Educational draft · awaiting clinical reviewThe full textbook explains uncertainty but does not replace live national or local guidance, specialist advice, or current prescribing information.
Bacterial vaginosis, candidiasis and trichomoniasis
Infectious diseases, microbiology and sexual health · UK scope · Sources checked 27 Aug 2026 · Clinical review pending
Action: Use ABCDE when unwell, perform pregnancy testing and urgent pelvic assessment, obtain STI and microbiology samples without delaying treatment, and involve gynaecology, maternity or critical care for pelvic inflammatory disease, obstetric complication or invasive infection.
Overview
Distinguish the common causes of vaginal discharge by symptoms, examination and point-of-care or laboratory testing, then use diagnosis-specific treatment, pregnancy precautions and partner management.
Bacterial vaginosis is vaginal dysbiosis with loss of hydrogen-peroxide-producing lactobacilli and overgrowth of anaerobic organisms. It is not classified simply as an STI, although sexual activity influences recurrence and associated STI risk.
Vulvovaginal candidiasis is inflammatory overgrowth of Candida, usually C albicans, after disruption by antibiotics, pregnancy, diabetes, immune factors or local irritation. Asymptomatic Candida colonisation does not require treatment.
Trichomoniasis is a sexually transmitted infection caused by the flagellated protozoan Trichomonas vaginalis. Untreated partners readily reinfect one another and infection is associated with other STIs and adverse pregnancy outcomes.
Diagnosis should explain the whole presentation. Odour, itch, discharge colour and microscopy patterns help, but pelvic pain, ulcers, urinary symptoms, retained foreign body, dermatitis and malignancy require separate assessment.
Key points
Bacterial vaginosis typically causes thin homogeneous grey-white discharge and a fishy odour with little vulval inflammation.
Practice
Two practice questions
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Question 1 of 2
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Infectious diseases, microbiology and sexual healthOriginal SBA
Pregnant patient with candidiasis
A pregnant adult has intense vulval itch, erythema and microscopy-confirmed Candida without pelvic pain or systemic illness. Which treatment is most appropriate?
Sources and review status4 sources · checked 27 Aug 2026 · clinical review pending+
Sources
Sources and review status
National guidance is shown before implementation-dependent detail. Typical adult dose examples remain subject to patient factors, contraindications and the live BNF or specialist protocol. Source check completed 27 Aug 2026; clinical approval remains outstanding.
Vulvovaginal candidiasis usually causes intense itch, soreness, erythema and thick white discharge; vaginal pH generally remains normal.
Trichomoniasis may cause offensive yellow-green or frothy discharge, vulval irritation and punctate cervical inflammation, but many infections are asymptomatic.
History and examination guide testing; symptoms alone are insufficient because mixed infection and non-infective vulval disease are common.
Use vaginal pH, microscopy or validated nucleic-acid testing according to the syndrome and local sexual-health pathway.
Treat bacterial vaginosis and trichomoniasis with a recommended nitroimidazole regimen, but trichomoniasis also requires partner treatment and sexual abstinence until completion.
Treat uncomplicated candidiasis with a topical azole or oral fluconazole; in pregnancy use topical azole therapy and avoid oral fluconazole.
Do not routinely treat asymptomatic bacterial-vaginosis partners or candidiasis partners unless they have symptoms.
Aetiology
01
Bacterial-vaginosis dysbiosis
Loss of protective lactobacilli permits Gardnerella-associated polymicrobial anaerobic biofilm and production of volatile amines that cause odour.
02
Candida overgrowth
Antibiotics, oestrogen exposure, diabetes, immune impairment and local disruption allow normally colonising yeast to provoke vulvovaginal inflammation.
03
Trichomonas transmission
Motile T vaginalis trophozoites pass through genital sexual contact and persist without a resistant environmental cyst phase.
04
Mixed disease
Concurrent dysbiosis, Candida and classical STIs can coexist, so one positive result may not explain every symptom.
Pathophysiology
1
Lactobacillus depletion
Reduced lactic-acid production raises vaginal pH and allows anaerobic organisms to adhere within a polymicrobial epithelial biofilm.
2
Candida inflammatory response
Yeast transition and epithelial interaction recruit neutrophils, producing itch, oedema, erythema, fissures and discharge despite infection remaining superficial.
3
Protozoal epithelial injury
T vaginalis adheres to genital epithelium, causes local cytotoxic inflammation and increases discharge and mucosal susceptibility.
4
Ascending inflammatory risk
Associated cervical infection can extend into upper genital tract, explaining why pain and cervical tenderness change the diagnosis.
Clinical features and red flags
Bacterial-vaginosis pattern
Thin smooth discharge coating the vaginal walls with fishy odour and minimal soreness favours bacterial vaginosis.
Candidal inflammation
Marked itch, vulval erythema, fissuring, soreness and thick curdy discharge favour candidiasis, although discharge may be absent.
Trichomonas syndrome
Frothy yellow-green discharge, offensive odour, dysuria, dyspareunia and a punctate cervix support trichomoniasis.
Upper-genital-tract featuresRed flag
Deep dyspareunia, intermenstrual bleeding, pelvic pain, fever or cervical excitation requires assessment for PID.
Complicated candidiasis
Recurrent episodes, severe oedema or fissuring, pregnancy, immunosuppression, uncontrolled diabetes or non-albicans species changes management.
Red flags requiring action
Pelvic pain, cervical motion tenderness or adnexal tenderness suggests pelvic inflammatory disease rather than isolated vaginitis.
Fever, hypotension, severe vulval pain, necrosis or rapidly spreading erythema requires emergency assessment for invasive infection.
Pregnancy with pain, bleeding, contractions or reduced fetal movements needs same-day maternity assessment.
Persistent vulval ulceration, bleeding, skin change or a mass requires examination and possible biopsy rather than repeated empirical thrush treatment.
Investigations
01Vaginal pHFirst step
Why
Differentiate normal-pH candidiasis from pH elevation in bacterial vaginosis or trichomoniasis.
Interpretation and limitations
A pH above 4.5 supports BV or trichomoniasis but is not diagnostic; semen, blood and cervical mucus can raise pH.
02Microscopy and Gram stain
Why
Identify clue cells, reduced lactobacilli, yeast forms, inflammation and motile trichomonads when facilities permit.
Interpretation and limitations
Nugent scoring provides a laboratory reference for BV; wet-mount sensitivity for trichomonas is limited, so a negative result may need NAAT.
03Trichomonas NAAT
Why
Detect T vaginalis sensitively from a vaginal swab or other locally validated specimen.
Interpretation and limitations
NAAT is preferred when available, especially after a negative wet mount with persistent suspicion; test other exposed sites only through validated pathways.
04Candida culture or speciation
Why
Confirm recurrent, severe, treatment-resistant or non-albicans candidiasis.
Interpretation and limitations
Growth alone can reflect colonisation; interpret with symptoms and inflammation and obtain susceptibility advice when repeated therapy fails.
05Chlamydia and gonorrhoea NAAT
Why
Differential diagnosis
01
Cervicitis and PID
Chlamydia, gonorrhoea or Mycoplasma genitalium causes cervical discharge, bleeding and pelvic pain rather than isolated vaginitis.
02
Retained foreign body
A tampon or other vaginal material produces offensive discharge and bleeding and requires careful examination and removal.
03
Vulval dermatosis
Contact dermatitis, eczema, lichen sclerosus and psoriasis cause itch and soreness with characteristic external skin change.
04
Genital ulcer infection
Herpes and syphilis produce erosions or ulcers that can resemble severe candidal fissuring; lesion PCR and syphilis testing distinguish them.
05
Malignancy
Persistent watery or blood-stained discharge, ulceration or a mass requires cervical, vaginal or vulval cancer assessment.
Additional chapter-specific clues
Mixed or alternative disease
Overlapping odour and itch, cervical discharge, ulcers or recurrent symptoms can represent mixed infection, STI or dermatosis.
Management
01ASSESSDefine the discharge syndromeFirst stepA patient presents with new vaginal discharge, odour, itch or vulval soreness.+
1Ask onset, discharge, odour, itch, pain, bleeding, urinary symptoms, pregnancy possibility, antibiotics, diabetes, products and sexual exposures.
2Examine vulva and use speculum assessment when clinically appropriate, checking cervix, retained foreign body, ulcers and upper-tract tenderness.
3Perform pH and microscopy or send vaginal NAAT and culture according to local capability and recurrence status.
4Offer site-appropriate STI testing and HIV or syphilis screening according to exposure rather than assuming all discharge is candidiasis.
1Give metronidazole 400 mg orally twice daily for five to seven days using the current local sexual-health formulary.
Complications
01
Recurrent symptoms
Persistent biofilm, repeated exposure or an incorrect initial diagnosis causes repeated discharge and substantial quality-of-life burden.
02
Pelvic infection association
BV and trichomoniasis correlate with acquisition of other STIs and adverse reproductive-tract outcomes during ongoing exposure.
03
Pregnancy morbidity
Symptomatic genital infection is associated with membrane, preterm and postpartum complications, so diagnosis and treatment must follow a pregnancy-specific pathway.
04
Candida fissuring
Severe inflammation causes painful erosions, urinary stinging and secondary skin breakdown without true genital ulcer disease.
05
Ongoing transmission
Untreated trichomonas partners maintain asymptomatic infection and repeatedly reinfect treated patients, so simultaneous partner treatment is essential to interrupt transmission.
Monitoring and follow-up
Review symptom resolution and check that pelvic pain, fever, bleeding or pregnancy complications have not developed.
For trichomoniasis confirm partner treatment and abstinence through treatment and arrange repeat testing according to local guidance.
For recurrent candidiasis document laboratory species, induction completion, maintenance adherence and modifiable diabetes or antibiotic factors.
Reassess BV recurrence without repeatedly treating unconfirmed odour; exclude retained foreign body and STI.
Monitor nitroimidazole or azole intolerance, hepatic symptoms, neuropathy and clinically important interactions.
Use maternity follow-up for symptomatic infection in pregnancy and any preterm-birth or membrane concerns.
Provide clear return advice for persistent ulceration, vulval skin change, pain, bleeding or systemic illness.
Special situations
pH separates imperfectly
Candidiasis usually preserves acidic vaginal pH, whereas BV and trichomoniasis often raise it above 4.5.
Candida can colonise
Finding yeast without itch, soreness or inflammation does not automatically establish symptomatic vulvovaginal candidiasis.
Trichomonas needs systemic therapy
The organism occupies urethral and vaginal sites that topical metronidazole does not reliably eradicate.
BV is dysbiosis
Sex influences recurrence, but routine treatment of male partners has not formed standard UK management.
Pregnancy changes azole choice
Topical azoles are used for longer courses, while oral fluconazole is avoided because of fetal safety concerns.
Repeated thrush labels harm
Dermatitis, lichen sclerosus, vulvodynia and neoplasia may be missed when every recurrent itch receives empirical antifungal treatment.
Common pitfalls
01
Do not diagnose the cause of discharge from colour or odour alone.
02
Do not use topical metronidazole to treat trichomoniasis.
03
Do not prescribe oral fluconazole during pregnancy.
04
Do not treat asymptomatic Candida growth without a compatible syndrome.
05
Do not omit partner treatment and wider STI testing after trichomoniasis.
06
Do not miss PID, retained foreign body or vulval dermatosis in persistent symptoms.
Recurrent candidiasis, severe infection or unusual species should prompt diabetes, immune and microbiological review.
Frothy discharge with urinary symptoms, exposure risk or a new partner warrants testing for trichomonas and other STIs.
Identify concurrent STI when exposure, cervical discharge, age or symptoms raise probability.
Interpretation and limitations
Take site-specific samples before antibiotics and obtain gonococcal culture if gonorrhoea is suspected for susceptibility.
06Pregnancy testing
Why
Guide differential diagnosis, medication selection and maternity referral.
Interpretation and limitations
A positive result makes oral fluconazole unsuitable and changes the urgency of pain, bleeding and pelvic infection.
07Glucose and immune assessment
Why
Investigate selected recurrent or severe candidiasis rather than every first episode.
Interpretation and limitations
Check diabetes, HIV or immunosuppressive treatment when history supports it; routine broad screening is unnecessary after one uncomplicated episode.
2Offer intravaginal metronidazole or clindamycin when an oral course is unsuitable and check product-specific pregnancy and latex precautions.
3Explain that routine partner treatment is not recommended and advise review if symptoms recur or pelvic pain develops.
4In pregnancy treat symptomatic disease through maternity or sexual-health guidance and avoid unsupported screening or treatment of asymptomatic low-risk patients.
03CANDIDATreat symptomatic candidiasisItch and vulval inflammation with microscopy, culture or a convincing uncomplicated syndrome supports Candida.+
1Use clotrimazole 500 mg intravaginally once or fluconazole 150 mg orally once for a suitable non-pregnant adult.
2In pregnancy use a topical azole for seven days and do not prescribe oral fluconazole.
3For recurrent disease confirm Candida and species before a longer induction and suppressive regimen and address diabetes or antibiotic drivers.
4Review persistent symptoms for dermatitis, vulvodynia, non-albicans Candida or another infection rather than repeating single doses.
04TRICHTreat trichomoniasis and contactsT vaginalis NAAT, microscopy or high-probability sexual-health assessment confirms infection.+
1Give metronidazole 400 to 500 mg orally twice daily for five to seven days using local guidance.
2Arrange treatment of current sexual partners and test for chlamydia, gonorrhoea, HIV and syphilis.
3Advise no sexual contact until the patient and partners have completed therapy and symptoms have resolved.
4Retest or obtain specialist advice for persistent infection, checking reinfection and adherence before resistance-directed therapy.
Key medicines and prescribing safety4 treatments · regimens, roles and cautions+
Suppresses anaerobic overgrowth and restores a lactobacillus-dominant vaginal environment in symptomatic BV.
Metronidazole for bacterial vaginosis
Give 400 mg orally twice daily for five to seven days, or use the exact locally approved intravaginal alternative.
Review warfarin and lithium interactions, liver disease and neurological toxicity; advise following the product alcohol warning and seek pregnancy-specific guidance.
Treats local Candida overgrowth without systemic azole exposure.
Clotrimazole for candidiasis
Give a 500 mg intravaginal pessary once for uncomplicated disease; in pregnancy use a topical azole course for seven days.
Intravaginal products may damage latex condoms or diaphragms temporarily; persistent or recurrent symptoms require examination and culture.
Provides convenient systemic azole treatment when topical treatment is unsuitable or declined.
Fluconazole for candidiasis
Give 150 mg orally as a single dose for uncomplicated vulvovaginal candidiasis in a suitable non-pregnant adult.
Do not use in pregnancy; check liver disease, QT risk and interactions including warfarin, statins and some antiepileptics.
Eradicates T vaginalis and reduces symptoms and transmission.
Metronidazole for trichomoniasis
Give 400 to 500 mg orally twice daily for five to seven days and ensure partners receive an effective regimen.
Topical metronidazole is inadequate for trichomoniasis; check adherence, reinfection, pregnancy, liver disease and interacting medicines before retreatment.