HIV
HIV (human immunodeficiency syndrome) is a retrovirus, meaning that it uses it’s viral RNA to produce a single strand of DNA called cDNA (copy DNA) inside the host cell. It carries the enzyme reverse transcriptase which synthesises a single strand of DNA from the viral RNA by reverse transcription and then directs the formation of a complementary double strand of DNA. The double stranded DNA is then inserted into a chromosome in the host cell, where it codes for the synthesis of viral proteins.
The HIV virus specifically attacks T-helper cells which are the bodies defence mechanisms. However once in the cell damage does not occur straight away, the virus remains latent as the infected cells first need to activated. This often occurs as a result of a secondary infection. Once activated the HIV starts to replicate and destroy the host T4 cell. By destroying them it reduces their numbers, this means that the bodies ability to fight disease is greatly reduced due to a lack of white blood cells. Eventually the infected individual can succumb to most forms of infection due to their immune system being so weak. AIDS is culmination of diseases gained due to a weakened immune system. People effectively die of a secondary often common disease.
Replication of HIV
First the HIV retrovirus attaches to a receptor site on the surface of the host cell, the envelope then fuses with the hosts cell membrane. Reverse transcriptase then makes a single strand of DNA from a single strand of viral RNA, a complementary strand is them made. The now double strand of DNA inserts itself into the chromosomal DNA of the cell. From here the viral DNA is the transcribed into mRNA which codes for the synthesis of proteins. New viruses are therefore made and leave the cell to infect other cells.
Modes of transmission
HIV is transmitted in either the blood or semen. Therefore it generally spreads via slight abrasions that may occur during sexual contact with an infected person or via the use of hypodermic needles that have been previously used by an infected person. It can also occur through blood transfusions of infected blood. However, in developed countries this is very rare due to sterilisation and screening for HIV antibodies before the use of blood for transfusions. Transmission in homosexuals is more likely as the lining of the rectum is more likely to be damaged during intercourse than the vagina wall, therefore it is more likely to bleed. Unborn babies can be at risk from the virus if the mother is infected as it can pass across the placenta to the foetus. Babies born to infected women are also at risk through breast feeding as the virus can pass via breast milk.
AIDS symptoms
Initially after infection, short lived flu like symptoms may develop. These may include swollen lymph glands, tiredness and a raised temperature. The person then enters a stage of no signs what so ever. This period can be months to years. Then gradually, as the immune system weakens diseases that are generally fought of easy enough begin to take hold. These general diseases could be a simple wart spreading all over the body, diarrhoea lasting for prolonged amounts of time or severe shingles may develop. The most common cause of death in AIDS suffers is pneumonia.
Treatment
There is currently no cure for AIDS, or no vaccine for HIV. Drugs are trying to be developed that will deactivate HIV the problem is that the drugs may damage the host lymphocytes anyway. This will in turn reduce the number in the blood and once again even without the virus the body is open to opportunistic infections. The most developed drug at the moment is AZT this acts by binding with reverse transcriptase blocking it s action therefore stopping HIV from replicating. However this can cause severe side effects such as anaemia. Vaccines for the virus are hard to develop as the HIV is able to change it’s surface proteins. As HIV is a retro virus it is also possible that any vaccine based on an attenuated whole virus could cause cancer.
Prevention
A very effective of preventing being infected by HIV is the use of a condom to provide a physical barrier between the two people. Limiting sexual partners can also easily decrease risk. The use and wide availability of clean needles can also help prevent transmission. Screening blood to be used for transmission and eliminating blood that is HIV positive can reduce transmission via transfusions. Sex education and HIV education and awareness programmes can also be effective in preventing the virus. Contact tracing is important in the control of the spread of the virus in the UK. Once diagnosed with the virus the infected person is asked to identify people they may have put at risk via intercourse or needles. These people are then offered a HIV test if they are found to be positive they then give the names of people the may have put at risk as so on.
