HIV risks and life cycle

Estimated infections per 10,000 exposures to an infected source are: 9,000 (blood transfusion); 2,500 (childbirth); 67 (needle sharing drug use); 50 (receptive anal intercourse without a condom; 30 (percutaneous needle stick); 10 (receptive penile-vaginal intercourse without a condom); 6.5 (insertive anal intercourse without a condom) and 5 (insertive penile-vaginal intercourse without a condom). Several factors determine the risk of HIV infection. Exposure to HIV in the anal cavity is more risky than through cells lining the vaginal cavity; both risks are higher than exposure to HIV through the mouth. The size of the HIV inoculum during exposure is also important in determining the risk of HIV infection and occurrence of illness after exposure. Persons with higher viral loads pose a higher risk than those with lower viral loads. Mother-to-child transmission is also enhanced in mothers with high viral loads. There are reports that the virulence of HIV is decreasing. Thus HIV is weakening and may not be killing its hosts as rapidly as it used to do a few years ago. This is typical of a parasite that wants to perpetuate itself: it kills slowly. The HIV life cycle can be divided into six phases: binding and entry, reverse transcription, integration, replication, budding, and maturation (see diagram). HIV changes its surface coat and escapes attack from the body’s immune cells. Immune cells fail to recognise HIV with new surface coats due to lack of immunological memory, putting HIV one step ahead of the immune system. Primary infection refers to the period of time when HIV first enters the body. At the time of primary infection with HIV, a person’s blood demonstrates a very high viral load, which means that there are many individual viruses in the blood. The number of copies of virus per milliliter of plasma or blood can exceed one million. Newly infected adults often experience an acute retroviral syndrome. Signs and symptoms of acute retroviral syndrome include fever, muscle pain, headache, nausea, vomiting, diarrhea, night sweats, weight loss, and rash. These signs and symptoms usually occur two to four weeks after infection, subside after a few days, and often are misdiagnosed as influenza or infectious mononucleosis. During primary infection, the virus targets CD4+ cells in the lymph nodes and the thymus. The CD4+ count in the blood decreases remarkably. During this time, the HIV-infected person is vulnerable to opportunistic infections. AIDS is a condition in which the immune system becomes severely weakened and loses its ability to fight infections. l Kazhila Chinsembu is a lecturer in the Department of Biology, University of Namibia. Email: kchinsembu@unam.na

June 2006
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