Australian scientists have identified two contributors to recurrent bacterial vaginosis infections, a common sexual health condition that affects about one in four women globally.

The condition, commonly called BV, is characterised by an imbalance of naturally occurring bacteria in the vagina, in which there is a reduction of “good” bacteria and an overgrowth of other mixed bacteria.

In March last year, researchers published a landmark study which found that BV can be sexually transmitted to women by male partners.

Concurrently treating women with the condition and their male sexual partners with antibiotics reduced BV recurrence rates by more than 60%, they found.

In new research, the same group of scientists set out to understand why about 20% of women treated concurrently with their partners were not cured of BV.

The study, published in The Lancet Obstetrics, Gynaecology & Women’s Health, involved 188 couples: the women were given a weeklong course of oral antibiotics, while the men were treated with both oral and topical antimicrobials.

It identified two distinct pathways to recurring BV: persistence of the condition despite a weeklong course of antibiotics, and re-infection.

Study lead author Dr Lenka Vodstrcil, a senior research fellow at Monash University and Bayside Health’s Melbourne Sexual Health Centre, said: “For about two-thirds of women, the day after they finish treatment, they return to … an optimal vaginal microbiome”, which is characterised by an abundance of healthy Lactobacilli bacteria.

The other third of women had microbiomes suggesting “they may not have cleared their BV” before couples resumed sex after treatment, and were nearly three times more likely to get the condition again within four weeks.

A better understanding of “persistence of infection … helps us develop new strategies to cure more women from BV,” Vodstrcil said.

BV can be asymptomatic or can cause vaginal itching, burning and discomfort. Treatment is essential as the condition can increase susceptibility to other infections and is linked to complications in pregnancy and childbirth.

The study – which the researchers believe is the largest to date involving heterosexual couples receiving concurrent BV treatment – found that persistence and reinfection from partners were “distinct, but not mutually exclusive, pathways”.

Of the female participants, 30% used an intrauterine device, which the study found “could enhance the persistence of bacterial vaginosis following treatment in some women” but noted that further research was needed.

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

“Our aim is to stop women from having two, three, four courses of antibiotics throughout the year, which was happening previously,” Vodstrcil said.

Study senior author Prof Catriona Bradshaw, head of research translation and mentorship at Monash University and the Melbourne Sexual Health Centre, said: “Using partner-treatment to mitigate re-infection and more intensive strategies that target persistence of BV-bacteria, we hope to create more effective treatment approaches that can be tailored to an individual and achieve higher levels of cure.”

Though the ethnicities of participants were “representative of urban Australia”, the study noted that its results “might not be generalisable to other global populations, including those in low-income and middle-income countries and in other settings with a high burden of bacterial vaginosis”.