As science learns more about the mystery that is the vagina, two seemingly benign bacteria could finally explain why some women are at a higher risk of contracting HIV and why the new 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 Clinical Infectious Diseases journal showing that inflammation at a cellular level along a woman's genital reproductive tract was linked with a higher risk of contracting HIV. What researchers couldn't answer was what was triggering this cellular reaction.
New research from the centre may have finally answered the question and provided health workers with a simple, cheap way of reducing this risk. The research was released at the International Aids Conference in Durban on Monday.
The average woman is likely to have tens of thousands of organisms present in her vagina and most of these are naturally occurring.
In the new Caprisa study, conducted among 120 women with HIV, those with cellular inflammation were much more likely to have high rates of one bacteria in particular, known as Prevotella bivia, according to Caprisa director Salim Abdool Karim. This bacterium releases 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 HIV positive, says 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 starts appearing in the vagina from the time a woman begins menstruating.
Normally, another bacteria — lactobacilli — helps keep Prevotella bivia in check by ensuring the pH levels in woman's vagina are low, resulting in an acidic environment hostile to bugs such as Prevotella bivia and HIV.
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," Karim says. "So they are sleeping with men who have HIV infection and, in instances, they may have an organism like Prevotella bivia and we see that they now have a higher vulnerability to getting HIV."
When lactobacilli aren't around to control Prevotella bivia, they are also not around to control the bacteria Gardnerella vaginalis.
In a double blow to women, Gardnerella vaginalis also decreases the effectiveness of one of the two drugs, tenofovir, currently being used in the HIV prevention pill, Truvada, as a form of pre-exposure prophylaxis (PrEP). Studies have shown that Truvada, an antiretroviral pill that contains the ingredients tenofovir and emtricitabine, 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 that the findings will trigger other scientists working on PrEP across the world to return to their labs and review past study samples to seek out these bad bugs. The research may then affect how other future HIV studies are designed and the world's understanding of the intersect between women's biology and HIV risk.
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