Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
208
As and when the immune system collapses further, it
becomes less effective in finding and destroying cancers.
A rare form of skin cancer, Kaposi’s sarcoma, caused by
a herpes-like virus, is associated with AIDS. Kaposi’s
sarcoma and cancers of internal organs are now the most
likely causes of death of people with AIDS in developed
countries, along with degenerative diseases of the brain,
such as dementias.
At about the same time that AIDS was first reported
on the west coast of the USA and in Europe, doctors
in Central Africa reported seeing people with similar
opportunistic infections. We have seen that HIV/AIDS
is now widespread throughout sub-Saharan Africa from
Uganda to South Africa. It is a serious public health
problem here because HIV infection makes people more
vulnerable to existing diseases such as malnutrition, TB
and malaria. AIDS is having an adverse effect on the
economic development of countries in the region, as it
affects sexually active people in their 20s and 30s who are
also potentially the most economically productive, and the
purchase of expensive drugs drains government funds. The
World Bank estimated that AIDS had reversed 10–15 years
of economic growth for some African states by the end of
the 20th century.
Treating HIV/AIDS
There is as yet no cure for AIDS and no vaccine for HIV.
No-one knows how many people with HIV will progress
to developing full-blown AIDS. Some people think it is
100%, although a tiny minority of HIV-positive people
do appear to have immunity (Chapter 11) and can live as
entirely symptomless carriers.
Drug therapy can slow down the onset of AIDS quite
dramatically, so much so that some HIV-positive people in
developed countries are adjusting to a suddenly increased
life expectancy. However, the drugs are expensive and
have a variety of side-effects ranging from the mild and
temporary (rashes, headaches, diarrhoea) to the severe and
permanent (nerve damage, abnormal fat distribution). If
used in combination, two or more drugs which prevent
the replication of the virus inside host cells can prolong
life, but they do not offer a cure. The drugs are similar to
DNA nucleotides (for example, zidovudine is similar to the
nucleotide that contains the base thymine). Zidovudine
binds to the viral enzyme reverse transcriptase and blocks
its action. This stops the replication of the viral genetic
material and leads to an increase in some of the body’s
lymphocytes. A course of combination therapy (taking
several drugs) can be very complicated to follow. The
pattern and timing of medication through the day must
be strictly followed. People who are unable to keep to such
a regimen can become susceptible to strains of HIV that
have developed resistance to the drugs.
Preventing HIV/AIDS
The spread of HIV/AIDS is difficult to control. The virus’s
long latent stage means it can be transmitted by people
who are HIV positive but who show no symptoms of AIDS
and do not know they are infected. The virus changes
its surface proteins, which makes it hard for the body’s
immune system to recognise it (Chapter 11). This also
makes the development of a vaccine very difficult.
For the present, public health measures are the only
way to stop the spread of HIV. People can be educated
about the spread of the infection and encouraged to
change their behaviour so as to protect themselves and
others. Condoms, femidoms and dental dams are the
only effective methods of reducing the risk of infection
during intercourse, as they form a barrier between body
fluids, reducing the chances of transmission of the virus.
Some countries have promoted the use of condoms as
well as other measures. As a result, infection rates in
these countries have slowed and the number of new cases
reported each year has either decreased or remained the
same year on year. It is estimated that the rate of HIV
infection across the world decreased by 25% between 2001
and 2009.
QUESTIONS
10.9 Suggest why the true total of AIDS cases worldwide
may be much higher than reported.
10.10 Suggest why condoms are not fully effective at
preventing HIV infection.
10.11 Suggest the types of advice which might be offered
as part of an HIV/AIDS education programme.
Contact tracing is an important part of controlling the
spread of HIV. If a person who is diagnosed as HIV positive
is willing and able to identify the people whom he or she
has put at risk of infection by sexual intercourse or needle
sharing, then these people will be offered an HIV test. This
test identifies the presence of antibodies to HIV, although
these only appear several weeks after the initial infection.
Injecting drug users are advised to give up their habit,
stop sharing needles or take their drug in some other
way. Needle-exchange schemes operate in some places to
exchange used needles for sterile ones to reduce the chances
of infection with HIV and other blood-borne diseases.