Cambridge International A Level Biology Revision Guide

Chapter 10: Infectious diseases
Deaths from
TB per 100000
no data 50
Figure 10.12 The global distribution of TB in 2010 (data from WHO).
do not complete their course of drugs, because they think
that when they feel better, they are cured. People who do
not complete their treatment may be harbouring drugresistant
bacteria and may spread these to others if the
bacteria become active.
Drug-resistant TB
Strains of drug-resistant M. tuberculosis were identified
when treatment with antibiotics began in the 1950s.
Antibiotics act as selective agents killing drug-sensitive
strains and leaving resistant ones behind. Drug resistance
occurs as a result of mutation in the bacterial DNA.
Mutation is a random event and occurs with a frequency
of about one in every thousand bacteria. If three drugs are
used in treatment, then the chance of resistance arising
to all three of them by mutation is reduced to one in a
thousand million. If four drugs are used, the chance is
reduced to one in a billion.
If TB is not treated or the person stops the treatment
before the bacteria are completely eliminated, the bacteria
spread throughout the body, increasing the likelihood that
mutations will arise, as the bacteria survive for a long time
and multiply. Stopping treatment early can mean that
M. tuberculosis develops resistance to all the drugs being
used. People who do not complete a course of treatment
are highly likely to infect others with drug-resistant forms
of TB. It is 10 to 15 others, especially if the person lives in
overcrowded conditions.
The WHO promotes a scheme to ensure that patients
complete their course of drugs. DOTS (direct observation
treatment, short course) involves health workers or
responsible family members making sure that patients
take their medicine regularly for six to eight months. The
drugs widely used are isoniazid and rifampicin, often in
combination with others. This drug therapy cures 95% of
all patients, and is twice as effective as other strategies.
Multiple-drug-resistant forms of TB (MDR-TB) now
exist. MDR-TB strains of TB are resistant to at least the two
main drugs used to treat TB – isoniazid and rifampicin –
which are known as first-line drugs. In 1995, an HIV unit
in London reported an outbreak of MDR-TB with a form
of M. tuberculosis that was resistant to five of the major
drugs used to treat the disease, including isoniazid, which
is the most successful drug. Extensively (or extremely)
drug-resistant TB (XDR-TB) has also emerged as a very
serious threat to health, especially for those people who
are HIV-positive . XDR-TB strains are resistant to firstline
drugs and to the drugs used to treat MDR-TB. These
resistant strains of TB do not respond to the standard sixmonth
treatment with first-line anti-TB drugs and can take
two years or more to treat with drugs that are less potent
and much more expensive. DOTS is helping to reduce the
spread of MDR strains of TB.
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