Vaginal bacteria plays a crucial role in heterosexual HIV transmission, according to findings presented at the 21st International AIDS Conference. This role, researchers say, has been neglected in efforts to prevent the virus' spread.
Like the mouth or gut, the vagina has a bacterial ecology. A healthy vaginal ecosystem is dominated by lactobacillus, but when something lowers lactobaciilus levels, other bacteria can move in. That these competing bacteria can cause a range of problems is well known, but two new studies have revealed just how serious these effects can be.
In one admittedly small study, women with high concentrations of Prevotella bivia were almost 20 times as likely to acquire HIV through sex as those with low or no P. bivia levels. The mechanism for this effect was identified as the bacteria causing an inflammation in the vagina, which in turn triggers the release of lipopolysaccharide (LPS). LPS is know to facilicate HIV infections.
Another study found that a different bacteria, Gardnerella vaginalis, makes a more indirect contribution to HIV's spread. G. vaginalis absorbs tenofovir gel. In its absence, the gel is an effective means of preventing HIV transmission, but where low lactobacillus levels allow G. vaginalis to move in, the gel's protective effect is lost.
"Reducing new HIV infections in young women is one of the greatest challenges in southern Africa," Professor Salim Abdool-Karim of Centre for the AIDS Program of Research in South Africa (CAPRISA) said in a statement. "Based on our results, implementing a combination of evidence-based targeted interventions to break the cycle of HIV transmission while effectively treating bacterial vaginosis could enhance HIV prevention in women in the highest HIV-burden region of the world."
Both Prevotella and Gardnerella make the vaginal canal more alkaline, so simple tests reveal if there is a problem. More sophisticated tests are also available to distinguish the nature of the infection.
At a press conference held to discuss HIV prevention and treatment among women and girls, Abdool-Karim said that the findings had yet to be replicated in other regions and that “within larger studies we need to do subgroup analysis” to see to what extent vaginal infections are contributing to the spread of HIV.
"Reducing new HIV infections in young women is one of [our] highest priorities," said Global AIDS Coordinator Dr Deborah Birx. "The CAPRISA findings provide us with a greater understanding of how to protect young women. This groundbreaking study suggests young women may have been very compliant with PrEP. This new insight will allow us to move forward with a different understanding of how best to protect women from HIV."
CAPRISA recently released results showing that, despite increased availability of antiretroviral therapy and HIV prevention strategies, infection rates have actually risen among pregnant South African women and girls aged 15 to 24. Between 2009 and 2013, average rates of infection were 39.3 percent in rural KwaZulu-Natal, with subgroups such as those with older partners, even more vulnerable.