Chlamydia pneumoniae (C. pneumoniae) is a common species of bacteria and a major cause of pneumonia around the world. Approximately 50% of adults have evidence of past infection by age 20, and reinfection later in life is common. Many studies have suggested a direct association between C. pneumoniae infection and other inflammatory diseases such as atherosclerosis, acute exacerbations of COPD, and asthma.
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Chlamydia pneumoniae are Gram-negative, aerobic, intracellular pathogens that have a very unique life cycle. The organism alternates between an infectious but non-replicating elementary body that is adapted to survive outside the host, and a non-infectious, reticulate body that reproduces inside host cells by means of transverse fission. The bacteria cannot survive for long outside host cells and as a result, it is difficult to diagnose by gram-staining. Furthermore, only in the reticulate stage is the pathogen susceptible to antibiotic therapy, making it difficult to treat. Most cases of C. pneumoniae infection cause no symptoms, or mild upper and lower respiratory tract infections. In school-age children, it is emerging as a frequent cause of mild pneumonia, and up to 10% of cases of community-acquired pneumonia can be attributed to this pathogen. Transmission occurs person to person via respiratory secretions and symptomatic patients can carry the bacteria in the nasopharynx for months after illness. Seroepidemiologic studies have shown an association between C. pneumoniae infection and atherosclerosis but the significance of this is not yet established. Possible links with Alzheimer’s disease, arthritis, and asthma are also postulated.
Diagnosis of C. pneumoniae infection is challenging due to the fastidious nature of the pathogen, the considerable seroprevalence, and the possibility of transient asymptomatic carriage. Established diagnostic laboratory methods include isolation of the organism in cell culture, serological assays and PCR. Microimmunofluorescence test (MIF), is the current “gold standard” for serological diagnosis, but the assay still lacks standardization and is technically challenging. Antibody immunoassays are the most common serology tests used and primary chlamydial infection is characterized by a predominant IgM response within 2 to 4 weeks and a delayed IgG and IgA response within 6 to 8 weeks. However, in reinfection, IgG and IgA levels rise quickly, often in 1-2 weeks whereas IgM levels may rarely be detected. For this reason, IgA antibodies have shown to be a reliable immunological marker of primary, chronic and recurrent infections especially when combined with the detection of IgM.
The chlamydia antigen test is not recommended for diagnosis of acute C. pneumoniae infection due to cross-reactivity with other Chlamydia species.
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