Transmission routes

Transmission routes

There are several ways in which someone can become infected with HIV. These transmission routes are well defined (see also Chapter "Post-Exposure Prophylaxis"). HIV infection can be transmitted through:

* unprotected sexual intercourse with an infected partner;

* injection or transfusion of contaminated blood or blood products (infection through artificial insemination, skin grafts and organ transplants is also possible);

* sharing unsterilized injection equipment that has been previously used by someone who is infected;

* maternofetal transmission (during pregnancy, at birth, and through breastfeeding).

Occupational infections of healthcare or laboratory workers may occur; however, a 1995 study estimated that although 600,000 to 800,000 needlestick injuries occurred among healthcare workers every year in the USA, occupational infection was not frequent. The risk of occupational HIV transmission from contaminated needles to healthcare workers was found to be 0.3 % in case series performed prior to the availability of potent ART.

There are sometimes concerns that there may be alternative routes of HIV transmission. It must be explicitly stated that HIV is NOT transmitted by mosquitoes, flies, fleas, bees, or wasps. HIV is NOT transmitted through casual every day contact. No case of HIV infection has been documented to arise from contact with non-bloody saliva or tears. Since HIV is not transmitted by saliva, it is not possible to contract it through sharing a glass, a fork, a sandwich, or fruit (Friedland 1986, Castro 1988, Friedland 1990). In the opinion of leading experts, exposure of intact skin to HIV-contaminated body fluids (e.g. blood) is not sufficient to transfer the virus.

Sexual intercourse

Unprotected sexual intercourse is the most important transmission route of HIV infection worldwide. Although receptive anal sex is estimated to produce the highest risk of infection, infection after a single insertive contact has also been described. The presence of other sexually transmitted diseases markedly increases the risk of becoming infected with HIV.

The lower the viral load, the less infectious the patient. A prospective study of 415 HIV-discordant couples in Uganda showed that of 90 new infections occurring over a period of up to 30 months, none was from an infected partner with a viral load below 1,500 copies/ml. The risk of infection increased with every log of viral load by a factor of 2.45 (Quinn 2000). It should be noted that the levels of viral load in blood and other body fluids do not always correlate with one another. Thus, individual risk remains difficult to estimate. In addition, HIV-infected patients are not protected from superinfection with new viral strains.

The higher the viral load, the more infectious the patient. This is especially true for patients during acute HIV infection. During acute HIV-1 infection, the virus replicates extensively in the absence of any detectable adaptive immune response, reaching levels of over 100 million copies of HIV-1 RNA/ml (see Chapter "Acute HIV-1 infection").