Merck Manual

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Chlamydial Infections


Margaret R. Hammerschlag

, MD, State University of New York Downstate Medical Center

Last full review/revision Jun 2019| Content last modified Jun 2019
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Chlamydia are bacteria that cause disease in humans, including sexually transmitted diseases (STDs), and infections of the eyes and respiratory tract.

Three species of Chlamydia cause disease in humans. They are

  • Chlamydia trachomatis

  • Chlamydia pneumoniae

  • Chlamydia psittaci

Chlamydia trachomatis

C. trachomatis can cause infections in many organs of the body such the urethra, cervix, and rectum. It is primarily spread sexually between adults and can also be transmitted from pregnant women to their infants. (See also Chlamydial and Other Infections.)

C. trachomatis is the most common bacterial cause of sexually transmitted diseases (STDs) in the United States. STDs are infections that are passed from person to person through sexual contact.

When spread through sexual contact, C. trachomatis can cause the following infections:

A pregnant woman who is infected with C. trachomatis can transmit the infection to her baby during childbirth, which can cause an eye infection (conjunctivitis) or lung infection (pneumonia) in the newborn. To prevent these infections in newborns, universal prenatal screening and treatment of pregnant women is done. These measures have greatly reduced the incidence of newborn conjunctivitis and pneumonia in the United States.

Certain strains of C. trachomatis cause an eye infection called trachoma. Trachoma is a prolonged infection of the conjunctiva of the eye and is the leading preventable cause of blindness in the world, especially in sub-Saharan Africa. Trachoma is very rare in the United States. The disorder occurs mainly in children, particularly those between the ages of 3 and 6. People become infected when they come into contact with discharge from the eyes or nose of an infected person, for example, through contact with contaminated hands, clothing, or towels. Also, insects can spread the disease.

Chlamydia pneumoniae

C. pneumoniae can cause lung infection (pneumonia). C. pneumoniae is spread from person to person when people with the infection cough or sneeze while in close contact with others who then breathe in droplets that contain the bacteria.

Many cases of pneumonia that develop outside of hospitals may be caused by C. pneumoniae. C. pneumoniae infection poses a particular risk for people in nursing homes, schools, military camps, prisons, and other closed populations. C. pneumoniae may also be a trigger of reactive airway disease (a diagnosis that doctors give when they suspect a person has asthma but have not yet confirmed it).

Chlamydia psittaci

C. psittaci causes psittacosis, which is an uncommon type of pneumonia. C. psittaci is present in birds including pet birds, such as parrots and cockatiels, and poultry, such as turkeys or ducks. People become infected if they inhale dust from the waste of infected birds. Outbreaks have occurred among workers who handle turkeys and ducks in poultry-processing plants.


Symptoms of a chlamydial infection vary depending on the species that is causing the infection.

Chlamydia trachomatis

Most people who have a sexually transmitted chlamydial infection, especially women, have no symptoms. When symptoms are present, they vary by sex and location of infection:

  • Women may have an abnormal vaginal discharge or a burning sensation while urinating.

  • Men may also notice a burning sensation while urinating. Additionally, there may be a discharge from the penis and pain or swelling in one or both testicles.

  • Men and women with a rectal infection may have rectal pain, discharge, and/or bleeding.

Chlamydia pneumoniae

People who have respiratory infections caused by C. pneumoniae may have hoarseness and a sore throat before developing a cough.

Chlamydia psittaci

People infected with C. psittaci may have fever, severe headache, and cough.

Complications of chlamydial infection

An undiagnosed sexually transmitted chlamydial infection in women can cause pelvic inflammatory disease, which can cause serious, permanent damage to a woman's reproductive system. This damage can result in infertility or a life-threatening ectopic pregnancy (one that occurs outside the womb, usually in a fallopian tube).


  • For C. trachomatis and C. pneumoniae, nucleic acid amplification tests

  • For C. psittaci, blood tests

C. trachomatis is best identified by nucleic acid amplification tests (NAATs) done on urine and on vaginal swabs. NAATs look for an organism's unique genetic material, its DNA or RNA (which are nucleic acids). NAATs use a process that increases the amount of the bacteria's DNA or RNA so that it can be more easily identified.

C. pneumoniae is diagnosed by doing NAATs or by taking swabs from the back of the throat and growing the organism in cell culture in a laboratory. .

C. psittaci is suspected mainly in people who had close contact with birds, usually parrots or parakeets. Doctors confirm the diagnosis by doing blood tests to detect antibodies, although NAATs are being developed.


Because chlamydial genital infection is so common and because many infected women have no or only mild symptoms, tests to screen for chlamydial infection and other STDs are recommended for certain sexually active women and men.

Women who are not pregnant (including women who have sex with women) are screened once a year if they have the following risk factors:

  • Sexually active and under age 25

  • A previous STD

  • Participation in high-risk sexual behavior (for example, have a new sex partner or multiple sex partners or engage in sex work)

  • A partner who has an STD or participates in high-risk sexual behavior

Pregnant women are screened during their first prenatal visit. Pregnant women who are under age 25 or who have risk factors are screened again during their 3rd trimester.

Men can be screened if their risk of chlamydial infection is increased—for example, if they have sex with men, are patients at an adolescent or STD clinic, or when they are admitted to a correctional facility.

Men who have sex with men are screened at least once a year and more often if they have HIV infection or risk factors such as multiple partners.


Screening and treatment of pregnant women is the most effective way to prevent transmission of C. trachomatis to newborns.

Safe sex practices are recommended to decrease risk of infection.


  • Antibiotics

Chlamydial infections are treated with antibiotics such as azithromycin, doxycycline, or, for some species, levofloxacin.

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