Clinical Overview of HIV Disease
Section 4: Transmission and Risk Factors
The primary method of spread of HIV infection worldwide is through sexual
exposure. In the United States and Europe, acquisition of the virus through
homosexual contact remains important, and there is some evidence of increasing
incidence of infection among young gay men and ethnic minorities.( 76 ) MSM,
however, now account for <50% of new infections in the United States.( 76 ) In
the areas of highest HIV prevalence globally, heterosexual intercourse is the
primary mode of transmission, accounting for approximately 70% of the overall
sexual transmission.( 77 )
HIV has been isolated from blood, seminal fluid, pre-ejaculate, vaginal
secretions, cerebrospinal fluid, saliva, tears, and breast milk of infected
individuals.( 78-81 ) HIV-1 DNA sequences have also been detected in
pre-ejaculatory fluid.( 82 ) In genital fluids, HIV may be found in both
cell-free and cell-associated compartments, but it is unknown which is
responsible for productive infection.( 83 ) Viral concentrations in tears and
saliva are comparatively low, and there are substances in saliva that appear to
inhibit infectivity. No cases of HIV infection have been documented to arise
from contact with nonbloody saliva or tears.
Transmission of HIV occurs more frequently through penile-anal intercourse and
penile-vaginal intercourse than through fellatio, although clear cases of
transmission through oral sex exist.( 310 ) Female-to-female HIV transmission
has been reported, but is rare.( 88 ) In a meta-analysis, the overall efficacy
of condoms in reducing HIV transmission was 69%.( 89 )
Sexual activity that is associated with exposure to infected blood increases the
risk of transmission, as does the presence of genital ulcers.( 90-92 ) Serum HIV
viral load is strongly associated with heterosexual transmission between HIV-serodiscordant
African sexual partners, where transmission was noted to be rare at viral loads
<1,500 copies/mL.( 93 ) The effect of viral load reduction with ART on HIV
transmission is being investigated. Intervention with antiretroviral medications
soon after high-risk sexual exposures has been proven to be safe and may be
effective in preventing transmission of HIV (as discussed in the chapter "
Prophylaxis Following Nonoccupational Exposure to HIV ").( 104 )
Nonsexual HIV transmission can occur through transfusion with contaminated blood
products, injection drug use, occupational exposure, or accidental needlesticks.
The risk from occupational needlesticks to health care workers from known
HIV-positive source patients in case series performed prior to the availability
of potent ART was found to be 0.33-0.5%.( 95,96 ) Factors increasing the risk of
HIV acquisition from an occupational needlestick include deep injury, injury
with a visibly bloody device, or injury with a device that had been previously
used in the source patient's vein or artery.( 96 ) Postexposure prophylaxis
(PEP) has been associated with a reduction of HIV transmission after
occupational needlestick events of approximately 80%.( 96,103 )
HIV transmission through transfusion of contaminated blood products was
recognized early in the epidemic.( 6 ) With current testing methods, the risk of
acquiring HIV from a unit of transfused blood in the United States is 1 in
676,000,( 102 ) but is significantly higher in many developing countries.
In the absence of interventions, mother-to-child transmission occurs in
approximately 25% of live births to HIV-infected mothers.( 97 ) Various regimens
of antiretrovirals can reduce the rate of perinatal transmission by 50% or
more.( 97-100,311 ). Breast-feeding is also a risk factor for HIV transmission.
Approximately one-third of cases of mother-to-child transmission result from
breast-feeding, and the risk increases with the duration of breast-feeding.( 101
) Thus, interventions to prevent mother-to-child transmission at delivery may be
largely negated if mothers are not provided with safe alternatives to
breast-feeding.
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