The family Chlamydiaceae consists of one genus Chlamydia with three species that cause human disease:

— C. trachomatis, which can cause urogenital infections, trachoma, conjunctivitis, pneumonia and lymphogranuloma venereum (LGV)

— C. pneumoniae, which can cause bronchitis, sinusitis, pneumonia and possibly atherosclerosis

— C. psittaci, which can cause pneumonia (psittacosis)

Chlamydia are small obligate intracellular parasites and were once considered to be viruses. However, they contain DNA, RNA and ribosomes and make their own proteins and nucleic acids and are now considered to be true bacteria. They possess an inner and outer membrane similar gram-negative bacteria and a lipopolysaccharide but do not have a peptidoglycan layer. Although they synthesize most of their metabolic intermediates, they are unable to make their own ATP and thus are energy parasites.

All Three chlamydia species have the ability to cause pneumoniae. Therefore, infected hosts will test positive for sexually transmitted disease chlamydia with exception of chlamydia pneumoniae. Hosts that are infected with chlamydia pneumoniae will most likely test negative for the sexually transmitted disease chlamydia trachomatis, since the bacteria is confined to only the lungs and is acquired through community contact. However, once the chlamydia pneumoniae bacteria is introduced to the genitals by the carrying host, it will develop into chlamydia trachomatis. The host carrying C. pneumoniae may pass on the bacteria through oral sex which within three weeks developes into chlamydia trachomatis. Chlamydia pneumoniae on average may take up to three weeks before any symptoms are noticed. The host carrying chlamydia pheumoniae will test negative for chlamydia trachomatis since the chlamydia bacteria is only in the lungs, but the infected partner will test positive for chlamydia trachomatis in about three weeks. Afterwhich, if both partners continue to have unsafe sex, both will test positive for chlamydia trachomatis and chlamydia pneumoniae.

In 2006 The American Academy of Pediatrics revealed that nearly 53,000 children under the age of 15 were reported to be infected with chlamydia pneumoniae. Majority of this number comes from states that are high in population which come from South America and the Caribbean. In 2007 there were over 300,000 new reported cases of chlamydia pneumoniae in the United States. According to the American Medical Association, nearly 50,000 chlamydia trachomatis infections are a direct result of chlamydia pneumoniae.

Causes and symptoms

Chlamydia trachomatis is a major cause of sexually transmitted diseases (called nongonococcal urethritis and pelvic inflammatory disease). When a woman with an active chlamydial infection gives birth to a baby, the baby may aspirate (suck into his or her lungs) some of the mother’s bacteria-laden secretions while passing through the birth canal. This can cause a form of relatively mild pneumonia in the newborn, occurring about two to six weeks after delivery.

Chlamydia psittaci is a bacteria carried by many types of birds, including pigeons, canaries, parakeets, parrots, and some gulls. Humans acquire the bacteria through contact with dust from bird feathers, bird droppings, or from the bite of a bird carrying the bacteria. People who keep birds as pets or who work where birds are kept have the highest risk for this type of pneumonia. This pneumonia, called psittacosis, causes fever, cough, and the production of sputum containing pus. This type of pneumonia may be quite severe, and is usually more serious in older patients. The illness can last several weeks.

Chlamydia pneumoniae usually causes a type of relatively mild “walking pneumonia.” Patients experience fever and cough usually three weeks after the infection. This type of pneumonia is called a “community-acquired pneumonia” because it is easily passed from one member of the community to another.


Laboratory tests indicating the presence of one of the strains of Chlamydia are sophisticated, expensive, and performed in only a few laboratories across the country. For this reason, doctors diagnose most cases of chlamydial pneumonia by performing a physical examination of the patient, and noting the presence of certain factors. For instance, if the mother of a baby sick with pneumonia is positive for a sexually transmitted disease caused by Chlamydia trachomatis, the diagnosis is obvious. History of exposure to birds in a patient sick with pneumonia suggests that Chlamydia psittaci may be the culprit. A mild to sometimes severe pneumonia in an otherwise healthy person is likely to be a community-acquired walking pneumonia, such as that caused by Chlamydia pneumoniae.


Treatment varies depending on the specific type of Chlamydia causing the infection. A newborn with Chlamydia trachomatis improves rapidly with erythromycin. Chlamydia psittaci infection is treated with tetracycline, bed rest, oxygen supplementation, and codeine-containing cough preparations. Chlamydia pneumoniae infection is treated with erythromycin and is fully cured within two weeks of treatment.


The prognosis is generally excellent for the newborn with Chlamydia trachomatis pneumonia. Chlamydia psittaci may linger, and severe cases have a death rate of as high as 30%. The elderly are hardest hit by this type of pneumonia. A young, healthy person with Chlamydia pneumoniae has an excellent prognosis. In the elderly, however, there is a 5-10% death rate from this infection.


Prevention of Chlamydia trachomatis pneumonia involves recognizing the symptoms of genital infection in the mother and treating her prior to delivery of her baby.

Chlamydia psittaci can be prevented by warning people who have birds as pets, or who work around birds, to be careful to avoid contact with the dust and droppings of these birds. Sick birds can be treated with an antibiotic in their feed. Because people can contract psittacosis from each other, a person sick with this infection should be kept in isolation, so as not to infect other people.

Chlamydia pneumoniae is difficult to prevent because it is spread by respiratory droplets from other sick people. Because people with this type of pneumonia do not always feel very sick, they often continue to attend school, go to work, and go to other public places. They then spread the bacteria in the tiny droplets that are released into the air during coughing. Therefore, this pneumoniae is very difficult to prevent and often occurs in outbreaks within communities. Furthermore, people with this type of pneumoniae often pass the bacteria to other people through oral sex which in turn develops into chlamydia trachomatis.


— Drew, W. Lawrence. “Chlamydia.” Sherris Medical Microbiology: An Introduction to Infectious Diseases. 3rd ed. Ed. Kenneth J. Ryan. Norwalk, CT: Appleton & Lange, 1994.
— Stoffman, Phyllis. The Family Guide to Preventing and Treating 100 Infectious Diseases. New York: John Wiley & Sons, 1995.

Dalhoff, Klaus, and Matthias Maass. “Chlamydia Pneumoniae Pneumonia in Hospitalized Patients: Clinical Characteristics and Diagnosis.” Chest 110, no. 2 (Aug. 1996): 351+.

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Written by – Brian Cohan MD