Bacterial Vaginosis
Dysbiosis of vaginal flora — loss of lactobacilli with overgrowth of anaerobes; thin gray discharge with fishy odor.
Also known as: BV, bacterial vaginosis, Gardnerella, vaginal dysbiosis
Overview
Polymicrobial alteration of the vaginal microbiome characterized by loss of protective Lactobacillus species and overgrowth of anaerobic bacteria including Gardnerella vaginalis, Prevotella, Atopobium vaginae, and Mobiluncus.
Epidemiology
Most common cause of vaginal discharge in reproductive-age women; prevalence ~20-30%. Higher rates in Black and Hispanic women; up to 50% in some populations.
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Risk factors
- New or multiple sex partners
- Female sex partners
- Douching, vaginal washing
- Cigarette smoking
- Copper IUD use (small increase)
- Lack of barrier contraception
Pathophysiology
Disruption of acidic Lactobacillus-dominant microbiome → rise in vaginal pH → overgrowth of facultative and obligate anaerobes producing biofilms and volatile amines (putrescine, cadaverine, trimethylamine) responsible for the characteristic fishy odor. Not classically considered an STI but is sexually associated.
Clinical presentation
Symptoms
- Thin, gray-white homogeneous vaginal discharge
- Fishy odor, often worse after intercourse or menses
- Minimal or no pruritus or inflammation (distinguishes from candidiasis/trichomoniasis)
- ~50% of cases are asymptomatic
Signs / physical exam
- Thin gray-white discharge coating vaginal walls
- Absence of significant erythema or edema
- No friability
Classic findings
Reproductive-age woman with thin gray discharge, fishy odor after intercourse, vaginal pH >4.5, clue cells on wet mount, positive whiff test.
Differential diagnosis
- Vulvovaginal candidiasis — Thick white 'cottage cheese' discharge, intense pruritus, vulvar erythema, normal pH (<4.5); KOH shows yeast/hyphae
- Trichomoniasis — Frothy yellow-green discharge, strawberry cervix, motile trichomonads on wet mount, pH >4.5
- Cervicitis (GC/CT) — Mucopurulent cervical discharge, friability; NAAT testing
- Atrophic vaginitis — Postmenopausal, dryness, dyspareunia; pale thin mucosa
- Foreign body (retained tampon) — Malodorous discharge that resolves with removal
- Allergic/irritant vaginitis — Pruritus and erythema after exposure to soap, douche, spermicide
Diagnostic workup
Diagnostic criteria
Amsel criteria (3 of 4): (1) thin homogeneous gray-white discharge, (2) vaginal pH >4.5, (3) positive whiff test with KOH, (4) clue cells on saline wet mount. Nugent score (gram-stain based) is research/lab gold standard.
Labs
- Vaginal pH (normal 3.8-4.5; BV >4.5)
- Wet mount: clue cells (epithelial cells coated with bacteria obscuring borders); <20% lactobacilli; absence of WBCs
- Whiff test: 10% KOH added to discharge produces fishy amine odor
- Optional commercial molecular tests (BD Affirm, NuSwab) for high-throughput diagnosis
Diagnostic algorithm
| Feature | BV | Candidiasis | Trichomoniasis |
|---|---|---|---|
| Discharge | Thin gray-white, homogeneous | Thick white 'cottage cheese' | Frothy yellow-green |
| Odor | Fishy, worse after sex/menses | None | Malodorous |
| Pruritus / erythema | Minimal | Marked | Moderate |
| Vaginal pH | >4.5 | <4.5 | >4.5 |
| Wet mount | Clue cells | Hyphae/pseudohyphae (KOH) | Motile trichomonads |
| Whiff test | Positive | Negative | Often positive |
| First-line Rx | Metronidazole PO or vaginal | Fluconazole 150 mg PO × 1 | Metronidazole 500 mg PO BID × 7d |
Treatment
First-line
- Metronidazole 500 mg PO BID × 7 days
- Metronidazole 0.75% vaginal gel 5 g intravaginally daily × 5 days
- Clindamycin 2% vaginal cream 5 g intravaginally at bedtime × 7 days
Pregnancy
- Treat all symptomatic pregnant women
- Metronidazole 500 mg PO BID × 7 days OR metronidazole vaginal gel × 5 days
- Treatment may reduce some adverse pregnancy outcomes
- Screening of asymptomatic pregnant women is NOT routinely recommended
Recurrent BV (≥3 episodes/year)
- Confirm diagnosis and exclude other causes
- Extended initial course followed by suppressive metronidazole gel twice weekly × 4-6 months
- Avoid douching, fragranced products
Second-line / adjunct
- Tinidazole 2 g PO daily × 2 days or 1 g PO daily × 5 days
- Clindamycin 300 mg PO BID × 7 days
- Secnidazole 2 g PO × 1 dose
Complications
- Increased risk of acquiring/transmitting HIV and other STIs
- Increased risk of post-procedural infection (post-hysterectomy cuff cellulitis, post-abortion endometritis)
- In pregnancy: associated with preterm birth, PROM, chorioamnionitis, postpartum endometritis (causal role debated)
- Increased PID risk (modest)
PANCE pearls
- BV is NOT classified as an STI — treatment of male partners does not reduce recurrence and is not recommended.
- Counsel patients to avoid alcohol with oral metronidazole and tinidazole (disulfiram-like reaction); the strict 'no alcohol' restriction has been called into question but is still standard advice.
- Asymptomatic BV does not require treatment except before invasive gynecologic procedures (hysterectomy, abortion, IUD insertion).
- Clindamycin cream is oil-based and weakens latex condoms — use alternative contraception during and 5 days after treatment.
- Recurrence is common (>50% within 6-12 months) — counsel about realistic expectations.
References
- CDC STI Guidelines 2021 — CDC Sexually Transmitted Infections Treatment Guidelines, 2021 (MMWR Recomm Rep 2021)
- ACOG PB 215 — ACOG Practice Bulletin No. 215: Vaginitis in Nonpregnant Patients
- Amsel Criteria — Amsel R et al., Nonspecific Vaginitis: Diagnostic Criteria and Microbial and Epidemiologic Associations (Am J Med 1983)
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