Bacterial vaginosis is associated with uterine cervical human papillomavirus infection: a meta-analysis

Bacterial vaginosis (BV) is the most prevalent cause of abnormal vaginal discharge, affecting women of reproductive age [1]. This infestation is characterized by a loss of indigenous (hydrogen peroxide-producing) Lactobacillus-predominant vaginal microflora, and a concurrent massive overgrowth of anaerobic bacteria. The most common include Gardnerella vaginalis, Mobiluncus species, Prevotella species, Mycoplasma hominis and Atopobium vaginae [2]. At least 50% of patients have no symptoms [3]. In the other half, it most often manifests clinically as a thin homogenous vaginal discharge, a vaginal pH of more than 4.5, presence of 'clue cells', and an amine odour after addition of 10% of potassium hydroxide [1, 2].

The etiopathogenesis of this condition remains subject of debate. Some risk factors have been associated with BV, including cigarette smoking, use of intrauterine devices, frequent vaginal douches, multiple sexual partners, early age at first intercourse, and black ethnicity [4, 5]. BV has been shown to increase the risk of obstetric and gynaecologic complications such as preterm labour and delivery, chorioamnionitis, post-caesarean endometritis, postabortion pelvic inflammatory disease, and cervicitis [6, 7]. Moreover, BV has been associated with many sexually transmitted infections (STIs), including infection with Chlamydia trachomatis, Neisseria gonorrhoeae, HSV-1 and 2, and an increased risk of HIV acquisition [4, 8, 9]. The leading hypothesis concerning these associations is that absence of protective lactobacilli increases biological susceptibility of acquiring an STI upon exposure. However, the temporal nature of the association between BV and acquisition of STIs remains an ongoing discussion. Although there is a large bulk of evidence favouring the plausibility that BV also incurs an elevated risk for human papillomavirus (HPV) acquisition, this remains a matter of debate.

It is well known that infection with oncogenic HPV, a sexually transmitted DNA virus, is the central etiological agent in the development of cervical cancer. Persistent HPV infection is a prerequisite for progression to high-grade lesions [10]. However, few HPV infections persist and progress to cervical cancer [11]. The vast majority cause no or only mild cytological abnormalities that may go undetected and regress to normalcy [11]. It is unknown why high risk HPV infection is cancerous in some women whereas in others it is eradicated. Individual differences in immunological defence may be one explanation [12]. Local cervical factors may determine the outcome of HPV. For this reason, there is a lot of interest in studying factors predisposing towards acquisition and persistence of this infection.

In contrast with cervical HPV infection, BV is associated with major changes in the vaginal environment. Because women with BV possess a Lactobacillus-poor flora, their changes in the vaginal ecosystem may provide biological plausibility for an increased risk or reactivation of HPV infection. Little is known about how the changed vaginal milieu in BV influences mucosal susceptibility for HPV, or vice versa, how infection with STIs in general influences the vaginal environment. The magnitude of association between BV and HPV has varied in epidemiological studies and remains controversial, yielding conflicting results and ranging from absence of any association [13] to a clear positive relationship [14].

To examine this controversial literature in more detail, a meta-analysis of available literature on the association between BV and cervical HPV infection was conducted. Estimates of association between BV and HPV are presented for HPV prevalence studies and analyzed for publication bias and heterogeneity.

To our knowledge, this review and meta-analysis with over 6,000 women is the first systematic evaluation of association between BV and cervical HPV infection. Although BV enhances acquisition of certain STIs, its relationship to cervical HPV infection is still an issue of controversy. Our results show evidence of a positive association between these two very common conditions, with an overall estimated odds ratio of 1.43.

Several hypotheses have been postulated, supporting this association. In BV-negative women, hydrogen peroxide-producing lactobacilli dominate the vaginal microflora and are part of the main defence mechanisms [1]. Loss of these protective micro-organisms and other changes in the vaginal milieu, related to BV, could facilitate survival of other sexually transmitted agents and are risk factors for developing vaginal infections. It is well recognised that BV renders women vulnerable to acquisition of Neisseria gonorrhoeae, Chlamydia trachomatis, HSV-1 and 2, and HIV [8, 9, 42]. Moreover, BV has been associated with a reduction in vaginal fluid levels of secretory leukocyte protease inhibitor (SLPI), able to block HIV infection in vitro [44]. It has been documented that BV propagates viral replication and vaginal shedding of HSV, thereby further enhancing spread of this STI [45].

Another hypothesis proposes that mucin-degrading enzymes are increased in vaginal fluid of women with BV. These enzymes, like sialidases, play a role in degradation of the gel layer coating the cervical epithelium, causing micro-abrasions or alterations of epithelial cells. The team of Briselden demonstrated positivity for sialidases in 84% of BV-positive women [46]. Such enzymes may promote virulence through destroying the protective mucosa barrier and hence increase susceptibility to cervical HPV infection by facilitating adherence, invasion and eventually incorporation of HPV oncogenes into the genome of cells of the transformation zone. Abnormal vaginal microflora could also be implicated in maintenance of subclinical HPV. Furthermore, changes in cervico-vaginal milieu resulting from co-infections may exert an influence on the natural history of cervical HPV infection.

It is also possible that BV is a cofactor involved in acquisition or reactivation of HPV infection by affecting immunological balance within the cervical tissue as a result of changes in production of factors, such as cytokines (interleukin-1ß, interleukin-10) [47]. Mucosal immune system activation represents a critical response against micro-organisms colonizing the reproductive tract. Neutrophil recruitment and activation is considered the main innate immune response against microbial and viral infections of vaginal mucosa [47]. Women harbouring clue cells show no inflammatory signs and neutrophils are typically relatively absent in BV smears subjected to microscopy [15]. Enzymes produced by anaerobic bacteria involved in the pathogenesis of BV can potentially alter immune signals and promote degradation of host factors, rendering women more susceptible of acquiring HPV.

These results, however, should be interpreted in light of a number of methodological limitations. The analysis suffers from the fact that most included studies had a cross-sectional design, where data on prevalence of BV and HPV infection were gathered simultaneously, instead of over time. Therefore this analysis is liable to reverse causation bias that would result from HPV infected women being more likely to acquire BV. This disadvantage prohibits concluding that BV increases risk of HPV acquisition or that there is a causal relationship. The sequence of infection is unknown and only a follow up study can determine which condition facilitates the other. In an incidence study by Watts et al., BV was significantly associated with detection of new HPV infection at follow-up visit (OR, 1.41; 95% CI 1.25-1.59) [17]. Association between BV and HPV persisted even after adjustment for number of sexual partners, suggesting that women with BV may be more susceptible for HPV and not simply because of shared risk factors. In contrast, another longitudinal study performed a time-lag analysis to evaluate which condition preceded the other [23]. The result suggested a temporal relationship, where BV was found to occur simultaneously with or after HPV infection, rather than ante-dating acquisition of HPV. Perhaps cervical HPV infection may favour changes in the vaginal milieu that enhances development of BV.

The question remains whether BV and cervical HPV infection are simply related because there is a biologic interaction between them, or because both occur frequently in sexually active women. A positive correlation between BV and HPV might be explained by the fact that sexual risk behaviour and promiscuity are found more often in women with BV than in comparison groups. Role of sexual transmission in causing or promoting BV continues to be a topic of debate, as e.g. highlighted by data in lesbians, who have a high prevalence of BV [48]. Although not considered an STI in its usual sense (e.g. treatment of the sexual partner has no effect on frequency or relapses), the epidemiological profile of BV mirrors an STI [49]. HPV is known to be one of the most common STIs, thus concerns regarding confounding by sexual behaviour certainly remain.

A number of variables are contributing to observed heterogeneity. Most prominent, prevalence of BV varied according to the population studied. Various social habits and ethno-geographical risk factors may explain the wide BV prevalence range observed (3%-47.2%). It is well recognized that prevalence of BV in African women is among the highest worldwide [1]. This meta-analysis did not include studies conducted in Africa. Considering the high prevalence of BV in this continent, it would be very interesting to evaluate the association between BV and cervical HPV infection in African women, since we may expect a more pronounced effect. Our unpublished data of a cross-sectional study including 820 HIV-negative female sex workers in Mombasa (Kenya) confirms this. In multivariate logistic regression, controlled for other STIs and behavioural characteristics, borderline significance was found between BV and high-risk HPV infection (AOR, 1.72; p = 0.06).

Technical biases (e.g. collection of specimen), subjectivity, sensitivity and specificity of diagnostic methods are also attributing to detected heterogeneity. HPV detection methods varied among included studies (e.g. FISH is less sensitive compared to PCR) and also distribution of HPV viral genotypes differed largely. However, high-risk genotypes 16 and 18 present in prophylactic vaccines were (when mentioned) always included.

Further, this meta-analysis was limited to that of published studies, which could have caused publication bias, resulting from tendency to selectively publish results that are statistically significant. However, this had probably little impact as there was no evidence of funnel plot asymmetry. In addition, most studies reported a non-significant effect, which makes publication bias highly unlikely.

Currently available vaccines targeting HPV types 16 and 18, accounting for 70% of cervical cancers worldwide, opened up new avenues in prevention of this important public health problem. If a longitudinal prospective study shows a cause - effect model, than it is clear that greater attention needs to be given to BV in the global fight against HPV infection and women with BV should be considered a priority group for prophylactic vaccination. Cervical screening remains of course a major preventive focus for the cancer control program. If BV is a risk factor for cervical HPV acquisition, it is clear that screening guidelines must adapt and implement a sensitive tool like HPV DNA testing in primary screening in BV-positive women, instead of cytological testing. Closer follow-up of these patients should be considered. Restoring the vaginal microflora should in that case be a promising answer to the high prevalence of HPV infections. Randomized clinical trials to determine effect of BV control measures on HPV acquisition may then be worth considering. In addition to the need to evaluate the potential of BV treatment to prevent HPV acquisition and transmission, a better understanding of its risk factors and determinants of recurrence is required.

This meta-analysis suggests a positive association between BV and cervical HPV infection. Considering that these conditions are very common among women worldwide, further research in this field is imperative. More data from prospective studies are needed to accurately evaluate temporal sequence of acquisition of both conditions in any attempt to determine a causal relationship and to identify specific sub-populations with a stronger association between BV and HPV.