Understanding Bacterial Vaginosis
Bacterial vaginosis (BV) is the most common cause of abnormal vaginal discharge in women of reproductive age, affecting approximately 30% of women at some point in their lives. Despite its prevalence, BV remains poorly understood by many patients and is often confused with other vaginal conditions such as thrush (candidiasis) or sexually transmitted infections. BV is not classified as a sexually transmitted infection, though sexual activity is associated with increased risk. The condition results from disruption of the normal vaginal microbiome, specifically a reduction in protective hydrogen peroxide-producing lactobacilli and corresponding overgrowth of anaerobic bacteria including Gardnerella vaginalis, Prevotella species, Mobiluncus species, and Atopobium vaginae.
The healthy vagina maintains an acidic pH (3.8-4.5) through lactobacilli production of lactic acid, which inhibits growth of potentially pathogenic bacteria. In bacterial vaginosis, this protective mechanism fails. Lactobacilli populations decline dramatically, vaginal pH rises above 4.5, and anaerobic bacteria proliferate, producing malodorous amines responsible for the characteristic fishy smell associated with BV. Whilst approximately 50% of women with BV remain asymptomatic, the condition increases susceptibility to sexually transmitted infections including HIV, chlamydia, gonorrhoea, and herpes simplex virus. BV also increases risk of pelvic inflammatory disease, post-surgical infections following gynaecological procedures, and pregnancy complications.
Clinical Presentation and Symptoms
When symptomatic, bacterial vaginosis most commonly presents with thin, homogeneous, grey-white vaginal discharge adherent to the vaginal walls. The discharge has a characteristic fishy or musty odour caused by volatile amines (putrescine, cadaverine, trimethylamine) produced by anaerobic bacterial metabolism. This odour intensifies after sexual intercourse when alkaline seminal fluid raises vaginal pH, volatilising the amines. Unlike vaginal thrush, BV rarely causes vulval itching, irritation, or inflammation. Some women experience mild dysuria (discomfort during urination) when urine contacts affected vaginal tissues.
Importantly, approximately half of women with laboratory-confirmed bacterial vaginosis report no symptoms whatsoever. Asymptomatic BV is typically detected incidentally during screening for other conditions or routine gynaecological examination. The clinical significance of asymptomatic BV remains debated. Most guidelines recommend treating symptomatic BV but suggest selective treatment of asymptomatic BV only in specific circumstances, particularly pregnant women at risk of preterm delivery or women undergoing gynaecological procedures where BV might increase infection risk.
Diagnostic Methods and Criteria
Bacterial vaginosis diagnosis relies on microscopic examination and pH testing of vaginal fluid. The Amsel criteria remain widely used, requiring three of four findings: thin homogeneous discharge, vaginal pH above 4.5, positive amine ("whiff") test (fishy odour when potassium hydroxide is added to vaginal fluid), and presence of clue cells on microscopy. Clue cells are vaginal epithelial cells with obscured borders due to adherent bacteria, pathognomonic for BV. The Amsel criteria provide rapid point-of-care diagnosis when appropriate equipment and expertise are available.
Gram staining of vaginal smears with Nugent scoring represents the laboratory gold standard for BV diagnosis. This method quantifies the ratio of lactobacilli (large Gram-positive rods) to Gardnerella and other BV-associated bacteria (small Gram-variable coccobacilli and curved Gram-negative rods). Nugent scores range from 0-10: scores 0-3 indicate normal flora, 4-6 indicate intermediate flora, and 7-10 indicate bacterial vaginosis. Gram staining enables standardised diagnosis independent of clinical symptoms and is particularly valuable for research and quality assurance. Some laboratories also employ molecular methods such as PCR testing for specific BV-associated bacteria.
Risk Factors and Causes
Multiple factors increase bacterial vaginosis risk, though causative mechanisms remain incompletely understood. Sexual activity represents the strongest risk factor—BV is more common in sexually active women and increases with number of sexual partners, new sexual partners, and female sexual partners. However, BV also occurs in women who have never been sexually active, and male partner treatment does not prevent BV recurrence, supporting classification of BV as non-sexually transmitted vaginal dysbiosis rather than a classic STI. Vaginal practices that disrupt normal flora increase BV risk, particularly vaginal douching, which removes protective lactobacilli and alters vaginal pH.
Use of intrauterine contraceptive devices (IUDs) associates with increased BV prevalence, possibly by providing surfaces for bacterial biofilm formation. Hormonal factors influence BV risk—the condition is most common during reproductive years, less frequent after menopause, and affected by menstrual cycle phase. Recent antibiotic use can precipitate BV by eliminating protective vaginal lactobacilli. Smoking increases BV risk through mechanisms that remain unclear. Conversely, hormonal contraceptive use (particularly oestrogen-containing methods) appears protective, likely by maintaining stable oestrogen levels that support lactobacilli growth. Genetic and ethnic factors also play roles, with BV being more prevalent in women of African descent.
Treatment Options and Management
First-line bacterial vaginosis treatment employs antibiotics effective against anaerobic bacteria, specifically metronidazole or clindamycin. Metronidazole is typically prescribed as 400-500mg oral tablets twice daily for 5-7 days, or as 0.75% vaginal gel applied once daily for 5 days. Alternatively, clindamycin 2% vaginal cream applied at night for 7 days provides effective treatment. Cure rates for first-episode BV exceed 80-90% with appropriate antibiotic treatment. Patients taking oral metronidazole must abstain from alcohol during treatment and for 48 hours afterwards to avoid severe disulfiram-like reactions (nausea, vomiting, headache, flushing).
Recurrent bacterial vaginosis represents a significant management challenge, affecting 50-70% of women within one year of successful treatment. Recurrence mechanisms remain poorly understood but likely involve failure to re-establish protective lactobacilli dominance after antibiotic therapy. Management of recurrent BV may involve extended antibiotic courses, suppressive metronidazole gel therapy (twice weekly for 4-6 months), or attempts to restore vaginal lactobacilli through probiotic supplementation, though evidence for probiotic efficacy remains limited. Sexual partners do not routinely require treatment, as partner therapy does not reduce recurrence rates. Women with recurrent BV should avoid douching and consider discontinuing IUDs if alternative contraception is acceptable.
BV in Pregnancy and Complications
Bacterial vaginosis during pregnancy associates with increased risk of adverse outcomes including late miscarriage (second-trimester pregnancy loss), preterm labour and delivery, premature rupture of membranes, low birth weight, and postpartum endometritis. The mechanisms involve ascending infection from the vagina to the uterus, with BV-associated bacteria triggering inflammatory responses that initiate early labour. BV also increases mother-to-child transmission risk for some infections. Given these risks, pregnant women with symptomatic BV require prompt treatment with pregnancy-safe antibiotics—oral metronidazole or clindamycin after the first trimester.
Screening and treatment of asymptomatic BV in pregnancy remains controversial. Current UK guidelines do not recommend universal BV screening in low-risk pregnant women, as trials have not demonstrated that treating asymptomatic BV prevents preterm birth in the general pregnant population. However, screening and treatment may benefit high-risk women with previous preterm delivery, particularly if that pregnancy was complicated by BV. Women planning gynaecological surgery (hysterectomy, abortion, IUD insertion) should be screened for BV and treated if positive, as BV increases post-procedural infection risk. Testing for BV should also be considered in women with recurrent or unexplained urinary tract infections.