What causes community-acquired pneumonia (CAP)?

Updated: Oct 28, 2019
  • Author: Fariba M Donovan, MD, PhD; Chief Editor: Thomas E Herchline, MD  more...
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A number of pathogens can give rise to CAP, generally categorized into typical and atypical pathogens.

Typical bacterial pathogens that cause CAP include Streptococcus pneumonia (penicillin-sensitive/resistant strains), Haemophilus influenza (ampicillin-sensitive/resistant strains), and Moraxella catarrhalis (all strains penicillin-resistant) and account for approximately 85% of CAP cases. CAP is usually acquired via inhalation or aspiration of a pulmonary pathogen into a lung segment or lobe. Less commonly, CAP results from secondary bacteremia from a distant source, such as Escherichiacoli urinary tract infection and/or bacteremia. Aspiration pneumonia is the only form of CAP caused by multiple pathogens (eg, aerobic/anaerobic oral organisms). Klebsiella pneumonia CAP occurs primarily in persons with chronic alcoholism and Staphylococcalaureus may cause CAP in patients with influenza. Pseudomonas aeruginosa is a cause of CAP in patients with bronchiectasis or cystic fibrosis. [1]

Atypical pathogen CAP manifests a variety of pulmonary and extrapulmonary findings (eg, CAP plus diarrhea). Atypical CAP can be divided into those caused by either zoonotic or nonzoonotic atypical pathogens. Zoonotic atypical CAP pathogens include Chlamydophila (Chlamydia) psittaci (psittacosis), Coxiella burnetii (Q fever), and Francisella tularensis (tularemia). Nonzoonotic atypical CAP pathogens include Mycoplasma pneumoniae, Legionella species, and Chlamydia pneumoniae. These organisms account for approximately 15% of all CAP cases. [1]

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