A species first isolated in 1986 and currently recognized as a common cause of pneumonia, bronchitis, rhinosinusitis, and pharyngitis in both adults and children.
Chlamydia pneumoniae is responsible for about 25% of cases of acute bronchitis and 10% of community-acquired pneumonia. It may also play a role in the genesis of
cardiovascular disease and late-onset Alzheimer dementia. Like C. trachomatis and C. psittaci, this organism is an occasional cause of myocarditis and endocarditis. Elevated levels of antibody to C. pneumoniae are found in patients with acute myocardial infarction (MI) and in those showing severe atheroma formation at autopsy significantly more often than in control groups. The organism has been detected by immunocytochemistry, polymerase chain reaction, and electron microscopy in macrophages and smooth muscle cells of atheromatous plaques of the aorta, coronary arteries, and carotid arteries (surgical and autopsy specimens), but not in normal arteries. The incidence of acute infection in MI patients, as detected by throat culture, is higher than in the general public. A retrospective review of medical records of patients with acute MI showed that they were less likely than matched controls to have been treated during the preceding 3 years with tetracycline or quinolone antibiotics, which are active against C. pneumoniae. To date, however, prospective studies have not shown an association between the presence of IgG antibody to C. pneumoniae and an increased risk of atherothrombotic disease. The current body of evidence favors infection with C.
pneumoniae as one of several factors capable of initiating changes that culminate in atherosclerosis. Limited studies suggest that antibiotic treatment may reduce the risk of recurrent coronary events, but have not shown benefits in stable coronary artery disease. Antibody to C. pneumoniae is also found in patients with severe hypertension at about twice the incidence rate for the general public, and has been linked statistically to accelerated loss of lung function in patients with asthma. In addition, the organism has been detected in microglia and astroglia of the hippocampus and temporal cortex in patients with late-onset Alzheimer disease with much greater frequency than in normal brains.