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Chlamydia pneumoniae

Chlamydia pneumoniae


Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues



Around half of the world's population is infected with C. pneumoniae, although it does not cause any obvious health problems in the majority. C. pneumoniae can, however, cause upper and lower respiratory tract infections, ranging from mild to severe and life-threatening. As the name suggests, pneumonia is the most common infection associated with C. pneumoniae. It can be difficult to diagnose, and there is, as yet, no standard diagnostic method for identifying its presence. Fortunately, however, it does respond to standard antibiotic treatment. In recent years, research has also suggested that C. pneumoniae infection may have a long-term impact on health, possibly playing a role in the development of heart disease.

Disease History, Characteristics, and Transmission

C. pneumoniae infection may affect both the upper and the lower respiratory tract. Bronchitis and pneumonia are the most common forms of C. pneumoniae infection. Sinusitis, pharyngitis (infection of the throat), and laryngitis (infection of the larynx or voice box) are less likely. Pneumonia may develop gradually with fever, hoarseness, and cough, although sometimes fever may be absent. C. pneumoniae may also make asthma symptoms worse. Transmission of C. pneumoniae is by hand-to-hand contact or by exposure to the aerosols created by coughing and sneezing.

Scope and Distribution

C. pneumoniae infection is common, and school-age children seem to be most susceptible. The Centers for Disease Control and Prevention (CDC) reports that about 50% of adults are affected by the time they are 20 years old. Reinfection throughout life is also common. Older adults are most at risk for complications from C. pneumoniae infection, which may account for 6–12% of all community-acquired pneumonia (CAP)— that is, as the name suggests, pneumonia acquired in the community (including retirement homes) rather than in hospital. Unlike influenza, C. pneumoniae is not seasonal in nature and does not “peak”in the winter months. Because C. pneumoniae is not easy to diagnose with certainty, there are neither precise figures for its incidence nor any definitive information about trends—that is, whether or not it is becoming more common. C.pneumoniae is not currently a notifiable infection.


AEROSOL: Particles of liquid or solid dispersed as a suspension in gas.

COMMUNITY-ACQUIRED INFECTION: Community-acquired infection is an infection that develops outside of a hospital, in the general community. It differs from hospital-acquired infections in that those who infected are typically in better health than hospitalized people.

NOTIFIABLE DISEASE: A disease that the law requires must be reported to health officials when diagnosed; also called a reportable disease.

PATHOGEN: A disease-causing agent, such as a bacteria, virus, fungus, etc.

Treatment and Prevention

Treatment of C. pneumoniae includes antibiotics in the tetracycline, macrolide, or fluoroquinolone classes. Like all infections spread by hand-to-hand contact or aerosol exposure, the best approach to prevention is frequent and thorough handwashing.

Impacts and Issues

C. pneumoniae is a significant contributor to CAP, which is a major public health problem. In the United States, pneumonia is the leading cause of death due to infectious disease and the sixth leading cause of death overall. Nearly half of all deaths from infection are caused by pneumonia and other respiratory infections, and most of these occur in people over the age of 65. There are 2–5 million cases of CAP in the United States each year, leading to around half a million hospitalizations, with associated healthcare costs. If a person with pneumonia is admitted to the hospital, the death rate from the disease goes up from one percent to 14%. For those admitted to intensive care, the mortality rate from pneumonia can be as high as 40%.

Research has suggested that C. pneumoniae may have other consequences for health. Atherosclerotic plaques are fatty deposits that are found lining the inner walls of the coronary and carotid arteries, the vessels serving the heart and brain, of those with heart disease. C. pneumoniae infection has been located within these plaques, possibly because the bacterium can infect many of the cells that make up the deposits. Research indicates that C. pneumoniae may aid in creating the inflammation and immune reaction within blood vessel walls that contribute to heart attacks and strokes. There is no definitive evidence, as yet, that a C. pneumoniae infection actually causes heart disease, merely that it is associated with it. There have been several clinical trials aimed at testing whether antibiotics can prevent heart disease by wiping out C. pneumoniae infection. So far it appears that anti-biotic therapy does not reduce overall mortality from heart disease or the overall rate of heart attack or stroke. The potential role of C. pneumoniae in heart disease continues to be explored, while significantly and for the first time, evidence points to an infectious agent as a risk factor for the number one killer in the U.S.

Other research has suggested that C. pneumoniae could also play a role in Alzheimer's disease, asthma, and arthritis.


The bacterium Chlamydia pneumoniae is one of the Chlamydia genus, a group that also includes C. trachomatis and C. psittaci. Together, these species are among the most common microbial pathogens (disease-causing organisms) in the world, although C. pneumoniae was only identified as such in 1983. A name change to Chlamydophila pneumoniae has been proposed for the bacterium to highlight its distinctiveness from other Chlamydia species associated with sexually transmitted disease (e.g. chlamydia infection).

See AlsoChlamydia Infection; Pneumonia; Psittacosis.



Gates, Robert H. Infectious Disease Secrets, 2nd ed. Philadelphia: Hanley and Beltus, 2003.

Wilson, Walter R., and Merle A. Sande. Current Diagnosis & Treatment in Infectious Diseases. New York: McGraw Hill, 2001.


Andraws, R., J.S. Berger, and D.L. Brown. “Effects of Antibiotic Therapy on Outcomes of Patients with Coronary Artery Disease: A Meta-analysis of Randomized Controlled Trials.” Journal of the American Medical Association. no. 293 (2005): 2641–2647.

Web Sites

Centers for Disease Control and Prevention (CDC). “Chlamydia pneumoniae.” Oct 6, 2005. <> (accessed Jan 30, 2007).

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