Sexually Transmitted Infections

Sexually transmitted infections (STIs) are infections spread predominantly through sexual contact, including vaginal, anal and oral sex, and also by mother to child transmission, blood, and non-sexual contact for some. They affect over 1 million people daily worldwide and cause large burdens in reproductive and neonatal morbidity.

Pathogens and common curable infections:

• Bacterial: Chlamydia trachomatis (serovars D to K: urethritis, cervicitis; L1 to L3: lymphogranuloma venereum), Neisseria gonorrhoeae, Treponema pallidum (syphilis), Haemophilus ducreyi (chancroid), Klebsiella granulomatis (donovanosis, granuloma inguinale), Mycoplasma genitalium, Ureaplasma urealyticum, and Gardnerella vaginalis (bacterial vaginosis, associated with sexual activity).
• Viral: HIV, herpes simplex virus (HSV-1 and HSV-2), human papillomavirus (HPV), hepatitis B virus, hepatitis C (especially in MSM and HIV), hepatitis A (oral-anal exposure), molluscum contagiosum, cytomegalovirus, and mpox.
• Protozoal: Trichomonas vaginalis, Entamoeba histolytica, Giardia lamblia.
• Ectoparasites: Sarcoptes scabiei (scabies), Phthirus pubis (pubic lice).
• Fungal: Candida (not strictly an STI).

Syndromic classification (used in low-resource settings):

• Urethral discharge, genital ulcer disease (syphilis, herpes, chancroid, LGV, donovanosis), vaginal discharge, lower abdominal pain (pelvic inflammatory disease), scrotal swelling (epididymitis), inguinal bubo, neonatal conjunctivitis, and persistent anogenital warts.

Epidemiology and risk:

• Highest burden in adolescents and young adults (15 to 24 years). Risk is increased with multiple or new partners, unprotected sex, concurrent partnerships, sex work, men who have sex with men (MSM), injecting drug use, and previous STI. Also, social determinants (poverty, stigma, limited access to care).
• Synergy with HIV: genital ulcers and inflammatory STIs raise HIV acquisition and transmission by 2 to 5 fold. HIV infection modifies the course of syphilis, HSV, HPV.

Consequences: pelvic inflammatory disease, infertility, ectopic pregnancy, chronic pelvic pain, adverse pregnancy outcomes (stillbirth, preterm birth, congenital syphilis, neonatal infections), cancer (HPV: cervical, anal, penile, oropharyngeal; hepatitis B: hepatocellular carcinoma), neurosyphilis, and psychological harm.

Antimicrobial resistance: widespread in N. gonorrhoeae (fluoroquinolone, penicillin, tetracycline, azithromycin resistance, emerging ceftriaxone resistance), M. genitalium (macrolide resistance 40 to 60% in many regions, fluoroquinolone resistance rising), and T. pallidum (macrolide resistance).

Many STIs are asymptomatic and are found only on screening. Clinical syndromes:

• Urethral discharge and dysuria (men): gonorrhoea, chlamydia, M. genitalium, trichomonas.
• Vaginal discharge: bacterial vaginosis (thin grey, fishy odour), trichomoniasis (frothy yellow-green, strawberry cervix), candidiasis (curdy, itch), cervicitis (mucopurulent).
• Genital ulcers:
• Herpes: multiple, painful, shallow vesicles and ulcers with tender lymphadenopathy.
• Primary syphilis: single painless indurated clean-based ulcer (chancre) with non-tender regional adenopathy.
• Chancroid: painful, ragged, soft ulcers with suppurative inguinal bubo.
• LGV: transient painless papule or ulcer then tender inguinal buboes (groove sign), or proctitis in MSM.
• Donovanosis: painless, beefy-red, bleeding ulcer, without adenopathy.
• Lower abdominal pain in women (PID): pelvic pain, cervical motion tenderness, adnexal tenderness, fever, discharge.
• Epididymo-orchitis: unilateral scrotal pain and swelling, fever.
• Proctitis: anorectal pain, discharge, tenesmus, bleeding (gonorrhoea, chlamydia, LGV, syphilis, HSV).
• Pharyngitis: gonorrhoea, chlamydia (usually asymptomatic).
• Skin and mucosa: warts (HPV), molluscum, scabies (nocturnal itch), pubic lice, rash (secondary syphilis, palms and soles), mpox lesions.
• Systemic: acute HIV (mononucleosis-like illness), hepatitis, secondary and tertiary syphilis, disseminated gonococcal infection, reactive arthritis.
• Neonatal: conjunctivitis (ophthalmia neonatorum), pneumonia (chlamydia), congenital syphilis, neonatal herpes.

Red flags: pregnancy, pelvic inflammatory disease, neurological or ocular symptoms (neurosyphilis, ocular syphilis), sexual assault, children (abuse), treatment failure, and immunosuppression.

Take a non-judgemental sexual history (the "5 Ps": partners, practices, protection from STIs, past STI history, prevention of pregnancy) and examine genitals, anus, mouth, skin, lymph nodes and pelvis as indicated.

Screening recommendations

• All sexually active women under 25 and women over 25 at risk: chlamydia and gonorrhoea NAAT annually.
• MSM: at least annual (every 3 to 6 months if high risk) testing at all exposed sites (urogenital, rectal, pharyngeal) for chlamydia, gonorrhoea, syphilis, HIV.
• Everyone: HIV at least once (routine opt-out, yearly if at risk), syphilis in at-risk groups and in all pregnant women (first visit, repeated at 28 weeks and delivery in high-risk), hepatitis B (all adults once), hepatitis C (all adults once), and cervical cancer screening.

Laboratory tests

• NAAT (urine, vaginal, endocervical, urethral, rectal, pharyngeal swabs) for chlamydia, gonorrhoea, trichomonas, M. genitalium (with resistance testing). Gonococcal culture and susceptibility testing.
• Syphilis: treponemal test (EIA/CLIA, TPPA) with a non-treponemal test (RPR/VDRL) for titre and monitoring (reverse sequence algorithm possible). Darkfield microscopy or PCR of ulcer exudate in primary syphilis. CSF analysis for neurosyphilis (VDRL, cell count, protein), ocular and otic evaluation when symptomatic.
• HIV: fourth-generation antigen/antibody assay, with HIV RNA in suspected acute infection.
• Herpes: PCR of ulcer swab (preferred), culture, and type-specific serology (limited role).
• Hepatitis B (HBsAg, anti-HBc, anti-HBs) and hepatitis C (anti-HCV with reflex RNA).
• Vaginal discharge: pH, wet mount, whiff test, Gram stain (Nugent score), culture or NAAT for trichomonas and Candida.
• Genital ulcer panel: PCR for HSV, T. pallidum, H. ducreyi, and C. trachomatis (LGV serovars).
• Pregnancy test, and other tests as appropriate.

Assessment of complications

• Pelvic ultrasound for tubo-ovarian abscess, scrotal ultrasound for epididymitis versus torsion (torsion must be excluded in acute scrotum), and imaging for disseminated disease.

Principles: treat at the first visit with effective regimens (single-dose or short course), treat syndromes empirically where tests are unavailable, provide partner notification and treatment, counsel and screen for HIV and other STIs, vaccinate, and follow-up.

Chlamydia

• Doxycycline 100 mg orally twice daily for 7 days (first line), azithromycin 1 g single dose (pregnancy), levofloxacin 500 mg daily for 7 days (alternative). LGV: doxycycline 100 mg twice daily for 21 days.

Gonorrhoea

• Ceftriaxone 500 mg IM single dose, with doxycycline if chlamydia not excluded. Alternatives as per the urethritis article.

Syphilis

• Primary, secondary and early latent (under 1 year): benzathine penicillin G 2.4 million units IM single dose. Late latent or unknown duration: 2.4 million units IM weekly for 3 doses. Neurosyphilis, ocular and otic: aqueous benzylpenicillin 18 to 24 million units daily (3 to 4 million units every 4 hours or continuous infusion) for 10 to 14 days. Penicillin allergy: doxycycline 100 mg twice daily for 14 days (early) or 28 days (late), desensitisation in pregnancy. Warn about the Jarisch-Herxheimer reaction (fever, myalgia within 24 hours; antipyretics, prednisolone 20 to 40 mg in some cases of ocular/neurological disease). Follow-up RPR titres at 3, 6, 12 and 24 months (a fourfold fall expected within 12 months).

Genital herpes

• First episode: valaciclovir 1 g twice daily for 7 to 10 days, or aciclovir 400 mg three times daily for 7 to 10 days (200 mg five times daily alternative), or famciclovir 250 mg three times daily. Recurrences: valaciclovir 500 mg twice daily for 3 days (or 1 g once daily for 5 days), aciclovir 800 mg three times daily for 2 days, started at the prodrome. Suppression (6 or more recurrences per year): valaciclovir 500 mg to 1 g daily or aciclovir 400 mg twice daily, reviewed yearly. Pregnancy: aciclovir or valaciclovir from 36 weeks, and caesarean delivery for lesions at labour.

Chancroid: azithromycin 1 g single dose, ceftriaxone 250 mg IM single dose, ciprofloxacin 500 mg twice daily for 3 days, or erythromycin 500 mg three times daily for 7 days. Aspirate buboes.

Donovanosis: azithromycin 1 g weekly or 500 mg daily for at least 3 weeks until healed. Doxycycline 100 mg twice daily is an alternative.

Trichomoniasis, bacterial vaginosis, candidiasis, M. genitalium, pelvic inflammatory disease:

• Trichomonas: metronidazole 500 mg twice daily for 7 days (women), 2 g single dose (men).
• Bacterial vaginosis: metronidazole 500 mg twice daily for 7 days or clindamycin cream.
• Candidiasis: fluconazole 150 mg single dose.
• M. genitalium: sequential doxycycline then moxifloxacin or azithromycin, guided by resistance testing.
• PID: ceftriaxone 500 mg IM single dose plus doxycycline 100 mg twice daily plus metronidazole 400 to 500 mg twice daily for 14 days. Severe: IV ceftriaxone 1 to 2 g daily with doxycycline and metronidazole.

Viral and ectoparasite infections

• HPV warts and cancer prevention: see anogenital HPV infection.
• Hepatitis B: vaccination, treatment with tenofovir or entecavir for chronic active disease.
• Hepatitis C: direct-acting antivirals (sofosbuvir/velpatasvir 400/100 mg daily for 12 weeks).
• Scabies: permethrin 5% cream applied from neck down, washed after 8 to 14 hours, repeated in 7 days, or ivermectin 200 micrograms/kg orally, repeated in 7 days. Pubic lice: permethrin 1% or malathion 0.5%, repeated in 7 days. Treat contacts and wash bed linen.
• Mpox: supportive care, tecovirimat for severe disease, and vaccination (Jynneos).

Prevention

• Behavioural: abstinence, mutual monogamy, reduction in partners, delay of sexual debut, correct and consistent condom use (reduces HIV, gonorrhoea, chlamydia and HSV by 50 to 90%), dental dams.
• Vaccination: HPV, hepatitis B, hepatitis A (MSM), meningococcal B (partial gonorrhoea protection), mpox.
• Biomedical: HIV pre-exposure prophylaxis (tenofovir disoproxil/emtricitabine 300/200 mg daily, or on-demand 2-1-1 for MSM, or long-acting injectables: cabotegravir 600 mg IM at months 0, 1 and then every 2 months, lenacapavir 927 mg SC every 6 months), post-exposure prophylaxis (tenofovir/emtricitabine plus dolutegravir 50 mg daily for 28 days, started within 72 hours), doxycycline post-exposure prophylaxis (200 mg within 72 hours), treatment as prevention (undetectable equals untransmittable), and male circumcision for HIV prevention.
• Screening programmes, expedited partner therapy, and partner notification.

Special populations

• Pregnancy: screening and treatment for syphilis, chlamydia, gonorrhoea, HIV, hepatitis B, HSV, and bacterial vaginosis, with infant prophylaxis (ophthalmia neonatorum: erythromycin ointment).
• Adolescents: confidential care, consent considerations, and abuse screening. Sexual assault: post-exposure prophylaxis, emergency contraception, and forensic care.
• Sex workers and key populations: community-based services.

Follow-up and counselling

• Retest for chlamydia and gonorrhoea at 3 months, test of cure for gonorrhoea at pharyngeal sites and in pregnancy, repeat syphilis serology, and HIV retest at 3 months after window period. Promote disclosure, reduce stigma, and provide psychological support.