Chlamydia

Introduction

Chlamydia trachomatis is the most commonly diagnosed bacterial sexually transmitted infection (STI) in the UK, with several hundred thousand new diagnoses reported annually, concentrated in people aged 15-24.¹ Genital infection is caused by serovars D-K, distinct from the L1-L3 serovars responsible for lymphogranuloma venereum (LGV), a distinct and more invasive syndrome seen predominantly in men who have sex with men.²

Its clinical significance lies less in the acute infection, which is usually mild or silent, than in its long-term consequences: chlamydia is the leading preventable cause of tubal factor infertility and ectopic pregnancy in the UK, which is why opportunistic screening of young adults is a public health priority.¹

Aetiology and transmission

Chlamydia trachomatis is an obligate intracellular Gram-negative bacterium that cannot replicate outside a host cell, which is why culture is impractical and diagnosis relies on nucleic acid amplification testing (NAAT) rather than microscopy or culture.

Transmission is through vaginal, anal or oral sexual contact with an infected partner, and vertically from mother to neonate during birth, causing neonatal conjunctivitis or pneumonia. The incubation period is typically 1-3 weeks, though many infections never become symptomatic at all.²

Risk factors

• Age under 25
• New or multiple sexual partners
• Inconsistent condom use
• Previous STI diagnosis
• Not having been screened in the past year

Clinical features

The defining feature of chlamydia is that most infections are asymptomatic - around 70% in women and 50% in men - which is why screening, rather than waiting for symptomatic presentation, is central to control.¹

In women

When symptomatic, women may report increased or altered vaginal discharge, dysuria, intermenstrual or post-coital bleeding, and deep dyspareunia. Ascending infection can cause pelvic inflammatory disease (PID), presenting with lower abdominal pain, fever and cervical motion tenderness.

In men

Men may present with urethral discharge (typically mucoid or clear, less purulent than gonorrhoea), dysuria, and testicular pain or swelling from epididymo-orchitis.

Extragenital and other sites

Rectal infection is usually asymptomatic but can cause proctitis with discharge, pain and tenesmus. Pharyngeal infection is almost always asymptomatic. Conjunctival inoculation (from genital secretions on the hands) causes chlamydial conjunctivitis.

Examination

Examination findings are often unremarkable, or subtle - mucopurulent cervicitis with contact bleeding on speculum examination in women, or a clear urethral discharge in men, expressed by gentle milking of the urethra if not spontaneously visible. Abdominal and pelvic examination should assess for cervical motion, adnexal or uterine tenderness suggesting PID, and the testes should be examined for tenderness or swelling if epididymo-orchitis is suspected.

Differential diagnosis

• Gonorrhoea: often co-infects with chlamydia; typically more purulent discharge
• Non-specific urethritis: urethritis without an identified organism
• Trichomonas vaginalis: frothy, malodorous discharge
• Urinary tract infection: dysuria without discharge, different urine findings
• Mycoplasma genitalium: clinically similar, requires separate NAAT testing

Investigations

NAAT is the diagnostic test of choice, with high sensitivity and specificity from a non-invasive sample.³ The sample taken depends on the sexual history: vulvovaginal swab (self-taken or clinician-taken) is preferred to a cervical swab in women, first-catch urine (the first 10-20 mL voided, not mid-stream) in men, and rectal or pharyngeal swabs where these sites have been exposed.

Because chlamydia and gonorrhoea often co-exist and share risk factors, dual NAAT testing for both is standard practice. Anyone testing positive for chlamydia should also be offered a full STI screen, including HIV and syphilis serology, since infections cluster.

Management

First-line: doxycycline 100 mg twice daily for 7 days.⁴ It is more effective than azithromycin for rectal chlamydia, which is one reason it has replaced azithromycin as first-line even for genital infection.

Alternatives: azithromycin 1 g as a single dose, followed by 500 mg once daily for two further days, is used where doxycycline is contraindicated or poorly tolerated. In pregnancy and breastfeeding, doxycycline is contraindicated, and azithromycin or amoxicillin 500 mg three times daily for 7 days is used instead.⁴

Patients should abstain from sexual intercourse until they and their partner(s) have completed treatment and, for single-dose regimens, for 7 days afterwards. A test of cure is not routinely required after treatment unless the patient is pregnant, symptoms persist, or reinfection is suspected, in which case retesting is done at least 6 weeks after treatment to avoid false positives from residual non-viable DNA.

Partner notification should cover all sexual partners from the preceding 6 months, or the most recent partner if longer ago than that. Partners should be tested and treated empirically without waiting for their own results, given the high transmission risk and consequences of reinfection.

Complications

Untreated chlamydia can ascend to cause pelvic inflammatory disease in women, with the risk of tubal damage, chronic pelvic pain, ectopic pregnancy and tubal factor infertility - the single most important reason chlamydia control matters at a population level.¹ In men, ascending infection can cause epididymo-orchitis and, rarely, contributes to infertility.

Reactive arthritis (formerly Reiter's syndrome) - a triad of arthritis, urethritis and conjunctivitis - can follow chlamydial infection, particularly in HLA-B27-positive individuals. Neonatal transmission during vaginal delivery can cause conjunctivitis (typically 1-2 weeks after birth) or pneumonia (4-12 weeks after birth) in the infant.

Red flags

Prevention

The National Chlamydia Screening Programme recommends opportunistic annual screening, and screening with each new sexual partner, for all sexually active people under 25 in England.¹ Consistent condom use reduces but does not eliminate transmission risk, since infection can occur from skin-to-skin or fluid contact not fully covered by a condom.

Prognosis

With prompt diagnosis and treatment, chlamydia resolves completely and reliably. The prognosis worsens the longer infection goes untreated, since the risk of tubal damage and subsequent infertility rises with repeated or prolonged untreated episodes, making early detection through screening the single biggest determinant of long-term outcome.