Chlamydia Infection (Sexually Transmitted)



  • Chlamydia trachomatis is an intracellular membrane-bound prokaryotic organism. Chlamydia derives from the Greek word for “cloak.”
  • Chlamydia is the most common bacterial sexually transmitted infection (STI) in the United States.
  • Transmitted through vaginal, anal, or oral sex; transmitted vertically during vaginal delivery
  • Most cases are asymptomatic, especially in people with female anatomy. Untreated disease can lead to pelvic inflammatory disease (PID), ectopic pregnancy, and infertility.
  • System(s) affected: reproductive

Pregnancy Considerations
Perinatal acquisition may result in neonatal pneumonia and/or conjunctivitis.



  • Mandatory reporting started in 1985; there has generally been a steady increase in incidence since then.
  • 1.64 million cases reported in 2020; ~1.8 million reported cases in 2019; incidence had been steadily increasing until 2020; lower reported numbers likely reflect reduced screening during COVID-19 pandemic rather than true decrease in actual infections.
  • Swedish new variant of C. trachomatis (nvCT) first reported in 2006; often produces false-negative tests; largely confined to Nordic countries


  • 495/100,000 people in the United States
  • Young females, ethnic minorities most affected
  • Highest prevalence in ages 20 to 24 years, followed by ages 15 to 19 years
  • Predominant sex: females > males; females have twice the reported incidence and prevalence than males, likely reflecting increased testing in females. The use of highly sensitive nucleic acid amplification test (NAAT) urine screening may increase identification in males.
  • Estimated ~2% of young sexually active individuals in the United States are affected.

Etiology and Pathophysiology

C. trachomatis serotypes D to K associated with genital tract infections. Chlamydia is an obligate intracellular organism. Chlamydia has biphasic life cycle. Extracellular elementary body (EB) is metabolically inactive and infectious. Once taken up by host cell (columnar epithelium of the genital tract), the EB prevents lysosomal phagocytosis and transforms to reticulate body (RB) which uses energy from host cell to synthesize RNA, DNA, and proteins. EBs are released and infect neighboring cells or spread through sexual contact.

Risk Factors

Risk correlates with:

  • Number of lifetime sexual partners and number of concurrent sexual partners
  • No use of barrier contraception during intercourse
  • Black/Hispanic/Native American and Alaskan Native ethnicity
  • Men who have sex with men (MSM) may be at higher risk for rectal and pharyngeal chlamydia than other groups; consider testing with NAAT when appropriate (1).

General Prevention

  • Screening is recommended if new or >1 sex partner in the past 6 months; attending an adolescent clinic, family planning clinic, STD or abortion clinic, or attending a jail or other detention center clinic; screen if rectal pain, discharge or tenesmus, testicular pain occurs; test all individuals with urethral or cervical discharge.
  • All sexually active women ≤25 years of age should be screened at least yearly. Repeat testing in ~3 months is recommended for those who screen positive because reinfection rate is high regardless of whether the sexual partner is treated (2)[A].
  • Consider screening sexually active men ≤25 years of age particularly in high-risk populations.
  • Screen high-risk MSM annually with genital and extragenital screening (3)[A].
  • NAAT is the preferred screening test in all circumstances except child sexual abuse involving boys or rectal/oropharyngeal testing in prepubescent girls. For these situations, culture and susceptibility testing is preferred (3)[A].
  • Acceptable to screen women for chlamydia on same day as intrauterine device (IUD) insertion—treat if positive (no need to remove IUD in this circumstance)
  • Doxycycline postexposure prophylaxis (Doxy-PEP) is an emerging prevention strategy for prevention of chlamydia, syphilis, and possibly gonorrhea; currently best studied in MSM and transgender women with a history of bacterial STI within the past year

Commonly Associated Conditions

  • Individuals with female anatomy
    • PID: ~10% develop PID within 12 months if untreated.
    • Infertility, ectopic pregnancy
    • Chronic pelvic pain
    • Urethral syndrome (dysuria, frequency, and pyuria in the absence of infection)
    • Arthritis (less common)
    • Spontaneous abortion
  • Individuals with male anatomy
    • Epididymitis and nongonococcal urethritis
    • Reiter syndrome (HLA-B27)
    • Proctitis
  • Neonates
    • Inclusion conjunctivitis (occurs in ~40% of exposed neonates)
    • Otitis media
    • Pneumonia
    • Pharyngitis
  • Diseases caused by other chlamydial species
    • Lymphogranuloma venereum (LGV): C. trachomatis serotypes L1 to L3
    • Trachoma: C. trachomatis serotypes A to C

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