Chlamydia is the most common bacterial sexually transmitted infection globally — caused by Chlamydia trachomatis (serovars D-K for urogenital infection). It is transmitted through vaginal, anal and oral sex. Crucially, most people with chlamydia have NO symptoms: approximately 70-80% of women and 50% of men are asymptomatic — making testing the only way to identify and treat infection before complications develop. Despite being easily curable with antibiotics, chlamydia causes approximately 127 million new infections per year globally and is setting new surveillance records in the EU.

What are the complications if chlamydia is untreated?

Women: pelvic inflammatory disease (PID) in approximately 10-15% of untreated women — causing fallopian tube scarring, which leads to tubal factor infertility, ectopic pregnancy and chronic pelvic pain. Repeated PID episodes cause cumulative damage. Reactive arthritis (formerly Reiter's syndrome): joints, eyes, urethra. Neonates born to infected mothers: conjunctivitis (50%), pneumonia (30%). Men: epididymo-orchitis (painful swelling of epididymis and testis); rarely infertility from scarring.

How is chlamydia tested?

NAAT (nucleic acid amplification test) is the gold standard — highly sensitive (>95%) and specific (>99%). Samples: women — vaginal self-swab (most sensitive, patient-collected) or endocervical swab by clinician; men — first-void urine (first 10-20mL of urine stream — not midstream). Rectal and pharyngeal swabs for MSM and anyone with relevant exposures. Multiplex STI panels simultaneously test for chlamydia, gonorrhoea, Trichomonas and other STIs from a single sample.

How is chlamydia treated?

2024 IUSTI/BASHH guidelines: first-line: doxycycline 100mg twice daily × 7 days (preferred — higher cure rates than azithromycin, especially for rectal chlamydia where azithromycin single dose showed lower efficacy). Alternatives: azithromycin 1g single dose (if doxycycline not tolerated or in pregnancy — azithromycin safer in pregnancy). LGV (serovars L1-L3): doxycycline 100mg BD × 21 days. Partner notification: all sexual partners in the preceding 60 days must be tested and treated simultaneously. Test of cure: recommended in pregnancy and for LGV.

What is the difference between chlamydia, gonorrhoea and LGV?

All three are STIs caused by different bacteria: Chlamydia trachomatis (D-K): usually asymptomatic, causes urethritis, cervicitis, PID, conjunctivitis. Very common. Gonorrhoea (Neisseria gonorrhoeae): urethral/cervical/rectal/pharyngeal discharge; AMR crisis; much higher symptomatic rate. LGV (C. trachomatis L1-L3): invasive — causes proctitis in MSM and lymphadenopathy (buboes) in heterosexual men; a separate disease despite being the same species. All three are NAAT-diagnosed; all three require partner treatment; chlamydia and LGV have different treatment durations despite same organism.

How does GMJ News cover chlamydia?

GMJ News covers WHO chlamydia burden data, ECDC record EU STI surveillance, PID and infertility complications, doxycycline 2024 treatment guidelines, LGV in MSM outbreaks, neonatal chlamydia, and partner notification. Chlamydia — the most prevalent bacterial STI — is a core topic for PHIG's STI surveillance programme and the Georgian Medical Journal.

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Chlamydia

GMJ News knowledge hub · last reviewed September 2026 · Georgian Medical Journal

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Chlamydia — caused by Chlamydia trachomatis — is the most common bacterial sexually transmitted infection globally with an estimated 127 million new cases per year (WHO 2020), setting new surveillance records in the EU in 2023-2024 (ECDC: chlamydia cases among the highest ever recorded) and causing disproportionate reproductive harm because approximately 70-80% of infections in women and 50% in men are entirely asymptomatic — enabling silent transmission and progression to pelvic inflammatory disease, ectopic pregnancy and tubal factor infertility (WHO). The emergence of lymphogranuloma venereum (LGV — serovars L1-L3) causing severe proctitis outbreaks in MSM across Europe and North America is a distinct and increasingly recognised complication that requires extended antibiotic treatment.

Key messages

127M new cases/year — most common bacterial STI

Chlamydia trachomatis causes approximately 127 million new cases per year globally — the most common bacterial sexually transmitted infection and one of the most prevalent infections of any kind. EU STI cases reached record highs in 2023-2024 (ECDC).

Mostly silent — 70-80% of women asymptomatic

Approximately 70-80% of women and 50% of men with chlamydia have no symptoms — enabling silent spread and untreated progression to pelvic inflammatory disease, ectopic pregnancy and infertility.

PID and infertility — the dangerous consequences

Ascending chlamydial infection causes pelvic inflammatory disease (PID) — affecting the fallopian tubes — in approximately 10-15% of untreated women. Each PID episode increases tubal infertility risk by 10-20% and ectopic pregnancy risk 6-10 fold.

Doxycycline 7 days — the 2024 first-line treatment

IUSTI/BASHH 2024 guidelines now recommend doxycycline 100mg BD × 7 days as first-line — replacing azithromycin single dose (which had lower cure rates for rectal chlamydia). Partner notification and testing are essential.

LGV — a distinct and more severe form

Lymphogranuloma venereum (LGV — serovars L1-L3) causes severe proctitis in MSM across Europe and North America, presenting as bloody diarrhoeal illness mimicking inflammatory bowel disease. Treatment: doxycycline 100mg BD × 21 days.

Neonatal infection — conjunctivitis and pneumonia

Vertical transmission of C. trachomatis from an infected mother during birth causes neonatal ophthalmia (conjunctivitis) in 20-50% of exposed neonates and neonatal pneumonia in 5-30%. Antenatal screening and treatment prevents vertical transmission.

Key statistics

127M

new chlamydia cases/year globally (WHO 2020)

WHO 2020

Record

EU chlamydia and STI cases in 2023-2024 (ECDC)

ECDC 2024

70-80%

of infected women are asymptomatic

WHO

10-15%

of untreated women develop PID

WHO

7 days

doxycycline 100mg BD — 2024 IUSTI/BASHH first-line recommendation

IUSTI 2024

21 days

doxycycline for LGV (longer course for invasive form)

IUSTI/WHO

Chlamydia vs gonorrhoea EU notification rates — ECDC trend 2015-2024

Source: ECDC. Chlamydia consistently the most notified STI; both at record levels in 2023-2024.

Glossary of key terms

Chlamydia trachomatis serovars
WHOC. trachomatis has multiple serovars with different tissue tropisms: A-C cause trachoma (eye NTD, separate hub); D-K cause urogenital chlamydia (most common STI form); L1-L3 cause lymphogranuloma venereum (LGV — invasive, causes proctitis, genital ulcers and lymphadenopathy).

NAAT (nucleic acid amplification test)
WHO/IUSTIGold standard diagnosis for chlamydia — highly sensitive and specific, detecting C. trachomatis DNA/RNA from genital, rectal and pharyngeal swabs, or first-void urine (men). Point-of-care NAATs are increasingly available. Multiplex STI panels simultaneously detect chlamydia, gonorrhoea and other STIs.

Pelvic inflammatory disease (PID)
WHO/RCOGAscending infection from the cervix to the upper genital tract — uterus, fallopian tubes, ovaries. Polymicrobial — chlamydia, gonorrhoea and vaginal anaerobes. Can be acute (fever, lower abdominal pain) or subclinical (silent PID). Repeated PID episodes cause cumulative tubal damage — infertility, ectopic pregnancy.

LGV (Lymphogranuloma venereum)
ECDC/IUSTIC. trachomatis serovars L1-L3 — more invasive than D-K. Classic presentation in heterosexual men: genital papule → unilateral painful lymphadenopathy (bubo) → resolution/suppuration. Contemporary European/North American presentation: severe haemorrhagic proctitis in MSM — mimicking inflammatory bowel disease. Treatment: doxycycline 100mg BD × 21 days.

Antenatal chlamydia screening
WHO/RCOGScreening pregnant women for chlamydia — and treating positive cases — prevents vertical transmission (neonatal conjunctivitis, neonatal pneumonia). Treatment in pregnancy: azithromycin 1g single dose (safe alternative to doxycycline, which is contraindicated in pregnancy).

Chlamydia screening programmes
ECDC/PHENational chlamydia screening programmes target sexually active young people (15-25 years) — who carry the highest burden. England's NCSP offers annual opportunistic testing. The main challenge: re-infection rates are high because partners are not always treated simultaneously (partner notification is key).