Epidemiology of Chlamydia pneumoniae.

Chlamydia pneumoniae is the most commonly occurring intracellular bacterial pathogen. It is frequently involved in respiratory tract infections and to a lesser degree in extrapulmonary diseases. According to seroepidemiologic surveys, C. pneumoniae infection seems to be both endemic and epidemic. Such studies indicate that C. pneumoniae infection is widespread, with frequent reinfection during a lifetime. In Western countries the highest rate of new infections occurs between the ages of 5 and 15. The antibody prevalence worldwide is higher in adult males than in females. Currently available data suggest that C. pneumoniae is primarily transmitted from human to human without any animal reservoir. Transmission seems to be inefficient, although household outbreaks with high transmission rates are reported. Most reports rank C. pneumoniae among the three most common etiologic agents of community-acquired pneumonia, with an incidence ranging from 6% to 25%, and generally presenting a mild and, in some cases, self-limiting clinical course. Recent reports also indicate a possible role for C. pneumoniae in severe forms of community-acquired pneumonia and in respiratory infections in immunocompromised patients. C. pneumoniae infection has also been implicated in the pathogenesis of asthma in both adults and children. The hypothesis that C. pneumoniae infection could lead to asthma is based on clinical studies and on the evidence of specific IgE production, direct epithelial damage, induction of T-cell immunopathologic diseases, and vascular smooth cell infection. Chronic C. pneumoniae infection seems to be common in patients with chronic bronchitis whether exacerbated or not, and is characterized by a strong humoral immune response to this intracellular microorganism, which is present in the majority of patients with severe chronic bronchitis. More than 60% of subjects with chronic bronchitis have specific C. pneumoniae antibody titers, and the microorganism may be identified by culture or PCR in almost 40% of these patients. This pathogen has also been recently associated with atherosclerosis and coronary heart disease (CHD). Seroepidemiological evidence indicates that the majority of patients with CHD present an anti-C. pneumoniae antibody pattern consistent with chronic infection. Furthermore, C. pneumoniae has been detected in atherosclerotic coronary plaques by several methods, including immunocytochemistry, transmission electron microscopy and molecular biology techniques. Recently, we detected C. pneumoniae DNA in a high percentage (51%) of aortic aneurysm plaques. Moreover, our serologic data support the hypothesis that a chronic C. pneumoniae antibody pattern may be a possible risk marker for atherosclerosis. Recently, C. pneumoniae has been isolated by culture from the coronary artery of a patient with coronary atherosclerosis, providing direct evidence of the presence of viable organisms in atheromatous lesions. Moreover, we recently demonstrated an association between C. pneumoniae reinfection and acute myocardial infarction.