Bacterial vaginosis

As is the case for other mucosal surfaces, the vagina may be colonised by a variety of bacteria, fungi, and other potential pathogens.

Within a population, the bacterial composition of the vaginal flora constitutes a spectrum. In some women, lactobacilli that produce hydrogen peroxide predominate (a normal vaginal flora). In others, the flora may include an increasing proportion of other bacterial species, such as Gardnerella vaginalis, Morbilincus species, Mycoplasma hominis, and Prevotella species. When there is an imbalance in the vaginal flora, this is considered an intermediate vaginal flora or bacterial vaginosis (BV). However, in many settings, over half of women have what is considered an ‘abnormal’ vaginal flora. Bacterial vaginosis is associated with a vaginal pH of more than 4.5 (more alkaline) and a thin whitish vaginal discharge.

Bacterial vaginosis has been shown to increase the risk of pre-term delivery, pelvic inflammatory disease and upper genital tract infections.

A 2008 meta-analysis¹ of the association between BV and HIV infection identified 21 cross-sectional studies. All but one of these found that HIV prevalence was higher in women with bacterial vaginosis. The review also identified four prospective studies of new HIV infections in four African populations. Pooling their data, bacterial vaginosis was associated with an increased risk of HIV acquisition (risk ratio: 1.61; 95% confidence interval 1.21 – 2.13).

An additional prospective study² found that women with BV or candidiasis were more likely to acquire HIV, especially if the condition was present at the same visit as the new HIV infection and the visit preceding it (hazard ratio 2.50 for BV, 2.97 for candidiasis). These relationships did not seem to be mediated by mucosal inflammation.

Although BV tends to be particularly prevalent in high-risk groups such as sex workers, the review found that the association between BV and HIV infection was strongest for lower-risk women.

The large number of women who have BV could mean that it makes a large contribution to the number of women who acquire HIV. Research is needed to find out if treatment for BV could reduce the risk of HIV infection.

There are several mechanisms by which bacterial vaginosis might enhance HIV acquisition. Loss of a lactobacilli-predominant flora is associated with an increased vaginal pH (which may be more conducive to the persistence of pathogens), an absence of hydrogen peroxide (which is toxic to many pathogens), a reduction in mucosal levels of secretory leukocyte protease inhibitor (an innate immune factor with anti-HIV activity), increased susceptibility to herpes simplex virus and other sexually transmitted infections, and substantial local increases in proinflammatory cytokine levels.³

Some vaginal practices, especially douching, may make bacterial vaginosis more common.