07 October 2013
HIV is an infection that can lead to a weakened immune system, referred to as acquired immunodeficiency syndrome or AIDS. In the years when HIV was first identified most people who became infected with HIV developed AIDS, as HIV treatment did not exist; today the situation is very different. With the availability of antiretroviral treatment, HIV has become a chronic, manageable condition.
However, many people remain unaware of their HIV status and access to treatment remains unavailable to half of those in need. As a result, many people continue to develop AIDS, one of the final stages of HIV infection.
HIV infects cells in the immune system and the central nervous system. One of the main type of cells that HIV infects is the T helper lymphocyte. These cells play a crucial role in the immune system, by coordinating the actions of other immune system cells. A large reduction in the number of T helper cells seriously weakens the immune system.
HIV infects the T helper cell because it has the protein CD4 on its surface, which HIV uses to attach itself to the cell before gaining entry. This is why the T helper cell is sometimes referred to as a CD4+ lymphocyte. Once it has found its way into a cell, HIV produces new copies of itself, which can then go on to infect other cells.
Over time, HIV infection leads to a severe reduction in the number of T helper cells available to help fight disease. The number of T helper cells is measured by having a CD4 test and is referred to as the CD4 count. It can take several years before the CD4 count declines to the point that an individual needs to begin antiretroviral treatment. Without treatment, the CD4 count continues to decline to very low levels, at which point the individual is said to have progressed to AIDS.
HIV infection can generally be broken down into four distinct stages: primary infection, clinically asymptomatic stage, symptomatic HIV infection, and progression from HIV to AIDS.
This stage of infection lasts for a few weeks and is often accompanied by a short flu-like illness. In up to about 20% of people the HIV symptoms are serious enough to consult a doctor, but the diagnosis of HIV infection is frequently missed.
During this stage there is a large amount of HIV in the peripheral blood and the immune system begins to respond to the virus by producing HIV antibodies and cytotoxic lymphocytes. This process is known as seroconversion. If an HIV antibody test is done before seroconversion is complete then it may not be positive.
This stage lasts for an average of ten years and, as its name suggests, is free from major symptoms, although there may be swollen glands. The level of HIV in the peripheral blood drops to very low levels but people remain infectious and HIV antibodies are detectable in the blood, so antibody tests will show a positive result.
Research has shown that HIV is not dormant during this stage, but is very active in the lymph nodes. A test is available to measure the small amount of HIV that escapes the lymph nodes. This test which measures HIV RNA (HIV genetic material) is referred to as the viral load test, and it has an important role in the treatment of HIV infection.
Over time the immune system becomes severely damaged by HIV. This is thought to happen for three main reasons:
Antiretroviral treatment is usually started once an individuals CD4 count (the number of T helper cells) drops to a low level, an indication that the immune system is deteriorating. Treatment can stop HIV from damaging the immune system, therefore, HIV-infected individuals on treatment usually remain clinically asymptomatic.
However, in HIV-infected individuals not receiving treatment or on treatment that is not working, the immune system fails and symptoms develop. Initially many of the symptoms are mild, but as the immune system deteriorates the symptoms worsen.
Symptomatic HIV infection is mainly caused by the emergence of certain opportunistic infections that the immune system would normally prevent. This stage of HIV infection is often characterised by multi-system disease and infections can occur in almost all body systems.
Treatment for the specific infection is often carried out, but the underlying cause is the action of HIV as it erodes the immune system. Unless HIV itself can be slowed down the symptoms of immune suppression will continue to worsen.
As the immune system becomes more and more damaged the individual may develop increasingly severe opportunistic infections and cancers, leading eventually to an AIDS diagnosis.
A clinical criteria is used by WHO to diagnose the progression to AIDS, this differs slightly between adults and children under five. In adults and children (aged 5 or over) the progression to AIDS is diagnosed when any condition listed in clinical stage 4 is diagnosed and/or the CD4 count is less than 200 cells/mm3 or a CD4 percentage less than 15%. In children younger than five, an AIDS diagnosis is based on having any stage 4 condition and/or a CD4 percentage less than 20% (children aged 12-35 months) and a CD4 percentage less than 25% (children less than 12 months). The criteria for diagnosing AIDS may differ depending on individual country guidelines.
The table below shows examples of common opportunistic infections and cancers and the body systems that they occur in.
|System||Examples of Infection/Cancer|
|Central/peripheral Nervous system||
In resource-poor settings, medical facilities are sometimes poorly equipped and tests to measure CD4 count and viral load are unavailable. In this case, another method to determine whether an individual should begin treatment is used. The World Health Organisation (WHO) developed a staging system for HIV disease based on clinical symptoms, which may be used to guide medical decision making.