Chlamydial ("kla-mid-ee-uhl") infection is the leading sexually transmitted disease (STD) in the United States today, with an estimated 4 million new cases occurring each year. Pelvic inflammatory disease (PID), a serious complication of chlamydial infection, has emerged as a major cause of infertility among women of childbearing age. Chlamydial infection is caused by a bacterium, Chlamydia trachomatis, and is transmitted during vaginal or anal sexual contact with an infected partner. A pregnant woman may pass the infection to her newborn during delivery, with subsequent neonatal eye infection or pneumonia. The annual cost of chlamydial infections and their sequelae is estimated to exceed $2 billion.


Men and women with chlamydial infections may experience abnormal genital discharge or pain during urination. These early symptoms of chlamydial infection may be absent or very mild. If symptoms occur, they usually appear within 1 to 3 weeks after exposure. One of every two infected women and one of every four infected men may have no symptoms whatsoever. As a result, the disease is often not diagnosed until complications develop.

In women, chlamydial infections can result in PID; in men, these infections may lead to pain or swelling in the scrotal area, which is a sign of epididymitis, an inflammation of a part of the male reproductive system located near the testicles. Left untreated, this condition, like PID in women, can result in infertility.

Chlamydial bacteria can cause proctitis (inflamed rectum) and conjunctivitis (inflammation of the lining of the eye). The bacteria have also been found in the throat as a result of oral sexual contact with an infected partner. A particular strain of chlamydia causes an uncommon STD called lymphogranuloma venereum (LGV), which is characterized by prominent swelling and inflammation of the lymph nodes in the groin. Complications may follow if LGV is not treated.


Chlamydial infection is easily confused with gonorrhea because the symptoms of both diseases are similar, and they often occur together. Until recently, the only way to diagnose chlamydial infection was to take a sample of secretions from a patient's genital area and attempt to grow the organism in specialized tissue culture in the laboratory. Although still considered the most definitive test, this method is expensive and technically difficult, and test results are not available for up to 3 days.

Scientists have developed several rapid tests for diagnosing chlamydial infection that use sophisticated techniques and a dye to detect bacterial proteins. Although these tests are slightly less accurate, they are less expensive, more rapid, and can be performed during a routine checkup.


A 7-day course of antibiotics such as tetracycline or doxycycline is the recommended treatment for chlamydial infection. Other antibiotics are effective, however, and can be used if tetracycline cannot be taken. For example, pregnant women should not take tetracycline, but rather can be treated with erythromycin. Penicillin, which is often used for treating some other STDs, is not effective against chlamydial infections. New medications are being developed that should greatly simplify treatment and help control the spread of chlamydia in the population. It is very important that a person with chlamydial infection take all of the prescribed medication, even after symptoms disappear. To be sure that the infection is cured, a follow-up visit to the doctor or clinic 1 to 2 weeks after finishing the medication may be necessary. All sex partners of a person with chlamydial infection should be evaluated and treated to prevent re- infection and further spread of the disease.

Pelvic Inflammatory Disease

Each year up to 1 million women in the United States develop PID, a serious infection of the reproductive organs. As many as half of all cases of PID may be due to chlamydial infection, and many of these occur without symptoms. PID can result in scarring of the fallopian tubes, which can block the tubes and prevent fertilization from taking place. An estimated 100,000 women each year become infertile as a result of PID.

In other cases, scarring may interfere with the passage of the fertilized egg to the uterus during pregnancy. When this happens, the egg may implant in the fallopian tube. This is called ectopic or tubal pregnancy. This very serious condition results in the loss of the fetus and is a major cause of maternal death in the United States.

Effects of Chlamydial Infection in Newborns

A baby who is exposed to chlamydial bacteria in the birth canal during delivery may develop conjunctivitis (eye infection) or pneumonia. Symptoms of conjunctivitis, which include discharge and swollen eyelids, usually develop within the first 10 days of life. Symptoms of pneumonia, including a progressively worsening cough and congestion, most often develop within 3 to 6 weeks after birth. Both conditions can be successfully treated with antibiotics. However, because of these risks to the newborn, many doctors now recommend routine testing of all pregnant women for chlamydial infection. Prevention

Because chlamydial infection often occurs without symptoms, people who are infected may unknowingly pass the bacteria to their sex partners. Many doctors recommend that all persons who have more than one sex partner, especially women under 25 years of age, be tested for chlamydial infection regularly, even in the absence of symptoms. Using condoms (rubbers) or diaphragms during sexual intercourse may help reduce the transmission of chlamydial bacteria.


The development of rapid diagnostic tests for chlamydia has marked an important breakthrough in the ability to detect chlamydial infections. Another technique has been developed that can detect chlamydial infection with greater accuracy than either culture or other current tests. This test uses polymerase chain reaction or PCR and results are available within 24 hours. Although PCR is not yet commercially available, it is being used in research studies and may be more widely used in the future in referral centers. However, one of the most urgent priorities with respect to chlamydial infection is the development of simple, inexpensive tests for use in clinics.

Scientists are also studying the basic process of how chlamydial bacteria cause disease in the body and why some people suffer more severe complications than others. Animal studies have shown that the body's immune response to the organism may be responsible for scarring associated with chlamydial infection. Investigators supported by the NIAID think that chlamydial bacteria stimulate production of a number of cellular substances that affect duration of infection and may be important in chronic inflammation and subsequent scarring of the fallopian tubes. These studies may lead to insights about how to prevent PID and PID-related infertility or other complications of chlamydial infection.

Prepared by:
Office of Communications
National Institute of Allergy and Infectious Diseases National Institutes of Health
Bethesda, Maryland 20892