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by Margaret R. Hammerschlag, MD

Three species of Chlamydia cause human disease, including sexually transmitted diseases and respiratory infections. Most are susceptible to macrolides (eg, azithromycin), tetracyclines (eg, doxycycline), and fluoroquinolones.

Chlamydiae are nonmotile, obligate intracellular bacteria. They contain DNA, RNA, and ribosomes and make their own proteins and nucleic acids. However, they depend on the host cell for 3 of their 4 nucleoside triphosphates and use host adenosine triphosphate (ATP) to synthesize chlamydial protein.

There are 9 species; 3 of them cause human disease:

  • Chlamydia trachomatis

  • Chlamydia pneumoniae

  • Chlamydia psittaci

All chlamydial species can cause persistent infection, which is often subclinical.

C. trachomatis has 18 immunologically defined serovars. Serovars A, B, Ba, and C cause trachoma; D through K cause sexually transmitted diseases (STDs) localized to mucosal surfaces; L1, L2, and L3 cause STDs that lead to invasive lymph node disease (lymphogranuloma venereum). In the US, C. trachomatis is the most common bacterial cause of STDs, including nongonococcal urethritis (see Chlamydial, Mycoplasmal, and Ureaplasmal Mucosal Infections) and epididymitis in men; cervicitis, urethritis, and pelvic inflammatory disease in women; and proctitis, lymphogranuloma venereum, and reactive arthritis (Reiter syndrome) in both sexes. Maternal transmission of C. trachomatis causes neonatal conjunctivitis and pneumonia. Universal prenatal screening and treatment of pregnant women have greatly reduced the incidence of infant C. trachomatis infection in the US. The organism can be isolated from the rectum and throat in adults (usually in men who have sex with men [MSM]). Rectal infection with L2 strains can cause severe proctocolitis that can mimic acute inflammatory bowel disease in HIV-positive MSM.

C. pneumoniae can cause pneumonia (especially in children and young adults) that may be clinically indistinguishable from pneumonia caused by Mycoplasma pneumoniae. In some patients with C. pneumoniae, pneumonia, hoarseness, and sore throat may precede coughing, which may be persistent and complicated by bronchospasm. From 6 to 19% of community-acquired pneumonia cases are due to C. pneumoniae; outbreaks of C. pneumoniae pneumonia pose a particular risk for people in closed populations (eg, nursing homes, schools, military installations, prisons). No seasonal variations in occurrence have been observed. C. pneumoniae has also been implicated as an infectious trigger of reactive airway disease.

C. psittaci causes psittacosis. Strains causing human disease are usually acquired from psittacine birds (eg, parrots), causing a disseminated disease characterized by pneumonitis. Outbreaks have occurred among workers that handle turkeys and ducks in poultry processing plants.


  • Nucleic acid–based testing

C. trachomatis is best identified in genital samples using nucleic acid amplification tests (NAATs) because these tests are more sensitive than cell culture and have less stringent sample handling requirements. NAATs for genital infection can be done using noninvasively obtained samples, such as urine or vaginal swabs obtained by the patient or clinician. Serologic tests are of limited value except for diagnosing lymphogranuloma venereum and psittacosis.

C. pneumoniae is diagnosed by culture of respiratory tract specimens or by NAAT testing. There is one commercially available FDA-approved NAAT for C. pneumoniae.

A primary clue to diagnosis of C. psittaci infection is close contact with birds, typically parrots or parakeets. Diagnosis is confirmed by serologic tests. Culture is not generally available. There are no FDA-approved NAATs for C. psittaci.


Because chlamydial genital infection is so common and often asymptomatic or causes only mild or nonspecific symptoms (particularly in women), routine screening of asymptomatic people at high risk of STDs is recommended by the CDC (see 2010 STD Treatment Guidelines ). People who should be screened include the following.

Nonpregnant women (including women who have sex with women) are screened annually if they

  • Are sexually active and ≤ 25

  • Have a history of a prior STD

  • Engage in high-risk sexual behavior (eg, have a new sex partner or multiple sex partners, engage in sex work, use condoms inconsistently)

  • Have a partner who engages in high-risk behavior

Women < 35 are screened when admitted to a correctional facility.

Pregnant women are screened during their initial prenatal visit; those ≤ 25 yr or with risk factors are screened again during the 3rd trimester.

Heterosexually active men are not screened except in settings with a high prevalence of chlamydial infection, including adolescent or STD clinics or at admission into correctional facilities.

Men who have sex with men are screened if they have been sexually active within the previous year (for insertive intercourse, urine screen; for receptive intercourse, rectal swab; and for oral intercourse, pharyngeal swab).


  • Azithromycin or doxycycline

Uncomplicated lower genital tract infection is typically treated with a single dose of azithromycin (1 g po) or with a 7-day regimen of doxycycline (100 mg po bid) or some fluoroquinolones (eg, levofloxacin 500 mg po once/day). Treatment of presumed chlamydial infection is routine when gonorrhea is present (see also Gonorrhea). Pelvic inflammatory disease, lymphogranuloma venereum, or epididymitis is usually treated with doxycycline for 10 days.

Specific infections are discussed elsewhere in T he M anual : Psittacosis and C. pneumoniae pneumonia Etiology, lymphogranuloma venereum and urethritis Lymphogranuloma Venereum (LGV), epididymitis Epididymitis, reactive arthritis Reactive Arthritis, neonatal conjunctivitis and neonatal pneumonia Neonatal Conjunctivitis and Neonatal Pneumonia, trachoma Trachoma, and inclusion conjunctivitis Adult Inclusion Conjunctivitis.

Key Points

  • C. trachomatis causes trachoma or STDs; maternal transmission can cause neonatal conjunctivitis and/or pneumonia.

  • C. pneumoniae can cause pneumonia (especially in children and young adults and in enclosed populations).

  • C. psittaci is a rare cause of pneumonia (psittacosis) that is usually acquired from psittacine birds (eg, parrots).

  • Diagnose using NAATs.

  • Screen high-risk, asymptomatic patients for sexually transmitted chlamydial infection.

  • Treat with azithromycin or doxycycline.

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