Community-acquired pneumonia

Background

• See also Respiratory infection in an immunocompromised person and Respiratory infection in unimmunized hosts

Microbiology

• Bacteria (15-30%)1
  • Typical organisms
    • Streptococcus pneumoniae (5-15%)
    • Haemophilus influenzae (1-7%)
    • Staphylococcus aureus (2-5%)
    • Group A streptococcus
    • Moraxella catarrhalis
    • Anaerobes and aerobic gram-negative bacteria
      • Klebsiella pneumonia
      • Pseudomonas aeruginosa (1-3%)
      • Acinetobacter baumannii
  • Atypical organisms
    • Legionella species (1-6%)
    • Mycoplasma pneumoniae (1-10%)
    • Chlamydia pneumoniae (1-3%)
    • Chlamydia psittaci
  • Others
    • Coxiella burnetti
    • Nocardia
    • Mycobacteria
• Viruses (25-60%)
  • Rhinovirus (8-17%)
  • Coronavirus (3-13%)
  • Influenza virus (1-6%)
  • Human metapneumovirus (1-8%)
  • Parainfluenza virus (1-8%)
  • RSV (1-8%)
  • Adenovirus (1-8%)
• Fungi (1-3%)
  • Pneumocystis jirovecii
  • Cryptococcus species
  • Histoplasma capsulatum
  • Coccidioides
• No cause identified (46-66%)
• Clinically-important but uncommon organisms
  • Legionella species
  • Influenza A and B, including avian influenza A H5N1 and avian influenza A H7N9
  • MERS-CoV and severe acute respiratory syndrome coronavirus (SARS-CoV)
  • Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA)

Clinical Manifestations

• Acute onset cough, dyspnea, and fever
• If effusion, consider Legionella

Severity

• Per IDSA guidelines2, severe CAP includes either one major criterion or three or more minor criteria
• Minor criteria
  • Respiratory rate ≥ 30 breaths/min
  • PaO2/FiO2 ratio ≤ 250
  • Multilobar infiltrates
  • Confusion/disorientation
  • Uremia (blood urea nitrogen level ≥ 20 mg/dl)
  • Leukopenia not due to chemotherapy (white blood cell count < 4,000 cells/μl)
  • Thrombocytopenia (platelet count < 100,000/μl)
  • Hypothermia (core temperature < 36°C)
  • Hypotension requiring aggressive fluid resuscitation
• Major criteria
  • Septic shock with need for vasopressors
  • Respiratory failure requiring mechanical ventilation

Investigations

• Always add Legionella testing in immunocompromised, critical illness, recent travel, significant alcohol, and consider when pleural effusion is present

Further Reading

• IDSA CAP guidelines 2007
• HQO CAP guidelines 2014