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Chlamydia pneumoniae is a human respiratory tract pathogen. Seroepidemiological studies indicate that C. pneumoniae infection is most common in school-aged children and infrequently detected in younger children.

The aims of this study were to further elucidate the prevalence of C. pneumoniae in paediatric populations and to describe the clinical implications of these infections.

The study population consisted of 367 children with respiratory tract diseases, 453 presumed healthy children at day-care, 69 children undergoing adenoidectomy and 1585 children from a population based cohort. Family members to infected day-care children were investigated. The laboratory methods used were polymerase chain reaction (PCR) on specimen from upper respiratory tract, serology by microimmunofluorescence (MIF), and immunohistochemistry (IHC) on adenoid tissue specimen. Personal data and medical history were obtained by the means of questionnaires and by the study of patient records.

In children younger than five years, the prevalence of C. pneumoniae was 17% as detected by PCR. This prevalence started to increase with increasing age from two years of age. The corresponding increase in serology as detected by MIF started at the age of four years. The prevalence at day-care centres varied from 4 to 39%. Both PCR and MIF underestimated the prevalence of C. pneumoniae detected by IHC. Families to infected children were investigated: mothers were more often infected than fathers were.

Most C. pneumoniae infections in small children were confined to the upper respiratory tract. These infections were usually mild or asymptomatic. Symptomatic disease may be of prolonged nature. No subsequent illness after C. pneumoniae infection was detected at follow-up after four years. In general, no association between C. pneumoniae and asthma was found, but C. pneumoniae may be of importance for asthma in some susceptible individuals. Previous C. pneumoniae infection reduced the risk for later atopy.

In conclusion, C. pneumoniae is a common finding in small children and most often causes relatively mild disease. If the acquisition of this infection early in life will have any implications for future health remains to be investigated.