Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
210
Some people become infected and develop TB quite
quickly, while in others the bacteria remain inactive for
many years. It is estimated that about 30% of the world’s
population is infected with TB without showing any
symptoms of the infection; people with this inactive
infection do not spread the disease to others. However, the
bacteria can later become active, and this is most likely
to happen when people are weakened by other diseases,
suffer from malnutrition or become infected with HIV.
Those who have the active form of TB often suffer from
debilitating illness for a long time. They have a persistent
cough and, as part of their defence, cells release hormonelike
compounds, which cause fever and suppress the
appetite. As a result, people with TB lose weight and often
look emaciated (Figure 10.11).
TB is often the first opportunistic infection to strike
HIV-positive people. HIV infection may reactivate
dormant infections of M. tuberculosis which may have
been present from childhood or, if people are uninfected,
make them susceptible to infection. TB is now the leading
cause of death among HIV-positive people. The HIV
pandemic has been followed very closely by a
TB pandemic.
Transmission of TB
TB is spread when infected people with the active form
of the illness cough or sneeze and the bacteria are
carried in the air in tiny droplets of liquid. Transmission
occurs when people who are uninfected inhale the
droplets. TB spreads most rapidly among people living in
overcrowded conditions. People who sleep close together
in large numbers are particularly at risk. The disease
primarily attacks the homeless and people who live in poor,
substandard housing; those with low immunity,
because of malnutrition or being HIV positive, are also
particularly vulnerable.
The form of TB caused by M. bovis also occurs in cattle
and is spread to humans in meat and milk. It is estimated
that there were about 800 000 deaths in the UK between
1850 and 1950 as a result of TB transmitted from cattle.
Very few now acquire TB in this way in developed countries
for reasons explained later, although meat and milk still
remain a source of infection in some developing countries.
The incidence of TB in the UK decreased steeply well
before the introduction of a vaccine in the 1950s, because
of improvements in housing conditions and diet. The
antibiotic streptomycin was introduced in the 1940s, and
this hastened the decrease in the incidence of TB. This
pattern was repeated throughout the developed world.
Figure 10.11 A TB patient undergoes treatment in a hospital
in India.
Once thought to be practically eradicated, TB is on the
increase. There are high rates of incidence all across the
developing world and in Russia and surrounding countries
(Figure 10.12). High rates are also found in cities with
populations of migrants from countries where TB is more
common. Parts of London, for example, have rates of TB
much higher than the rest of the UK. The incidence in such
areas is as high as in less economically developed countries.
This increase is due in part to the following factors:
■■
■■
■■
■■
some strains of TB bacteria are resistant to drugs
the HIV/AIDS pandemic
poor housing in inner cities and homelessness
the breakdown of TB control programmes; partial
treatment for TB increases the chance of drug
resistance in Mycobacterium.
Treating TB
When a doctor first sees a person with the likely symptoms
of TB, samples of the sputum (mucus and pus) from their
lungs are collected for analysis. The identification of the
TB bacteria can be done very quickly by microscopy. If
TB is confirmed, then patients should be isolated while
they are in the most infectious stage (which is at two to
four weeks). This is particularly if they are infected with
a drug-resistant strain of the bacterium. The treatment
involves using several drugs to ensure that all the bacteria
are killed. If not killed, drug-resistant forms remain to
continue the infection. The treatment is a long one (six to
nine months, or longer), because it takes a long time to
kill the bacteria, which are slow growing and are not very
sensitive to the drugs used. Unfortunately, many people