal life is disrupted by
war and by natural catastrophes such as earthquakes.
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