Although HIV has been isolated from a variety of body fluids, including blood, semen, vaginal secretions, breast milk, urine, saliva, and tears, the risk of transmission is a consequence of the amount of virus present and the type of exposure to infected bodily fluids. HIV is found in such small quantities in tears, saliva, and urine that casual contact with these fluids is a theoretically possible but very unlikely mode of transmission.

Epidemiological studies indicate that semen, cervical and vaginal secretions, breast milk, and blood and blood products are the predominant, if not exclusive, vehicles for viral transmission (Staprans and Feinberg 1997). Although uncommon, infection is possible through the exposure of cuts in skin or mucous membranes to HIV-infected blood.

HIV is typically spread by sexual contact, exposure to infected blood (transfusions, blood products, percutaneous or intravenous injections with contaminated syringes or needles, etc.) and through perinatal transmission from mother to child (Warner 1997). Penile­vaginal and penile­anal intercourse are considered the highest-risk sexual behaviors, in addition to activities that cause a rupture of tissue and the presence of blood. Although the risk of infection is somewhat higher for the recipient of semen than for the insertive partner, transmission has been documented in both directions.

Because HIV is present in vaginal secretions, preseminal fluid, and semen, oral sex has also been documented as a mode of transmission for the recipient of fluids, as has the sharing of sexual toys, although to a lesser extent. Certain cofactors enhance the risk of sexual transmission of HIV, including the presence of sexually transmitted infections such as syphilis and chlamydia, and genital lesions or genital or mucous membrane bruising during sexual activity.

Sharing needles or other equipment during injection is a very efficient means of transmitting HIV and amounts to a direct inoculation of viral particles from the infected to the noninfected person. As with other routes of transmission, the likelihood of transmission increases with the size of the viral inoculum. Even noninjection substance use may increase the risk for HIV by increasing the chance that an individual will engage in high-risk behaviors due to lowered sexual inhibitions, impaired judgment, increased impulsiveness, or the exchange of sex for drugs or money to buy drugs.

Transfusion with infected blood and the use of infected blood products almost always results in acquisition of HIV, although testing of donated blood and blood products has practically eliminated the chances of this occurrence in the industrialized countries.

HIV Testing
The most commonly used HIV test-enzyme-linked immunosorbent assay (ELISA) followed, if positive, by Western blot-detects the presence of antibodies produced by the host as an immune response to certain genetic components of the virus, but not the presence of the virus itself. Although sensitive and specific, these tests can give false negative or indeterminate results, especially early in the course of infection.

False positive results may occur as well, which is why blanket testing is problematic when carried out in a population in which the prevalence of HIV infection is very low. There is a period following initial infection and before antibody development when an individual is infectious but has negative ELISA and Western blot test results.

The current generation of ELISA tests has reduced this “window period” to 3 or 4 weeks in most patients, but an occasional individual may take months to develop a positive test result (Corbitt et al. 1991). However, direct measurement of viral presence using the polymerase chain reaction (PCR) assay will usually show extremely high levels of virus during this window period.

Viral Load and Resistance Assays
Viral load, a quantifiable measurement of how many viral particles are present in a cubic centimeter of blood, can be determined by several different assays, some of which detect as few as 50 viral particles per cubic centimeter of blood. Below that threshold of measurement, the result is reported as “nondetectable.” This does not indicate that there is no virus present in blood at all, nor does it measure the amount of virus in lymphoid tissue or in the central nervous system (CNS).

Viral load is a strong predictor of disease progression in untreated patients. For those on antiretroviral therapy, CD4 T cell counts are the stronger predictor of clinical outcome and are useful in monitoring clinical response to therapy (O’Brien et al. 1996). Predicting responses to antiviral medications can be accomplished by testing for drug resistance, a problem increasingly seen in clinical practice.

Resistance assays fall into two classes: genotyping examines the virus for mutations known to confer drug resistance, whereas phenotyping tests the susceptibility of the patient’s virus to specific medications.

Milton L. Wainberg, M.D.
Francine Cournos, M.D.
Karen McKinnon, M.A.
Alan Berkman, M.D.

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