Common bug in vagina makes it easier to get HIV
In the vagina’s ecosystem, mundane bacteria matter more than you think.
As science learns more about the vagina’s mysteries, it has found that two seemingly benign bacteria could explain why some women are at a higher risk of contracting HIV than others and why the HIV-prevention pill, Truvada, may fail them.
Last year, researchers from the Centre for the Aids Programme of Research in South Africa (Caprisa) published findings in the journal Clinical Infectious Diseases showing that inflammation at a cellular level along a woman’s reproductive tract was linked to a higher risk of contracting HIV. What researchers couldn’t answer was what was triggering this cellular reaction.
The results of the Caprisa study, which was conducted on 120 women infected with HIV, were released last week at the International Aids Conference in Durban. It reveals that those with cellular inflammation were more likely to have high rates of one bacterium in particular: Prevotella bivia. These bacteria release a molecule that helps HIV infection establish itself widely in cells. As a result, women with high levels of the bug were about 20 times more likely to be susceptible to HIV, says Caprisa’s director Salim Abdool Karim.
Caprisa researchers now posit that an overabundance of this bacteria in women’s vaginas is behind about 40% of HIV infections in women.
Up to 15% of women are thought to carry this naturally occurring bug, which is not sexually transmitted but is one of a host of bacteria that usually start appearing from the time a woman begins menstruating.
Normally, another bacteria — lactobacillus — helps to keep Prevotella bivia in check by ensuring the pH levels in a vagina are low, resulting in an acidic environment hostile to bugs.
When the good lactobacilli aren’t present in high enough concentrations to control bad bugs, Prevotella bivia take over and increase a woman’s risk of contracting HIV.
“Now we have a sense that young women in our community are being exposed to lots of men, who have HIV that they recently acquired themselves,” says Karim says. “So they are sleeping with men who have HIV infection and, in instances, they may have an organism such as Prevotella bivia; we see that they now have a higher vulnerability to HIV.”
When lactobacilli aren’t around to control Prevotella bivia, they are also not around to control the bacterium Gardnerella vaginalis, which decreases the effectiveness of one of the two ingredients, tenofovir, currently being used in the HIV prevention pill Truvada as a form of preexposure prophylaxis (PrEP). Studies have shown that Truvada can reduce HIV transmission in couples, where one person is HIV-positive, by about 75%, if taken daily.
The good news is that health workers can use a simple, cheap test to diagnose pH levels, cutting women’s risk of contracting HIV and allowing them to use PrEP effectively.
Karim expects the findings will trigger other scientists working on PrEP to review past samples to seek out these bad bugs. The research may affect how other HIV studies are designed and how the world understands the intersect between women’s biology and HIV risk.