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HIV transmission Prevention

Updated: Thursday, Mar 19,2009, 11:11:29 AM
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1. Transmission

Sexual practices involving no exposure to bodily fluids are safe. Other practices, such as fellatio and cunnilingus appear to be relatively, but not absolutely, safe. The greatest risk is through genital intercourse, especially anal-receptive intercourse. Sexual practices producing mucosal trauma before or during intercourse increase the risk. Use of latex condoms or vaginal barriers decreases but does not eliminate risk. Oil-based lubricants decrease the protection provided by latex condoms because they dissolve them.

HIV transmission requires contact with body fluids containing infected cells or plasma. HIV may be present in any fluid or exudate that contains plasma or lymphocytes, specifically blood, semen, vaginal secretions, breast milk, saliva, or wound exudates. Although theoretically possible, transmission by saliva or droplet nuclei produced by coughing or sneezing is extremely rare, if it occurs. HIV is not transmitted by casual contact or even by the close nonsexual contact that occurs at work, school, or home. The most common means of transmission is direct transfer of bodily fluids either through sharing contaminated needles or sexual relations.
Infected cells or free virions can reach target cells in a new host via blood transfusion, accidental injection, or mucous membrane exposure. The role of mucous membrane inflammation is illustrated by the effect of other sexually transmitted diseases (STDs) on susceptibility to HIV infection. HIV transmission is definitely increased by chancroid and may be more likely in the presence of herpes, syphilis, trichomoniasis, and possibly other STDs.
Transmission of HIV by needle-stick injury, estimated at about 1/300 incidents, is much less frequent than transmission of hepatitis B, presumably because of the relatively lower number of HIV virions in the blood of most infected patients. Risk of HIV transmission appears to be increased by deep wounds or injection of blood, such as when hollow-bore needles containing blood penetrate the skin.
Use of enzyme-linked immunosorbent assay (ELISA) to screen blood donors has vastly reduced the risk of acquiring HIV by transfusion. However, persons in the early stages of HIV infection, who have not yet mounted an antibody response, may have transiently negative ELISA and Western blot results while yielding positive results for HIV p24 antigen in plasma. These persons may account for the very low, but continuing, risk of transfusion-associated HIV infection (estimated at between 1/10,000 and 1/100,000 per unit transfused). Currently mandated screening for both antibody and p24 antigen may further reduce this risk.
2. Prevention

Multiple strategies are being developed to induce protective immunity in persons not infected with HIV. Immunogens include attenuated live and whole killed HIV, genetically engineered HIV proteins and peptides (e.g., from the viral envelope), and vaccinia virus genetically modified to express HIV viral proteins. These efforts are hampered by the lack of a measurable marker of protective immunity, such as the neutralizing antibody engendered by polio vaccine, or of a convenient animal model. Nevertheless, vaccines continue to be developed and tested for safety and immunogenicity.
All pregnant women should be offered a test for antibody to HIV. HIV-infected women should be advised to consider deferring pregnancy at least until management of HIV in pregnancy is better studied. The risk of transmission in utero, intrapartum, or postpartum transmission to the fetus is estimated to be 30 to 50%, but zidovudine (ZDV or AZT) alone reduces intrapartum infection by 2/3, and combinations of drugs may be more effective. Given the low, but real, risk of transmission even with treatment and the uncertainty of the effects on the fetus of drugs needed for their own health, termination of pregnancy may be an alternative for many HIV-infected pregnant women.
Confidential testing for antibody to HIV should be offered to anyone requesting it, but only in conjunction with pretest and posttest counseling. Persons who are at high risk for contracting HIV infection--even those with negative HIV antibody test results--should not donate blood or organs for transplantation because of the small risk they may have been recently infected and be infectious but antibody-negative.
Isolation of hospitalized patients with HIV infection is unnecessary, except when their complicating infections (e.g., suspected or proven TB) are communicable. Surfaces contaminated by blood or other body fluids should be cleaned and disinfected. HIV is readily inactivated by heat and many disinfectants, including peroxide, alcohols, phenolics, and hypochlorite. The body fluids and tissues of HIV-infected patients should be handled with extreme care.
Medical and dental professionals should wear gloves when examining all patients if contact with mucous membranes or other wet surfaces may occur. Because needle-stick accidents are common, health care workers must be taught how to avoid them.
Postexposure prophylaxis with immediate antiretroviral therapy after penetrating injuries involving HIV-infected blood (needle sticks) or heavy mucous membrane (eye or mouth) contamination is believed to reduce transmission. Combinations of a protease inhibitor with two nucleoside reverse transcriptase inhibitors are currently recommended for postexposure prophylaxis of relatively high-risk exposures. Zidovudine (ZDV or AZT) appeared to reduce risk of transmission after needle-stick injuries in one study, which provided the only evidence that prophylaxis works. Because of the low risk of infection for most injuries, controlled prospective studies of the effectiveness of prophylaxis are not practical. Cancers or birth defects from the brief exposures to these drugs have not been found in the small numbers of otherwise healthy persons who have used ZDV for this purpose. Because some women in early pregnancy will be offered postexposure prophylaxis before their pregnancy is suspected or confirmed, special caution must be exercised in treating potentially pregnant women. Additional problems arise when the source or HIV status of blood is unknown, but identification of the source and testing of that person for HIV infection should be vigorously pursued.

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