Why Bacterial Vaginosis Keeps Returning: Two Causes Found

Australian scientists have identified two distinct reasons bacterial vaginosis (BV) so often returns: bacteria surviving standard antibiotics and reinfection after initial clearance. The findings come from a study of 188 couples published in The Lancet Obstetrics, Gynaecology & Women's Health The Guardian.

Study lead author Dr Lenka Vodstrcil is a senior research fellow at Monash University and Bayside Health's Melbourne Sexual Health Centre. Study senior author Prof Catriona Bradshaw is head of research translation and mentorship at Monash University and the Melbourne Sexual Health Centre.

BV affects about one in four women globally. It is a polymicrobial syndrome, meaning many bacterial species are involved at once, with the vaginal microbiota — the community of microbes in the vagina — shifting from an optimal, healthy balance to a non-optimal one. It is the most common cause of vaginal discharge with a foul odor. It increases the risk of HIV acquisition and other sexually transmitted infections. It also increases the risk of adverse birth outcomes, and it can cause infertility, premature births and newborn deaths.

Why it keeps coming back

About two-thirds of women returned to an optimal vaginal microbiome the day after finishing treatment. The other third had microbiome patterns suggesting they may not have cleared BV. That group was nearly three times more likely to develop the condition again within four weeks.

The new study named persistence despite antibiotics and reinfection as two distinct pathways to recurrence. Thirty percent of female participants used an intrauterine device (IUD), a small device placed in the uterus to prevent pregnancy, which the study found could enhance persistence following treatment in some women.

In the study, women were given a weeklong course of oral antibiotics while men were treated with both oral and topical antimicrobials, meaning drugs taken by mouth plus drugs applied to the skin.

How the trials worked

Earlier research by the same group found that concurrently treating women and their male sexual partners with antibiotics reduced BV recurrence rates by more than 60%. That earlier work was a 164-couple trial in monogamous relationships, published in the New England Journal of Medicine Monash University. Couples took antibiotics for one week and were followed up for 12 weeks. The trial was stopped early after recurrence was halved in the partner-treatment group compared with treating women alone.

In the 164-couple trial, male partners in the treatment group received both an oral antibiotic and a topical antibiotic cream for one week. The control group received female-only antibiotic treatment, which is the globally recommended practice. No previous male-partner treatment trials had used a combination of oral and topical antibiotics to clear BV-associated bacteria in men, especially from the penile-skin site.

In the male-partner treatment study, 34.7% of women in the both-partners-treated group experienced BV recurrence compared with 63.2% of women in the comparison group. More than 50% of women have recurrent BV within three months after usual treatment with oral antibiotics. Post-treatment recurrence is common, with more than 50% of women experiencing recurrence within 3 to 12 months.

The 2020 trial protocol led by L.A. Vodstrcil aimed to determine the effect of concurrent male partner treatment for preventing BV recurrence compared with standard care. The partner-treatment study reported the first conclusive evidence that BV is sexually transmissible and that reinfection from sexual partners is a major driver of recurrence.

Men can harbor BV-associated bacterial species on penile skin and inside the penis. BV has a similar incubation period after sex — the time between exposure and symptoms — to most sexually transmitted infections. Change in sexual partner and not using condoms are risk factors, similar to chlamydia. Melbourne Sexual Health Centre treatment guidelines state post-treatment recurrence is associated with exposure to an ongoing sex partner and lack of condom use for penile-vaginal sex.

The U.S. Centers for Disease Control and Prevention states that BV most often occurs in those who are sexually active CDC. The agency also states that researchers do not know the cause of BV. A 2016 study by J.E. Bilardi and colleagues conducted semi-structured interviews — guided but open-ended conversations — with 35 women of varying sexual orientation who had experienced recurrent BV in the past 5 years. Women with recurrent disease often felt confused about why they were experiencing recurrence and frustrated at their lack of control over recurrence.

The scientists now plan to trial longer treatment lengths and combinations of multiple antibiotics for BV.

What could change

The broader context here is a shift in clinical logic. For decades, female-only therapy was standard even as epidemiological signals pointed to sexual transmission. Partner treatment closes that gap by addressing a reservoir that standard care left untouched. Persistence points to a separate pharmacologic problem, inadequate clearance in a subset of women, potentially compounded by IUD presence.

Looking at what this means for practice, two questions will shape guidelines. First, whether dual-route therapy for male partners, oral plus topical to cover cutaneous and urethral sites, becomes routine for penile-vaginal partnerships. Second, whether initial therapy for women needs stratification, with longer duration or multi-agent regimens for those with microbiome signatures of incomplete clearance at test-of-cure. Both carry antimicrobial stewardship trade-offs that expert panels will need to weigh against the high baseline recurrence rate and the reproductive effects documented in the literature.