Bacterial Vaginosis

Introduction

Bacterial vaginosis (BV) is the most common cause of abnormal vaginal discharge in women of reproductive age. It results from a disturbance of the normal vaginal microbiome: a fall in the population of protective, lactic-acid-producing lactobacilli allows overgrowth of anaerobic organisms, most notably Gardnerella vaginalis, alongside Mycoplasma hominis, Prevotella and Atopobium species.¹

Unlike chlamydia or gonorrhoea, BV is not classed as a sexually transmitted infection - it is an imbalance of a woman's own vaginal flora rather than an infection acquired from a partner - though it is more common in sexually active women and shares some behavioural associations, which sometimes causes confusion in how it is discussed and managed.

Pathophysiology

The healthy vagina is dominated by lactobacilli, which metabolise glycogen in vaginal epithelial cells to produce lactic acid, maintaining an acidic vaginal pH (typically below 4.5) that suppresses the growth of other organisms.¹ Anything that reduces the lactobacilli population - reduced oestrogen, vaginal douching, receptive oral sex (semen is alkaline), certain hygiene products - allows the vaginal pH to rise, creating conditions favourable for anaerobic overgrowth.

Gardnerella vaginalis forms a biofilm on the vaginal epithelium that further displaces lactobacilli and provides a scaffold for other anaerobes, which is thought to explain why BV can be difficult to eradicate fully and why recurrence is common even after apparently effective treatment.

Risk factors

• Vaginal douching or use of vaginal deodorants/scented products
• Recent change of sexual partner, or multiple sexual partners
• Receptive oral sex
• Presence of an intrauterine device (IUD)
• Black ethnicity (higher reported prevalence, reasons not fully understood)
• Smoking
• Absence of these risk factors does not exclude BV; some women develop it with none of them present

Notably, BV is less common in women taking the combined oral contraceptive pill and in circumcised male partners, and condom use appears somewhat protective, likely by reducing exposure to semen's alkalinising effect.

Clinical features

The classic symptom is a thin, white or grey, homogeneous vaginal discharge with an offensive 'fishy' odour, often more noticeable after sexual intercourse (semen further raises vaginal pH, accentuating the smell) or during menstruation.² Crucially, BV is characteristically not associated with itch, soreness or vulval inflammation - the presence of these features should raise suspicion of candidiasis or another cause instead.

Up to half of affected women are asymptomatic, with BV identified incidentally on a swab taken for another reason. This is an important point given that treatment decisions hinge on symptom status.

Diagnosis

Amsel's criteria

Clinical diagnosis is made using Amsel's criteria - at least 3 of the following 4 must be present:³

1. Thin, white/grey, homogeneous vaginal discharge
2. Vaginal pH greater than 4.5 (tested with pH paper against the lateral vaginal wall)
3. Positive 'whiff test' - a fishy odour released on adding potassium hydroxide (KOH) to a sample of discharge
4. Clue cells seen on microscopy of a wet mount or Gram stain

Clue cells are vaginal epithelial cells so heavily coated with adherent bacteria that their normally sharp cell border becomes stippled and indistinct - they are the single most specific microscopic finding for BV.

Hay/Ison criteria

An alternative, laboratory-based method that grades a Gram-stained vaginal smear according to the relative proportion of lactobacilli versus Gardnerella/anaerobe morphology, often used in genitourinary medicine clinics and reported as a grade (I to III).

Differential diagnosis

• Candidiasis (thrush): itchy, thick, curd-like discharge; normal/low pH; no fishy odour
• Trichomonas vaginalis: frothy, yellow-green, offensive discharge; strawberry cervix; raised pH (can overlap with BV features)
• Chlamydia/gonorrhoea: may cause discharge but typically with other features (intermenstrual bleeding, pelvic pain); test if any risk of STI
• Physiological discharge: clear/white, non-offensive, varies with cycle

Management

Treatment is indicated for symptomatic women, and for asymptomatic women before certain gynaecological procedures (e.g. surgical termination of pregnancy, IUD insertion, hysterectomy) because of an increased risk of post-procedure infection. Asymptomatic BV found incidentally outside these situations does not routinely require treatment.²

Partner treatment is not routinely recommended, as BV is not classed as an STI and treating male partners has not been shown to reduce recurrence. This is a frequently tested contrast with the management of chlamydia, gonorrhoea and trichomoniasis.

Recurrent BV

Recurrence is common. Management includes reviewing and addressing modifiable precipitants (stopping douching, scented products, and considering whether an IUD is contributing), and, for frequent recurrence, longer or maintenance courses of topical metronidazole under specialist guidance.

Bacterial vaginosis in pregnancy

BV in pregnancy is associated with an increased risk of late miscarriage, preterm birth, preterm prelabour rupture of membranes, and postpartum endometritis, thought to relate to ascending infection and local inflammatory changes at the cervix.⁴

Symptomatic BV in pregnancy is treated with oral metronidazole in the same way as outside pregnancy - it is not teratogenic at standard doses used for this indication. Routine screening for and treatment of asymptomatic BV in unselected pregnant women is not recommended, as trial evidence has not shown a clear benefit in reducing preterm birth in this group; management should be individualised, particularly in women with a previous preterm birth.

Complications

• Increased susceptibility to sexually transmitted infections, including HIV, due to disruption of the normal protective vaginal flora
• Increased risk of post-operative infection after gynaecological procedures if untreated beforehand
• Pregnancy complications as above
• Psychological and relationship impact of recurrent or persistent symptoms

Prognosis

BV responds well to first-line antibiotic treatment in most women, with cure rates generally high in the short term. However, recurrence within 12 months is common, reflecting how readily the vaginal microbiome can shift back once lactobacilli dominance is lost, and repeated or chronic recurrence may need a longer-term, individualised approach with specialist input.