Sexual networks, HIV, race and bacterial vaginosis

In their recent article, Buvé et al.[1] argue that the higher HIV prevalence in ‘black populations’ is due, in large part, to racial differences in the vaginal microbiome. They base this argument on two findings. Firstly, bacterial vaginosis (BV) prevalence tends to be higher in black populations. Secondly, a number of studies have found an increased incidence of HIV following the diagnosis of BV. We would like to advance a sexual network based explanation that we believe provides a better fit to the observed patterning of BV, sexually transmitted infections (STIs), sexual behaviour and race. We illustrate our argument by way of an analogy with John Snow's insights into the forces underpinning cholera transmission in London in 1854. Snow [2] showed that persons whose houses were supplied by the water-pipes from the Southward water company had nine times the cholera-related mortality of those supplied by the Lambeth company. He argued that this was likely due to Lambeth, unlike Southward, drawing its water upstream from the sewage contamination. Faecal contamination of Southward's water network put the Southward supplied houses at a high risk for exposure to entero-pathogens, including cholera and other faecal-oral transmitted illnesses such as typhoid. It is also possible that there could have been an association between typhoid and cholera infections at an individual level. However, the higher prevalence of cholera in Southward, together with the association between typhoid and cholera, would have best been explained by their common source (a contaminated water network) rather than by typhoid potentiating the transmission of cholera. In a similar vein, black as opposed to white populations in the USA, UK and South Africa have been found to have higher prevalences of BV, HIV and other STIs [3–5]. The most parsimonious explanation in each case is that the black populations in these countries have more connected sexual networks [3,4,6]. This results from a number of factors, including the higher prevalence of partner-concurrency observed in black populations in each of these countries [3–5]. A more connected sexual network represents a higher risk network for all STIs entering the network [3]. These network-level properties could in turn explain a part of the observed higher prevalence of STIs in black populations in these countries. Network factors could also explain the observed association between different STIs at an individual level. Persons who contract one STI are by virtue of this more likely to be connected to a high-risk part of the sexual network and therefore more likely to contract other STIs. This effect is very difficult to control for in individual-level analyses [3]. Just as in the case of entero-pathogens in the Lambeth versus Southward populations, it would not be appropriate to assume that an association between STIs represented a causal relationship. This sexual network-level explanation is supported by a variety of types of evidence. Studies at both individual [7,8] and ecological levels [9] have found an association between partner-concurrency and BV prevalence. BV prevalence is not just increased in blacks but in a range of nonblack populations such as Greenland, Aboriginals in Canada and Aymara speakers in Peru [10–12]. In each of these cases, the populations with a high BV prevalence had markers of higher-risk sexual behaviour such as a high prevalence of other STIs [10–12]. This is commensurate with the findings of a systematic review and meta-analysis of the relationship between sex and BV that found that various forms of multiple partnering were associated with an increased incidence of BV [13]. A further crucial problem for hypothesis by Buvé et al.[1] is that the available data suggest that black populations with low-risk behaviour and low prevalence of other STIs have a low BV prevalence. In our systematic review of the global epidemiology of BV, we found that Burkina Faso, which has a relatively low prevalence of HIV and other STIs [14], has a very low prevalence of BV, 6.4% and 7.9% according to two large, high-quality studies [15]. This fits with findings from studies from other sub-Saharan African countries that find considerable differences between HIV and STI prevalence between different black ethnic groups that is strongly associated with differences in sexual behaviour [5,6,16]. We have enough evidence to conclude that the vaginal microbiome varies considerably according to a myriad of factors considered at the levels of individuals, couples and sex networks. Longitudinal studies that sample the genital microbiomes of women and their partners from the time of sexual-debut and in multiracial communities are required to assess whether any of these variations can be attributed to race. Acknowledgements Conflicts of interest There are no conflicts of interest.

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