Chlamydial Infections

ByMargaret R. Hammerschlag, MD, State University of New York Downstate Medical Center
Reviewed/Revised Apr 2023 | Modified Aug 2023

Chlamydia are bacteria that cause disease in humans, including sexually transmitted infections (STIs), 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 and can also be transmitted from pregnant women to their infants. (See also Chlamydial and Other Nongonococcal Infections.)

C. trachomatis is the most common bacterial cause of sexually transmitted infection (STI) in the United States. STIs 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 (the thin membrane lining the surface of the eye and inside of the lids) 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 the 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 crowded conditions. 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. Psittacosis is a rare infection.

C. psittaci is present in birds, including pet birds, such as parrots, parakeets, and cockatiels, and poultry. Psittacosis can occur in pet owners, veterinarians, and workers in the poultry industry exposed to 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 Chlamydial Infections

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 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

If women have an undiagnosed chlamydial infection of the reproductive system, it 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).

Diagnosis of Chlamydial Infections

  • For C. trachomatis, nucleic acid amplification tests

  • For C. pneumoniae, culture or nucleic acid amplification tests

  • For C. psittaci, blood tests or polymerase chain reaction testing

C. trachomatis is best identified by nucleic acid amplification tests (NAATs) done on urine samples and on vaginal swabs. NAATs are used to 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, and also in veterinarians and workers in turkey and duck processing plants. Doctors confirm the diagnosis by doing blood tests to detect antibodies, and certain laboratories are equipped to do the polymerase chain reaction (PCR) technique. For PCR, a sample of blood is used. The PCR technique increases the amount of the bacteria’s DNA and thus enables doctors to identify the bacteria more rapidly.


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 STIs are recommended for certain sexually active adolescents and adults. For information, see Screening for Chlamydia.

Treatment of Chlamydial Infections

  • Antibiotics


Current sex partners should be treated also. Infected people and their sex partners should abstain from sexual intercourse until at least 1 week after they have completed treatment.

Prevention of Chlamydial Infections

Safer sex practices are recommended to decrease risk of spreading C. trachomatis to other people. (See also Prevention of Chlamydia.)

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

More Information

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

  1. Centers for Disease Control and Prevention (CDC): Chlamydia—CDC Fact Sheet: A resource covering information about chlamydia, including risk factors, prevention, and treatment

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