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Margaret R. Hammerschlag

, MD, State University of New York Downstate Medical Center

Last full review/revision May 2021| Content last modified May 2021

Three species of Chlamydia cause human disease, including sexually transmitted infections and respiratory infections. All 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.

The genus Chlamydia now contains 12 species; 3 of them cause human disease:

  • Chlamydia trachomatis
  • Chlamydia pneumoniae
  • Chlamydia psittaci

Chlamydial species can cause persistent infection, which is often subclinical.

C. trachomatis

C. trachomatis has 18 immunologically defined serovars:

  • A, B, Ba, and C cause the eye disease trachoma.
  • D through K cause sexually transmitted infections (STIs) localized to mucosal surfaces.
  • L1, L2, and L3 cause STIs that lead to invasive lymph node disease (lymphogranuloma venereum).

In the US, C. trachomatis is the most common bacterial cause of STIs, including

Maternal transmission of C. trachomatis causes neonatal conjunctivitis and neonatal pneumonia. Universal prenatal screening and treatment of pregnant women have greatly reduced the incidence of infant C. trachomatis infection in the US (1).

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

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

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 who handle turkeys and ducks in poultry processing plants.

General reference

  • 1. Kohlhoff S, Roblin PM, Clement S, et al: Universal prenatal screening and testing and Chlamydia trachomatis conjunctivitis in infants. Sex Transm Dis Dec 15, 2020. doi: 10.1097/OLQ.0000000000001344

Diagnosis of Chlamydia

  • For C. trachomatis and C. pneumoniae, nucleic acid–based testing
  • For C. psittaci, blood tests

C. trachomatis STIs are 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. Currently, 6 NAATs are approved by the U.S. Food and Drug Administration (FDA) for testing genital and urine specimens from adults and adolescents. NAATs for genital infection can be done using noninvasively obtained samples, such as urine or vaginal swabs obtained by the patient or clinician. Several commercially available NAATs have been approved for extragenital infections (eg, those in the rectum or pharynx).

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. Two FDA-approved NAATs for C. pneumoniae are available as part of panels that simultaneously test for multiple respiratory pathogens.

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.

Screening for chlamydia

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 STIs is recommended by the Centers for Disease Control and Prevention (see 2015 Sexually Transmitted Diseases Treatment Guidelines and see the complement 2020 Recommendations for Providing Quality Sexually Transmitted Diseases Clinical Services).

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 years of age
  • Have a history of a prior STI
  • Engage in high-risk sexual behavior (eg, have a new sex partner or multiple sex partners, engage in sex work)
  • Have a partner who has an STI or engages in high-risk behavior

Pregnant women are screened during their initial prenatal visit; those < 25 years or with risk factors are screened again during the 3rd trimester (1).

Heterosexually active men are not screened except in settings with a high prevalence of chlamydial infection, including adolescent or STI 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 anal intercourse: Urine screen
  • For receptive anal intercourse: Rectal swab
  • For oral intercourse: Pharyngeal swab

Diagnosis reference

  • 1. Kohlhoff S, Roblin PM, Clement S, et al: Universal prenatal screening and testing and Chlamydia trachomatis conjunctivitis in infants. Sex Transm Dis Dec 15, 2020. doi: 10.1097/OLQ.0000000000001344

Treatment of Chlamydia

Uncomplicated lower genital tract infection is typically treated with one of the following oral regimens:

  • A single dose of azithromycin 1 g
  • A 7-day regimen of doxycycline 100 mg 2 times a day
  • A 7-day regimen of some fluoroquinolones (eg, levofloxacin 500 mg once a day)

When gonorrhea is present and chlamydial infection has not been excluded (uncommon, because all currently available NAATs for gonococcal infection are duplexed with a test for C. trachomatis), concurrent treatment for chlamydia is recommended. Pelvic inflammatory disease, lymphogranuloma venereum, or epididymitis is usually treated with doxycycline for 10 days.

Screening and treatment of pregnant women is the most effective way to prevent neonatal chlamydial infection, including conjunctivitis and pneumonia. Neonatal ocular prophylaxis with erythromycin or other preparations does not prevent neonatal chlamydial conjunctivitis.

Specific infections are discussed elsewhere in THE MANUAL: Psittacosis and C. pneumoniae pneumonia, lymphogranuloma venereum and urethritis, epididymitis, reactive arthritis, neonatal conjunctivitis and neonatal pneumonia, trachoma, and inclusion conjunctivitis.

Key Points

  • C. trachomatis causes trachoma or STIs; 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 C. trachomatis and C. pneumoniae infections using nucleic acid amplification tests.
  • Screen high-risk, asymptomatic patients for sexually transmitted chlamydial infection.
  • Treat with azithromycin or doxycycline.

More Information

The following are English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

  • Centers for Disease Control and Prevention: 2015 Sexually Transmitted Diseases Treatment Guidelines
  • Centers for Disease Control and Prevention: 2020 Recommendations for Providing Quality Sexually Transmitted Diseases Clinical Services

Drugs Mentioned In This Article

Drug Name Select Trade
erythromycin ERY-TAB, ERYTHROCIN
azithromycin ZITHROMAX

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