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Chlamydia pneumoniae
Chlamydia pneumoniae is a common cause of lower respiratory tract diseases, including pneumonia in children and bronchitis and pneumonia in adults.
Etiology
Chlamydiae are obligate intracellular pathogens that have established a unique niche in host cells. Chlamydiae cause a variety of diseases in animal species at virtually all phylogenic levels. The most significant human pathogens are C. pneumoniae and Chlamydia trachomatis (see Chapter 253 ). Chlamydia psittaci is the cause of psittacosis, an important zoonosis (see Chapter 254 ). There are now 9 recognized chlamydial species.
Chlamydiae have a gram-negative envelope without detectable peptidoglycan, although recent genomic analysis has revealed that both C. pneumoniae and C. trachomatis encode proteins forming a nearly complete pathway for synthesis of peptidoglycan, including penicillin-binding proteins. Chlamydiae also share a group-specific lipopolysaccharide antigen and use host adenosine triphosphate for the synthesis of chlamydial proteins. Although chlamydiae are auxotrophic for 3 of 4 nucleoside triphosphates, they encode functional glucose-catabolizing enzymes that can be used to generate adenosine triphosphate. As with peptidoglycan synthesis, for some reason these genes are turned off. All chlamydiae also encode an abundant surface-exposed protein called the major outer membrane protein. The major outer membrane protein is the major determinant of the serologic classification of C. trachomatis and C. psittaci isolates.
Epidemiology
C. pneumoniae is primarily a human respiratory pathogen. The organism has also been isolated from nonhuman species, including horses, koalas, reptiles, and amphibians, where it also causes respiratory infection, although the role that these infections might play in transmission to humans is unknown. C. pneumoniae appears to affect individuals of all ages. The proportion of community-acquired pneumonias associated with C. pneumoniae infection is 2–19%, varying with geographic location, the age group examined, and the diagnostic methods used. Several studies of the role of C. pneumoniae in lower respiratory tract infection in pediatric populations have found evidence of infection in 0–18% of patients based on serology or culture for diagnosis. In 1 study, almost 20% of the children with C. pneumoniae infection were coinfected with Mycoplasma pneumoniae. C. pneumoniae may also be responsible for 10–20% of episodes of acute chest syndrome in children with sickle cell disease, up to 10% of asthma exacerbations, 10% of episodes of bronchitis, and 5–10% of episodes of pharyngitis in children. Asymptomatic infection appears to be common based on epidemiologic studies.
Transmission probably occurs from person to person through respiratory droplets. Spread of the infection appears to be enhanced by close proximity, as is evident from localized outbreaks in enclosed populations, such as military recruits and in nursing homes.
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