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Human immunodeficiency virus (HIV, giving rise to AIDS)

(Life: Viruses: Retroid viruses: Retroviridae: Lentivirus)

HIV parasitises A parasite lives off a host organism (it parasitises it) without giving it anything beneficial in return. 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.

 

AIDS helpline in South Africa: phone 0800 012322 or 0800 012312 (toll free)

Relationship of HIV to other viruses

(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).

How HIV causes AIDS 

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.

The AIDS epidemic in Africa

(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):

  1. There needs to be unified national planning.
  2. 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. 
  3. 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.
  4. 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. 

Questions about AIDS 

Links

References

  • McCune, J.M. 2001. The dynamics of CD4+ T-cell depletion in HIV disease. Nature 410: 974-979.

  • Piot, P., Bartos, M., Ghys, P.D., Walker, N. & Schwartländer, B. 2001. The global impact of HIV/AIDS. Nature 410: 968-973.

  • Weiss, R.A. 2001. Gulliver's travels in HIVland. Nature 410: 963-967.

  • 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 


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