Whilst reducing infectivity is not considered a reason for treatment in itself by most doctors and people with HIV, it is an important potential benefit of treatment. However, predicting reduced infectivity at the individual level is difficult because several factors influence the infectivity of genital fluids, the most important of which are:

• Drug penetration into the genital tract.

• The presence of sexually transmitted infections.

Drug activity in the genital tract of men

Although most studies show that the majority of men treated with antiretroviral drugs experience parallel declines in plasma and seminal viral load, all studies have shown considerable individual variation in responses. This means that some men may still have infectious HIV in their semen after their viral load tests indicate that HIV is undetectable in the blood.

The following patterns have been observed:

• Viral load becomes undetectable in plasma weeks or months before doing so in semen.

• Viral load becomes undetectable in semen but not in plasma.

• Plasma viral load rebounds after a period of undetectability but seminal viral load remains undetectable.

In the first case, prolonged HIV production in the genital tract may be explained by the fact that long-lived cells which have been infected by HIV continue to pump out virus copies because anti-HIV drugs cannot adequately penetrate these particular cells. Another explanation might be that virus production continues because latently infected cells are triggered into virus production by the presence of infections or inflammation.

However, even when HIV is 'undetectable' by branched-chain DNA testing in semen, this does not mean that it has disappeared. The viral load test cannot detect viral load below 1000 copies/ml in semen, and more sensitive tests must be used to establish whether semen still contains infectious HIV. Furthermore, as this test looks for HIV's DNA, it suggests the presence of actively replicating infectious HIV. One small study found that three out of 11 patients on antiretroviral therapy with undetectable viral load in semen still had detectable HIV DNA when more sensitive tests were conducted.

A larger study of over 100 men on treatment examined blood and semen samples for HIV RNA and DNA. Only two treated men (2%) had HIV RNA in their semen in comparison to 67% of untreated controls. Of the 53 treated men tested for cell-associated HIV DNA, nine (17%) had detectable HIV DNA in contrast to 21 (38%) of 55 untreated controls. These statistically significant differences suggest that antiretroviral therapy does reduce the amount of HIV in semen. However, some HIV may remain in semen and HIV transmission through the exchange of sexual fluids remains a possibility despite treatment (Vernazza 2000).

Another study of 52 HIV-infected men has found a substantial minority do not have a correlation between seminal and blood plasma viral load. Twenty men had seminal viral load equivalent to or higher than their blood plasma viral load (Tachet 1999). Italian researchers found that viral load tended to rebound first in plasma during a treatment interruption, and was followed by seminal rebound in all patients (Liuzzi 2003).

Whilst some doctors have argued that reducing viral load in semen will have a beneficial effect on transmission rates, others caution that this is not yet proven (Albrecht 1998; Vernazza 1997).

A 58-week study of eleven men taking antiretroviral therapy found that different viral variants emerged in plasma and semen respectively in seven out of eight men in whom drug-resistant mutations could be identified. These findings suggest that different forms of drug pressure are operating in the plasma and the male genital tract, and lend some support to the view that protease inhibitors do not penetrate the male genital tract in high enough concentrations to prevent the development of resistance (Eron 1998). Analysis of mutations in HIV taken from semen and blood has led some researchers to propose that a separate seminal HIV reservoir exists in some men.

Separate resistance patterns in plasma and semen have been observed by a French group in treatment-experienced men failing treatment. Thirty percent exhibited different genotypic resistance patterns in virus isolated from the two compartments (Ghosn 2003).

This reservoir is in large part an effect of poor drug penetration into the male genital tract. The ability of treatment to suppress HIV in the semen to undetectable levels has been attributed to the high concentrations of the NRTIs (Pereira 2000). However, there is considerable inter-class variability regarding which drugs achieve active levels in the semen.

AZT, 3TC, nevirapine and efavirenz have been shown to achieve tenfold higher concentrations in semen compared to plasma, although one study has shown ineffective efavirenz (Sustiva) concentrations in semen (Ghosn 2004). Indinavir (Crixivan), amprenavir (Agenerase) and d4T (stavudine, Zerit) also achieve active levels, while concentrations of saquinavir (Invirase / Fortovase), ritonavir (Norvir), lopinavir and nelfinavir (Viracept) are lower in semen than in plasma (Kashuba 1999).

Other studies of protease inhibitor concentrations in the semen have found:

• Amprenavir and indinavir alone reach active levels in the semen (Pereira 2002; Taylor 2000; Solas 2003).

• Ritonavir and saquinavir, when taken as sole PIs, have low seminal concentrations (Pereira 2002; Taylor 2000).

• Concentrations of nelfinavir and lopinavir in the semen are well below concentrations achieved in the blood plasma (Ghosn 2004; Solas 2003).

Poor penetration of protease inhibitors is probably due to the protein binding of the drugs and the high protein content of semen, or to the low levels of P-glycoprotein (Pgp), a substance which pumps drugs, including protease inhibitors out of cells. Pgp is present at very low levels in cells of the brain and testes. Sub-therapeutic levels of PIs might encourage the development and transmission of drug-resistant virus.

T-20 (enfuvirtide, Fuzeon) was not detectable in semen in a study of four men receiving the drug as part of a salvage regimen despite good plasma concentrations (Ghosn 2004).

Other conditions such as urethritis (inflammation of the urethra) may be associated with detectable HIV in semen. One study of 77 HIV-positive men found that seven had asymptomatic urethritis, four of whom had detectable virus in their semen, despite anti-HIV treatment (Winter 1999).

HIV and drug activity in the genital tract of women

Several large studies have found a strong association between the level of viral load in blood and the level of viral load in cervicovaginal fluid (CVF; Cu-Uvin 1998; Hoesley 1998; Hart 1999; Reichelderfer 2000). The initiation of antiretroviral therapy significantly reduced HIV viral load in vaginal fluid in the Women First study. In this study, nine of 56 women had detectable HIV RNA in the genital tract at baseline, with a mean of 1240 copies/ml. After three and 12 months treatment, all had undetectable HIV in the genital tract. Higher plasma viral load and lower CD4 cell counts were associated with detectable virus in the genital tract (Squires 1999).

However, there is some evidence that antiretroviral therapy may have discrepant effects on blood and on cervicovaginal fluid, especially when a genital infection is present. An American study of 11 women on antiretroviral therapy showed that viral load in vaginal fluid was significantly higher. However, the presence of genital tract infections such as candidiasis (thrush) or inflammatory lesions of the vulva or cervix was predictive of a discrepant response to anti-HIV treatment. The authors of the study have noted that their results should be treated with caution due to the small sample size and the fact that none of the women was receiving protease inhibitor treatment (Stephens 1998).

Recent findings have suggested that the levels of different anti-HIV drugs in the CVF is highly variable, with levels of indinavir (Crixivan) around fivefold higher than in the blood, but those of nevirapine (Viramune), amprenavir (Agenerease), lopinavir, efavirenz (Sustiva), saquinavir (Invirase / Fortovase) and ritonavir (Norvir) being lower, albeit with a high degree of variability. Although only seven women were tested, this variability may be of importance in determining the risk of HIV transmission during sex and from a mother to her child (Min 2004).

A small study found that HIV in blood and in CVF is genetically distinct, suggesting that HIV in CVF is being produced by activation of local immune cells (Lawn 2001). This was confirmed in a larger study examining paired blood and cervicovaginal secretions from 41 HIV-1-infected women. The majority of women (94%) had a higher viral load in blood plasma than CVF. Gene sequence data from 13 paired samples indicated genetic diversity between virus present in the blood and genital tract of all 13 women. Antiretroviral drug resistance reflecting drug use history was found in three of the women (23%). Two of these had concordant and one discordant resistance mutations in the blood and genital tract (Mullen 2004).

These data support the theory of compartmentalisation of the virus, and suggest that selection of drug resistant variants in the blood and genital tract can differ. It cannot be assumed, therefore, that resistance profiles in the two compartments are the same, which could impact on post exposure prophylaxis following perinatal and sexual exposure.

The amount of virus in the CVF has been shown to be correlated with advanced disease stage (CD4 cell counts below 200 cells/mm³ and viral loads above 50,000 copies/ml) in one study of 56 women from the American Women's Interagency HIV-1 Study cohort. Thirty percent of the women had an active vaginal infection, but this only had a marginal effect on the amount of virus in the CVF (Kovacs 1999). This has been confirmed in further studies, including one reporting CVF viral load below 400 copies/ml in 69% of 36 women before treatment and in 89% after one year of treatment. A bout of bacterial vaginosis was associated with a cervicovaginal viral load above 400 copies/ml (Cu-Uvin 2001).

Another study determined viral load level discrepancies in paired plasma and cervicovaginal secretions over a period of 36 months among HIV-positive women on or off highly active antiretroviral therapy (HAART), finding that viral load was the most significant factor associated with genital tract virus shedding. Plasma viral load was more likely to rebound first or at the same time as genital tract viral load, and both plasma and genital tract viral load were more likely to be detectable when HIV-1 RNA was detected in either or both compartments at a previous visit (Cu-Uvin 2004).

HIV viral load in the female genital tract and fluids also varies during the course of a menstrual cycle, even among women on treatment. A prospective study of 55 HIV-infected women with CD4 cell counts below 350 cells/mm³ was conducted to assess the impact of the menstrual cycle on viral load and CD4 cell counts. CVF viral loads were highest during menses, lowest immediately after menses, and rising in between. In contrast, HIV viral load in the cervical fluid was highest in the week prior to menses when it was higher than in the blood plasma. Viral load and CD4 cell count in the blood did not change during the menstrual cycle (Reichelderfer 2000). This was confirmed in a second study that took vaginal swabs from 17 women with HIV each day for a month. This study found that viral load levels in vaginal fluid tended to peak at the time of menstruation and fell to the lowest level just prior to ovulation (Benki 2004).

Forty-eight weeks of micronutrient supplementation had no effect on viral shedding in the female genital tract in a placebo controlled trial conducted in Thailand (Jiamton 2004).

HIV in the rectum

Several studies have shown that detectable levels of HIV may persist in anorectal tissue even after HIV becomes undetectable in the blood.

Undetectable plasma HIV RNA was strongly correlated with undetectable HIV RNA in anorectal specimens in one study. However, 28% of men with undetectable plasma viral load had HIV DNA in anorectal tissue specimens. Treatment with antiretroviral therapy was associated with a reduced rate of detectable HIV RNA in anorectal specimens but not with reduced HIV DNA levels (Lampinen 2000).

In a second study which looked at levels of viral load in secretions from rectal mucosa of 27 men taking HAART, rectal secretions had a median viral load of 3980 copies/ml, compared with 200 copies/ml in the blood and 1000 copies/ml in semen. For every 1 log₁₀ reduction in plasma viral load, viral load in rectal secretions appeared to decline by only 0.5 log₁₀ (Zuckerman 2004).

These findings imply that men who believe themselves to have undetectable viral load and who are the receptive partner in unprotected anal intercourse may have a high risk of transmitting HIV.

Sexually transmitted infections

Sexually transmitted infections are important co-factors in the spread of HIV. Not only can they enhance the sexual transmission of HIV by increasing the rate of viral shedding, but HIV infection can also increase susceptibility to sexually transmitted infections. However, studies examining the effect of antibiotic treatment of sexually transmitted infections on HIV infection within African communities have produced mixed findings.

In one study, viral load in seminal and blood plasma was measured in 135 HIV-positive men in Malawi. The men with inflammation of the urethra (urethritis) had seminal plasma viral loads eight times higher than those in HIV-positive men without urethritis, despite similar CD4 cell counts and blood plasma viral load levels. Gonorrhoea was associated with the greatest concentration of HIV-1 in semen. Antibiotic therapy directed against sexually transmitted infections reduced the seminal viral load threefold after two weeks (Cohen 1997).

However, monthly prophylactic treatment of female sex workers in Kenya with the antibiotic azithromycin (Zithromax) failed to reduce the incidence of HIV-1 infection despite reducing the incidence of other sexually transmitted infections, including gonorrhoea, chlamydia and bacterial vaginosis. The study's authors offer a number of explanations for this apparent discrepancy. Firstly, the women in this study were receiving higher standards of screening and care than normal for the study region, possibly reducing the number of HIV infections that could be attributed to sexually transmitted infections. They also propose that other pathways may account for the observed association between HIV infection and the presence of other infections, including increased viral shedding in male clients with sexually transmitted infections, or a high prevalence of sexually transmitted infections amongst HIV-positive men. This would lead to female sex workers being exposed to high levels of both HIV and other sexually transmitted infections, but without treatment reducing the risk of HIV infection (Kaul 2004).

Studies have also been carried in the developed world. Twenty British HIV-positive gay men not on HAART with urethritis - nine gonococcal, three chlamydial, one combined gonococcal and chlamydial, and seven non-specific urethritis (NSU) - and 35 controls of similar age, pre-study blood plasma viral loads and CD4 cell counts had blood and semen measured for viral load. The men with urethritis were treated with appropriate antibiotics and all men had blood plasma and seminal viral load checked one and two weeks after the first visit.

Although at the first study visit there were no differences seen in blood viral loads between those with and without urethritis, mean seminal viral loads were higher among those with chlamydia and gonorrhoea compared with controls but not in those with NSU. Seminal viral loads among those with gonorrhoea and chlamydia were similar. Following antibiotic treatment, seminal viral load fell by an average of 0.25 log₁₀ in those with chlamydia and gonorrhea, whereas there was no difference in those with NSU (Sadiq 2004).

This study implies that average seminal viral load levels appear to be much lower amongst United Kingdom gay men not on therapy than their heterosexual counterparts in sub-Saharan Africa. The implications of this on the effects of sexually transmitted infections on sexual transmission of HIV-1 in the developed world remain unclear and require further study.

Researchers in the United Kingdom have also shown that even where viral load in semen is controlled by antiretroviral treatment, sexually transmitted infections can cause viral load rebound in semen, and conversely, even when viral load is rising in the blood, viral load in semen can be brought under control if a sexually transmitted infection is treated (Sadiq 2002). This reinforces the view that the blood and genital tract are largely independent compartments.

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