Lucan · Conditions

Chlamydia trachomatis infection

Australian guidance: ASHM Doxy-PEP client letter / decision-making tool (doxycycline post-exposure prophylaxis for chlamydia and syphilis PREVENTION only); treatment per ASHM Australian STI Management Guidelines (sti.guidelines.org.au, captured 2026-10-05)

1. Definition and epidemiology

Chlamydia trachomatis infection is the most common bacterial sexually transmissible infection (STI) in Australia, caused by an obligate intracellular gram-negative bacterium. Australian surveillance data consistently record over 100,000 diagnoses annually, with the highest burden in individuals aged 15 to 29 years; notification rates are disproportionately elevated in Aboriginal and Torres Strait Islander communities, where rates are four to six times those of non-Indigenous Australians. Most infections are asymptomatic, driving ongoing transmission and the consequent risk of long-term complications in those who go undetected.

2. Pathophysiology

Chlamydia trachomatis serovars D through K cause urogenital infection. The organism is an obligate intracellular pathogen that exists in two morphological forms: the infectious elementary body (EB), which is metabolically inert and structurally rigid, and the reticulate body (RB), the metabolically active intracellular replicating form. After mucosal attachment via heparan sulphate proteoglycan receptors on columnar and transitional epithelium, EBs are internalised into membrane-bound endosomes called inclusions, where they differentiate into RBs. RBs replicate by binary fission and evade lysosomal fusion, exploiting host cell ATP and biosynthetic intermediates. After 48 to 72 hours, RBs condense back into EBs and are released by exocytosis or cell lysis to infect adjacent cells. The resulting host immune response, dominated by CD4+ T helper cells and interferon-gamma-mediated activation of macrophages, triggers a chronic inflammatory state characterised by submucosal fibrosis and scarring. This fibroproliferative response, rather than direct cytopathic injury, underlies tubal scarring, pelvic adhesions, and the sequelae of pelvic inflammatory disease (PID) (Robbins Ch 8).

3. Clinical presentation and diagnostic criteria

The majority of infections in both sexes are asymptomatic, which is the central challenge in control. When symptomatic in males, urethritis presents with a mucopurulent or clear urethral discharge and dysuria, typically mild relative to gonorrhoea; epididymo-orchitis may complicate untreated urethral infection and presents with unilateral scrotal pain, erythema, and tenderness. In females, the primary site of infection is the endocervix, causing cervicitis with mucopurulent discharge, postcoital bleeding, or intermenstrual spotting; urethral co-infection produces dysuria and frequency. Ascending infection produces PID manifesting as bilateral lower abdominal or pelvic pain, cervical motion tenderness (chandelier sign), uterine tenderness, and adnexal tenderness on bimanual examination, often with fever. Rectal infection, occurring in men who have sex with men and women after receptive anal intercourse, causes proctitis with anorectal discomfort, discharge, or tenesmus. Pharyngeal infection is usually asymptomatic.

4. Investigations and AU-specific cutoffs

The ASHM chlamydia capture (2026-10-05) supplies the testing method: NAAT is the only recommended test for chlamydia. Specimens: first-pass urine in people who do not have a vagina or if an endocervical or self-collected vaginal swab cannot be taken (less sensitive than a self-collected vaginal swab); self-collected vaginal swab as the best test if no speculum examination; clinician-collected endocervical swab as the best test if examined; anorectal swab for any patient with anorectal symptoms and all men who have sex with men; pharyngeal swab for all men who have sex with men. Asymptomatic patients can collect most samples themselves, including vaginal, anorectal and throat swabs, and concurrent gonorrhoea testing should accompany chlamydia testing. The capture gives no numeric sensitivity or specificity figure (it describes NAATs only as highly sensitive), so the pre-grounding note's NAAT sensitivity above 95% and specificity above 99% remain removed rather than restated.

5. Management

What this repo's source verifies — Doxy-PEP prevention only. An ASHM-endorsed patient letter (created November 2025) describes doxycycline post-exposure prophylaxis (Doxy-PEP) as a preventive intervention: 200 mg doxycycline (two 100 mg tablets) taken within 72 hours after sex, which multiple clinical trials have shown reduces the incidence of syphilis and chlamydia by approximately 80%, citing the Australian consensus statement on Doxy-PEP (MJA, doi.org/10.5694/mja2.52258). Doxy-PEP does not prevent HIV and is described as unlikely to be effective against gonorrhoea in Australia because of local tetracycline resistance patterns. Current evidence, per this letter, supports its use in cisgender men who have sex with men and transgender women, and it may be considered for people with a recent (e.g. within the last year) syphilis diagnosis, two or more recent bacterial STI diagnoses, increased risk of serious consequences of chlamydia or syphilis (including transmission to women or people with a uterus), or ongoing higher-risk sexual practices. The letter notes Doxy-PEP is prescribed via private script as a non-PBS item, and that an ASHM Doxy-PEP decision-making tool (ashm.org.au/doxy-dmt) supports individual risk-benefit assessment, ongoing STI/HIV screening, periodic review of continued need, and discussion of antibiotic stewardship and side effects. As of this document's creation (November 2025), it states that national Doxy-PEP prescribing guidelines were expected in early 2026 — this repo has no later document confirming whether those have since been published, so check for an updated national guideline before relying on this letter as current practice.

Treatment of established infection - ASHM Australian STI Management Guidelines (sti.guidelines.org.au), chlamydia page, captured 2026-10-05 (sources/ashm-sti/chlamydia.md).

Principal treatment options:

• Uncomplicated genital or pharyngeal infection: doxycycline 100 mg PO, BD for 7 days. Alternative: azithromycin 1 g PO, stat., to consider where adherence to daily treatment is likely to be poor, especially where anorectal infection is less likely.
• Anorectal infection: doxycycline 100 mg PO, BD for 7 days if asymptomatic, but 21 days if symptomatic (see anorectal syndromes; the anorectal syndromes capture treats symptomatic proctitis empirically with doxycycline 100 mg PO, BD for 21 days, sources/ashm-sti/anorectal-syndromes.md). Alternative: azithromycin 1 g PO, stat. and repeat in 12-24 hours.

(BD: twice a day; PO: orally; stat.: immediately.)

Treatment advice: see urethritis for immediate management of urethritis symptoms. Immediate treatment is not recommended for all sexual contacts of chlamydia; offer testing of exposed anatomical sites and await results. If contact treatment is initiated, use the recommended treatment, and only use azithromycin if adherence is likely to be poor or it matches the index case treatment. If a patient has an IUD, leave it in place and treat as recommended; seek specialist advice as needed.

Other immediate management: perform a full STI check-up including HIV and syphilis serology if not done as part of initial testing; advise no sexual contact for 7 days after treatment is commenced, or until the course is completed and symptoms resolved, whichever is later; advise no sex with partners from the last 6 months until the partners have been tested and treated if necessary; arrange contact tracing and patient delivered partner therapy; provide the patient with a factsheet; notify the state or territory health department.

Special treatment situations (consider seeking specialist advice before treating any complicated presentation):

• Pregnant people: azithromycin 1 g PO, stat.
• Allergy to the principal treatment choice: if both principal treatment options are unsuitable, seek specialist advice.
• Rectal co-infection with gonorrhoea: treat both infections, i.e. ceftriaxone 500 mg IMI, stat. in 2 mL 1% lignocaine, PLUS doxycycline 100 mg PO, BD for 7 days if asymptomatic, but 21 days if symptomatic (see anorectal syndromes).

Contact tracing: important to prevent re-infection and reduce transmission. All partners should be traced back for 6 months. The diagnosing doctor is responsible for initiating and documenting a discussion about contact tracing. Offer testing of exposed anatomical sites to all sexual contacts. Consider presumptive treatment if there has been sexual contact within the past 2 weeks, or when the person's individual circumstances mean later treatment may not occur.

Patient delivered partner therapy (PDPT): a partner notification and treatment method whereby antibiotic treatment is prescribed or supplied for the sexual partner/s of a patient diagnosed with chlamydia (the index patient), and the index patient delivers the prescription or treatment to their partner/s. Consider it for heterosexual index patients with anogenital or oropharyngeal chlamydia whose partners are unlikely to seek chlamydia testing or treatment, or with repeat infections whose partners have not been treated. It is approved in some jurisdictions; PDPT guidance is available in Victoria, NSW, and the NT.

Follow up: confirm patient adherence with treatment and assess symptom resolution, confirm contact tracing procedures have been undertaken or offer more support, and educate about condom use, contraception, HIV PrEP/PEP, safe injecting practices, consent, CST and vaccinations for HAV, HBV and HPV as indicated. Test of cure (TOC): not routinely recommended, except for pregnant people and anorectal infection treated with azithromycin. TOC by NAAT in these situations should be performed no earlier than 4 weeks after treatment is completed, to prevent a false positive result due to persistent chlamydia DNA. Re-infection is common; retesting at 3 months is recommended to detect re-infection, and consider testing for other STIs if not undertaken at first presentation or retesting after the window period.

6. Pharmacology and AU PBS notes

The only sourced pharmacology point is that Doxy-PEP is dispensed via private script as a non-PBS item - this is single-dose prevention dosing (200 mg once, within 72 hours of sex), not a chlamydia treatment course, and the two should not be confused. The ASHM capture now supplies the treatment doses (section 5: doxycycline 100 mg PO, BD; azithromycin 1 g PO, stat.), but no mechanism content and no PBS listing status or item number for doxycycline or azithromycin used to treat an established chlamydia infection; the pre-grounding version of this note stated PBS item specifics that do not trace to any file read here and remain removed.

7. Complications and monitoring

Untreated or inadequately treated chlamydia carries significant long-term morbidity. In females, ascending infection causing PID can result in tubal factor infertility (affecting approximately 10 to 15 percent of women after a single episode of PID), ectopic pregnancy (risk is six to ten times background after PID), and chronic pelvic pain from adhesion formation. Fitz-Hugh-Curtis syndrome, perihepatitis causing right upper quadrant pain and hepatic capsule inflammation, occurs in up to 30 percent of women with PID. Vertical transmission during vaginal delivery causes neonatal conjunctivitis (ophthalmia neonatorum, onset 5 to 12 days after birth) and neonatal pneumonitis (onset 3 to 16 weeks after birth with a characteristic staccato cough and bilateral interstitial infiltrates on chest X-ray).

In males, epididymo-orchitis can cause subfertility through epididymal obstruction or impaired spermatogenesis. Reactive arthritis (formerly Reiter syndrome), classically the triad of urethritis, conjunctivitis or uveitis, and asymmetric oligoarthritis affecting large joints, is an immune-mediated complication occurring weeks after urogenital or enteric infection and is associated with HLA-B27 positivity.

Retesting at 3 months is recommended to detect re-infection, per the Australian STI Management Guidelines (captured 2026-10-05). All positive results must be notified to the relevant state or territory health department, as chlamydia is a notifiable condition in every Australian jurisdiction.

Written for Australian practice and grounded in the guidance named above. Study material only — not medical advice. Always check against current guidelines.