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--- "Edward E. Rylander, M.D." <[log in to unmask]>
wrote:
>
> Table of Contents
>
> HIV Transmission
> Intervention Strategies
> Biological HIV Prevention Strategies: Reducing
> Infectiousness,
> Susceptibility, and the Efficiency of Transmission
> Antiretroviral Therapy and HIV Transmission
> Conclusion
> References
> Appendix: Talking About Safer Sex With Your Patients
>   _____
>
>
> HIV Transmission
>
>
> Epidemiology of HIV Transmission
>
> HIV is spread from human to human by 3 routes[1]:
> *       blood transmission (contaminated
> transfusions, needle sharing during drug
> use, needle-stick injuries)
> *       vertical transmission (mother to offspring
> during parturition or
> breastfeeding)
> *       sexual transmission
> Sexual transmission of HIV accounts for more than
> 75% of infections
> worldwide.
> The probability of transmission of HIV by different
> sexual routes per
> episode of intercourse is summarized in Figure 1,
> using data generated by
> epidemiologists and mathematical modeling.[1]
> Transmission of HIV from men
> to their partners is more efficient than from women
> to men.[2,3]
> Transmission of HIV through anal intercourse is more
> efficient than other
> sexual behaviors. In addition, transmission per
> episode of intercourse may
> well be affected by the stage of disease of the
> infected subject or innate
> or acquired immunity of the exposed person as
> discussed below. Oral sex
> between women (cunnilingus) appears to confer almost
> no risk, and fellatio
> appears to have limited risk relative to sexual
> intercourse. However, HIV
> acquisition among gay men has been reported as a
> result of fellatio
> alone.[4] Among 122 individuals with primary HIV
> infection, Dillon and
> coworkers[5] attributed 6.6% of cases to oral sex.
> In addition, simian
> immunodeficiency virus (SIV) has been transmitted to
> macaques through oral
> inoculation using similar concentrations of virus as
> those required for
> vaginal infection.[6,7]
>
> Figure 1. Per-contact probability of HIV
> transmission. The infectivity
> ranges for sexual contact are derived from a
> comprehensive review of the
> literature (lower and upper bounds are from modeling
> per-contact
> transmission in different study populations with
> different modeling
> techniques). Each infectivity estimate for the other
> routes of infection
> originates from one representative study. The routes
> of infection are as
> follows: sexual intercourse, with indicating
> female-to-male transmission,
> indicating male-to-female transmission, and
> indicating male-to-male
> transmission; needle stick; needle sharing;
> transmission from mother to
> infant with and without perinatal zidovudine
> treatment; and transfusion.
> Royce RA, Sena A, Cates W Jr, Cohen MS. Current
> concepts: sexual
> transmission of HIV. N Engl J Med.
> 1997;336:1072-1078. Copyright 1997.
> Massachusetts Medical Society. All rights reserved.
> The spread of HIV can be assessed at a population
> level as well. Anderson
> and May[8] have described the risk of secondary
> (new) cases of HIV as Ro,
> where Ro = beta x C x delta, with beta representing
> the efficiency of
> transmission, C the number of sexual partners, and
> delta the duration of
> infectiousness of the index case. When Ro exceeds 1,
> new, secondary cases of
> HIV occur, and the epidemic continues. Successful
> prevention strategies must
> reduce Ro to less than 1 and include lowering the
> rate of partner change,
> reducing the efficiency of transmission, and
> shortening the duration of
> infectiousness. This model offers an excellent
> conceptual framework to
> approach HIV prevention and a tool to examine the
> success of interventions.
>
> Biology of HIV Transmission
>
> The efficiency of transmission of HIV represents a
> biological event;
> transmission either does or does not occur. HIV
> transmission must depend on
> the infectiousness of the index case (reviewed in
> the paper by Vernazza and
> coworkers[9]) and the susceptibility of the exposed
> host (reviewed in the
> paper by Buchacz and colleagues[10]). A schematic
> representing the
> transmission of HIV from a male to a female partner
> is provided in Figure 2.
>
> Figure 2. Male-to-female transmission of HIV.
> Infectiousness of HIV. HIV can be recovered from
> seminal cells (CD4+
> macrophages and lymphocytes) and seminal plasma.
> Detection of HIV in seminal
> plasma by RNA polymerase chain reaction
> amplification techniques has been
> used as a surrogate for the concentration of HIV in
> semen, although
> procedures to eliminate inhibition of amplification
> must be used.[11] When
> the concentration of HIV in seminal plasma exceeds
> 10,000 copies/mL, HIV can
> usually be grown in seminal cells.[12] However, it
> is unclear whether HIV is
> transmitted from seminal cells or seminal plasma;
> cell-free virions in
> seminal plasma may be defective (and unfit).
> The recovery of HIV from cervical mucus and
> cervicovaginal lavage fluid is
> similar to semen, although it has only recently been
> possible to quantify
> the copy number.[13,14] HIV can be recovered from
> cervicovaginal lavage
> fluid cells when the concentration exceeds 10,000
> copies/mL.[15]
> Several lines of evidence suggest that the
> concentration of HIV in genital
> secretions can be correlated with risk of sexual
> transmission. First, there
> is overwhelming evidence that the concentration of
> HIV in an infected
> mother's blood determines the risk of vertical
> transmission,[16-18] although
> one must realize that the biologic mechanisms of
> vertical and sexual
> transmission are different. Second, a correlation
> has been demonstrated
> between increased concentrations of HIV in blood and
> enhanced transmission
> by all routes.[18-21] In a remarkable study in
> Uganda, Quinn and
> coworkers[22] demonstrated that HIV transmission in
> HIV serodiscordant
> couples could be correlated directly with the blood
> plasma HIV RNA level in
> the infected subject. No HIV transmission was
> observed when blood plasma HIV
> was less than 1500 copies/mL. In a similar study
> among serodiscordant
> couples in Rakai, Uganda, Gray and colleagues[23]
> calculated that when the
> blood plasma viral load was less than 3500
> copies/mL, the transmission
> probability was 0.0001 (1 per 10,000 episodes of
> intercourse). When blood
> viral burden was greater than 50,000 copies/mL, the
> transmission probability
> was calculated to be 0.0051 (5.1 per 1000 episodes
> of intercourse). It
> should be noted the concentration of HIV in blood
> can be directly (but
> imperfectly) correlated with the concentration of
> HIV in semen[11,12,24] and
> female genital secretions.[15]
> Third, the concentration of HIV in genital tract
> secretions from males and
> females is increased at times when enhanced
> transmission is suspected, such
> as in primary infection or in later stages of HIV
> disease,[11,12,15,24,25]
> and in patients with classic sexually transmitted
> diseases (STDs) (reviewed
> in the paper by Fleming and Wasserheit[26]).
> However, recent data suggest
> that the concentration of HIV in semen during
> primary infection may not be
>
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