Watts, D. Heather MD
Altered vaginal flora, labeled as bacterial vaginosis (BV), whether diagnosed by clinical examination, Gram stain of vaginal secretions, or molecular typing of bacteria, has been associated with an increased rate of multiple adverse events including sexual HIV acquisition and potentially transmission,1,2 acquisition of genital herpes simplex virus,3 human papilloma virus,4 Neisseria gonorrhoeae,5 Chlamydia trachomatis,5 pelvic inflammatory disease,6 postabortal and postoperative infections,7,8 and among pregnant women, an increased risk of preterm birth,9–11 chorioamnionitis,12 and postpartum infections.13 The alterations in vaginal flora include decreased levels of hydrogen-peroxide producing lactobacilli, increased prevalence and levels of Gardnerella vaginalis, Mycoplasma hominis, and several logarithm increases in levels of anaerobic bacteria including Prevotella spp, Porphyromonas spp., and Mobiluncus spp.14,15 Clinically, the diagnosis may be made by the presence of increased homogenous vaginal discharge, vaginal pH above 4.5, amine odor with the addition of potassium hydroxide to vaginal secretions, and the presence of more than 20% clue cells, or vaginal epithelial cells coated with coccobacillary bacteria, on microscopic examination of vaginal fluid.16 The gold standard for diagnosis in research has been the Nugent score, which evaluates Lactobacillus morphotypes (scored inversely 0–4), small Gram-negative rods (scored 0–4), and Mobiluncus (scored 0–2) to provide a score of 0–10, with 0–3 being normal, 4–6 being intermediate, and 7–10 representing BV.17,18 More recently, molecular studies have confirmed the association of G. vaginalis and anaerobic bacteria with BV, but have identified additional species associated with the clinical and Gram stain diagnosis of BV, including Atopobium, Leptotrichia, Megasphaera, Eggerthella-like species, and the newly described BV-associated bacteria 1, 2, and 3.19,20
Although BV has classically not been associated with signs of inflammation or increased white blood cells in the vaginal discharge, leading to the name vaginosis rather than vaginitis, there still are associated cytokine and immune cell changes in the genital tract that may facilitate acquisition of other infections or lead to complications such as preterm labor. BV is associated with higher vaginal concentrations of interleukin (IL)-1beta, tumor necrosis factor (TNF), interferon-gamma, IL-2, and IL-4, IL-6, and IL-10.21,22 TNF is probably triggered by the lipopolysaccharide in the walls of the Gram-negative bacteria found in abundance in BV. Increased TNF leads to increased IL-1beta and other inflammatory cytokines.21 Interferon gamma induces macrophage activation, which may increase susceptibility to HIV-1 infection in the genital tract.23 IL-2 induces T and B lymphocyte activation, again potentially increasing HIV susceptibility or levels in the genital tract.24 The anti-inflammatory cytokines, IL-4 and IL-10, are also increased in BV, potentially balancing the effects of the cytokines discussed above and limiting inflammation.21 Of note, levels of IL-8, a potent neutrophil chemoattractant, are not increased in BV, potentially explaining the lack of white blood cells in the vagina with BV.25
BV has been associated with both increased risk of acquisition of HIV and among infected women, with an increased risk of genital tract detection.1,26 In studies evaluating incident HIV, BV was associated with an 1.61-fold increased risk of HIV (95% confidence interval: 1.21 to 2.13).1 Changes in cytokines discussed above that increase genital tract target cells along with elevated pH, loss of hydrogen peroxide production, and decreased secretory leukocyte protease inhibitor and other protective protein levels may all enhance susceptibility.22,24 In addition, BV has been associated with increased genital tract levels and rate of detection of genital HIV among infected women2,26; and increased genital HIV RNA levels have been shown to predict risk of heterosexual transmission.27
Given the association of BV and increased genital tract levels of HIV, it is not surprising that BV might also be associated with an increased risk of vertical transmission of HIV. In an early study among women not receiving antiretroviral prophylaxis, an increased risk of perinatal transmission of HIV among women with altered vaginal flora was suggested, but inconclusive.28 In that study, BV was significantly increased among HIV-infected women with preterm birth compared with those with term birth.28 In a study from Kenya in which women received zidovudine beginning at 34–36 weeks of gestation, BV was associated with an adjusted odds ratio of 3.0 (95% confidence interval 1.2 to 7.0) for in-utero transmission of HIV.29 Frank et al,30 as reported in this issue, found an association between bacterial communities associated with BV and mother to child transmission of HIV among women receiving no antiretroviral therapy or zidovudine from 36 weeks of gestation. Increased abundance of Gardnerella, Sneathia, Atopobium, Prevotella, and Mycoplasma species and decreased abundance of lactobacilli were significantly associated with perinatal HIV transmission.
Most striking was the association between in-utero transmission of HIV and BV-associated flora changes both in the studies of Farquhar et al and Frank et al.29,30 Although BV might be expected to increase the risk of intrapartum transmission by increasing the levels of HIV in the genital tract to which the infant is exposed at delivery, the potential pathogenesis of in-utero transmission is less clear. As discussed above, BV is associated with alterations in the vaginal milieu including interferon gamma and IL-2, which may lead to local increases in macrophages and activated T lymphocytes potentially infected with HIV, possibly increasing the chance of infected cells traversing the placenta and membranes. BV has been associated with an increased risk of preterm delivery and with upper tract infection.9,10 Multiple studies have found an association between infection and preterm birth with the risk of amniotic fluid and chorioamnion infection, and inflammation inversely related to gestational age. Evidence of upper tract infection is present in up to 75% of deliveries at 23 weeks of gestation, declining to about 10% at 34 weeks.31 Histologic chorioamnionitis follows a similar inverse pattern with gestational age although chorioamnionitis is present in as many as 21% of women with term delivery.12
Preterm infants are more likely to have in-utero HIV infection compared with infants born to HIV-infected women at term, possibly because of HIV infection triggering preterm birth.32,33 Alternately, the upper tract infection and inflammation leading to preterm birth may also increases the risk of HIV acquisition by the fetus. Studies in the pre-antiretroviral era have shown an association between acute histologic chorioamnionitis, manifest by polymorphonuclear leukocyte infiltration in placental membranes, and transmission of HIV.34,35 More recently, in the era of intrapartum NVP prophylaxis, acute chorioamnionitis was not associated with increased risk of transmission, possibly because of antiretroviral mediation of infection risk. However, chronic chorioamnionitis, as defined by the presence of >10 monocytes/high-power field, was associated with increased risk of in-utero transmission.36 Chronic chorioamnionitis has been associated with chronic subclinical choroamnion infection, often associated with BV.37,38 Unfortunately in the study by Chi et al,36 vaginal flora was not assessed, and in the study by Frank et al,30 placental membrane histology was not reported. Although preterm birth was not associated with vaginal bacterial results in the study by Frank et al,30 numbers were small. The risk of preterm delivery with BV varies according to genetic make-up, suggesting that host response plays an important role.39 Genetic variations may allow some women to have chronic subclinical upper tract infection, potentially facilitating HIV transfer, without preterm delivery. In the only study to date evaluating antibiotics to prevent chorioamnionitis and reduce HIV transmission, the rate of chorioamnionitis and HIV transmission were not different between those receiving the metronidazole/erythromycin regimen or placebo at 20–24 weeks of gestation along with maternal and infant peripartum nevirapine.40 Further studies of placentas from infants born with and without in-utero HIV infection and correlation with maternal genital tract infection may help to clarify risks. A better understanding of the systemic and vaginal cytokine milieu in normal pregnancy and pregnancies with BV along with changes in these compartments in HIV-infected pregnant women is needed to help elucidate the pathophysiology of both preterm birth and HIV transmission.
Maternal BV increases the risk of seroconversion to HIV, and seroconversion late in pregnancy or during breastfeeding is associated with an increased risk of transmission to the fetus because of high maternal HIV RNA levels without maternal antibody.41,42 The duration of HIV infection was not assessed in the study by Frank et al,30 and HIV RNA levels were not available. Although BV increases the risk of HIV acquisition, it is unlikely that recent infection explains all of the increased risk seen in this study. Further studies of methods to decrease the risk of HIV acquisition in pregnancy including potentially identification and antiretroviral treatment of infected partners, pre-exposure prophylaxis, vaginal microbicides, and treatment of BV and other genital tract infections are needed. Thus far, studies of treatment of genital tract infections for HIV prevention have had mixed results,43 but treatment of BV may not only decrease HIV acquisition risk but may also decrease pregnancy complications.
Although the study findings are intriguing, they must be considered preliminary at best. Women included in this study were an opportunistic sample and represented a small proportion of women in the study overall and may not be representative of the general population of pregnant HIV-infected women. The number of infected infants was small, and data on numerous other risk factors such as maternal HIV RNA levels, duration of infection, and clinical and histological chorioamnionitis are not available. In addition, the standard of care for pregnant women has evolved considerably since this study was done, with antiretroviral drugs beginning earlier in pregnancy now available to HIV-infected pregnant women who are identified through counseling and testing. Whether BV is associated with increased risk of transmission in the presence of maternal antiretroviral use needs to be evaluated. If the risk is primarily for in-utero transmission, elevated risk could remain among those infants with bacterial infection-mediated preterm birth. In addition, the association of increased levels of genital tract HIV in the presence of BV may not be mitigated if antiretroviral drugs with high genital tract levels are not included in the treatment or prophylaxis regimen. Thus further evaluation of the effects of BV on genital tract HIV levels, genital tract cytokines and immune cells, upper tract infection, and mechanisms of increased preterm birth are warranted. Molecular studies will allow evaluation of a broad range of bacteria for their role in pregnancy complications. Future studies should also include more readily available markers of altered genital flora such as clinical examination and vaginal Gram stains in addition to molecular markers to allow correlation of findings so that more practical means of diagnosis can be used in the clinical setting if screening and interventions are found to be effective.
Although BV has been associated with an increased risk of adverse pregnancy outcomes in multiple studies, thus far studies of routine screening and treatment of BV during pregnancy have not shown consistent benefit in reducing complications.44 Some speculate that the timing and type of antibiotics studied have not been appropriate and that better results may be obtained with earlier treatment with a more broad-spectrum antibiotic than metronidazole, the most commonly evaluated agent.45 Whether screening for and treating BV in pregnancy may have additional benefits in HIV-infected women is unclear, although the British HIV Association Guidelines currently recommend routine screening and treatment of BV in pregnant women.46 Clearly much work remains to be done investigating the relationship of BV to adverse pregnancy outcome and to the potential increased risk of perinatal HIV transmission.
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