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Human immunodeficiency virus
(HIV, giving rise to
AIDS) (Life: Viruses:
Retroid viruses: Retroviridae: Lentivirus)
HIV parasitises 
T-helper cells of the immune system and uses their genetic machinery to produce
new copies of itself. The end result is that it kills these infected
cells and also reduces the body's ability to produce new cells to replace them.
The depletion of these T-helper cells reduces a person's ability to fight off
disease and when the number of T-helper cells drops to below 200 per mm3
a person is regarded as having AIDS.
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AIDS helpline in South Africa: phone
0800 012322 or 0800
012312 (toll free)
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(quoted from Fact
Sheet of National Institute of Allergy and
Infectious Diseases (NIAID))
"HIV belongs to a class of viruses called retroviruses,
which have genes composed of ribonucleic acid (RNA) molecules. The genes of
humans and most other organisms are made of a related molecule, deoxyribonucleic
acid (DNA). Like all viruses, HIV can replicate only inside cells, commandeering
the cell's machinery to reproduce. However, only HIV and other retroviruses,
once inside a cell, use an enzyme called reverse transcriptase to convert their
RNA into DNA, which can be incorporated into the host cell's genes. HIV belongs
to a subgroup of retroviruses known as lentiviruses,
or "slow" viruses. The course of infection with these viruses is
characterized by a long interval between initial infection and the onset of
serious symptoms. Other lentiviruses infect nonhuman species. For example, the
feline immunodeficiency virus (FIV) infects cats and the simian immunodeficiency
virus (SIV) infects monkeys and other nonhuman primates. Like HIV in humans,
these animal viruses primarily infect immune system cells, often causing
immunodeficiency and AIDS-like symptoms. These viruses and their hosts have
provided researchers with useful, albeit imperfect, models of the HIV disease
process in people."
Humans inherited HIV from chimpanzees and mangabeys
and since doing so, HIV has diversified and continues to do so. The way HIV infects people today might not be the exclusive way it does so 10 years
hence. For instance, it is possible that blood-sucking insects might eventually become vectors of HIV
although at present the chances of being infected with HIV from an insect bite
are remote (See Iqbal 1999).
Mainly from Fact
Sheet of National Institute of Allergy and
Infectious Diseases (NIAID),
with additional information included.
HIV disease is characterized by a gradual
deterioration of immune function. Most notably, crucial immune cells called CD4+
T cells (or "T-helper cells") are disabled and killed during the typical course of infection.
Not only does HIV destroy the cells it infects, it also seems to reduce the
body's ability to produce new ones (McCune 2001). CD4+
T cells play a central role in the
immune response, signalling other cells in the immune system to perform their
special functions. A healthy, uninfected person usually has 800 to 1,200 CD4+
T cells per cubic millimeter (mm3) of blood. During HIV infection, the
number of these cells in a person's blood progressively declines. When a
person's CD4+
T cell count falls below 200/mm3, he or she becomes
particularly vulnerable to the opportunistic infections and cancers that typify
AIDS, the end stage of HIV disease. People with AIDS often suffer infections of
the intestinal tract, lungs, brain, eyes and other organs, as well as
debilitating weight loss, diarrhea, neurologic conditions and cancers such as
Kaposi's sarcoma and lymphomas. Most scientists think that HIV causes AIDS by
directly killing CD4+
T cells or interfering with their normal function, and by
triggering other events that weaken a person's immune function. For example, the
network of signalling molecules that normally regulates a person's immune
response is disrupted during HIV disease, impairing a person's ability to fight
other infections. The HIV-mediated destruction of the lymph nodes and related
immunologic organs also plays a major role in causing the immunosuppression seen
in people with AIDS.
(information mainly from Piot et al.
2001)
At the end of the year 2000 there were estimated to be about 36 million
people worldwide infected with HIV and in total about 20 million people had died
of AIDS by this time. Of the 36 million people infected, 25.3 million (70%) are
in sub-Saharan Africa and of these 1.1 million are children under 15 years old,
90% of whom were born with HIV. An estimated 8.8% of people in sub-Saharan
Africa aged 15-49 are infected with HIV.
In southern Africa, average life expectancy increased from 44 years in the
early 1950s to 59 years by the late 1980s but in the decades to come, HIV will
return life expectancy to under 45 years. AIDS is affecting the economies
of these countries. It is predicted that South Africa's Gross Domestic Product
(GDP) by 2010 will be 17% less than it would be without AIDS.
The ingredients needed to effectively control the epidemic
There are four main lessons to be learnt from countries that have managed to
contain the AIDS epidemic (based on Piot et al. 2001):
- There needs to be unified national planning.
- Proven strategies for reducing HIV infections need to be put into
practice on a scale that matches the extent of the epidemic. For
strategies to be effective, they need to be adapted to local community
circumstances.
- People need to have ready access to essential drugs and equipment (e.g.
condoms) for prevention of HIV infection and for care of those infected with
HIV. Prevention and care need to operate in synergy.
- There needs to be a positive attitude by the public to those people
infected with HIV and those most at risk. HIV-infected people are vital
in the process of educating those not infected.
Effect of HIV on other infectious diseases
(see Weiss (2001)
- HIV-infected people can be conduits of other
diseases. HIV-infected people are susceptible to over 100 opportunistic
infections by viruses, bacteria, fungi and protozoa. The increased
prevalence of these diseases in the HIV-infected population means that there
is an increased possibility of virulent strains of these other diseases
being spread to people who are not infected by HIV.
- Public-health immunization programmes are
detrimentally affected by HIV infections. Live 'attenuated' vaccines
such as vaccinia, measles and oral polioviruses can be dangerous in people
with lowered immunity and these people can take a very long time to build up
immunity to these vaccines. This means that in populations where there is a
high incidence of HIV, it is difficult to eradicate these diseases because
not everyone can be immunised against them.
- Minor diseases could become major diseases by
becoming adapted in people with poor immunity. The fact that there are so many people with HIV means that other diseases
currently of minor importance are having the opportunity of infecting people and
becoming better adapted to the physiology of humans. In this way, they
might become widespread diseases that are sufficiently adapted to humans to
be able to infect normal healthy people as well.
References
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McCune, J.M. 2001. The dynamics of CD4+
T-cell depletion in HIV disease. Nature 410: 974-979.
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Piot, P., Bartos, M., Ghys, P.D., Walker, N.
& Schwartländer, B. 2001. The global impact of HIV/AIDS. Nature
410: 968-973.
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Weiss, R.A. 2001. Gulliver's travels in
HIVland. Nature 410: 963-967.
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Iqbal M.M. 1999. Can we get AIDS from mosquito
bites? J La State Med Soc 151: 429-433. See
abstract.
Text by Hamish Robertson |
