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Pneumonia, an infection of the lung parenchyma, is classified as community-acquired (CAP) or health care–associated (HCAP). The HCAP category is subdivided into hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP). HCAP is associated with current hospitalization for ≥48 h, hospitalization for ≥2 days in the prior 3 months, residence in a nursing home or extended-care facility, antibiotic therapy in the preceding 3 months, chronic dialysis, home infusion therapy, home wound care, and contact with a family member who has a multidrug-resistant (MDR) infection.
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Microorganisms gain access to the lower respiratory tract via microaspiration from the oropharynx (the most common route), inhalation of contaminated droplets, hematogenous spread, or contiguous extension from an infected pleural or mediastinal space.
Before disease manifests, the size of the organism burden must overcome the ability of macrophages and other components of innate immunity (e.g., surfactant proteins A and D) to clear bacteria.
Classic pneumonia (typified by that due to Streptococcus pneumoniae) presents as a lobar pattern and evolves through four phases characterized by changes in the alveoli:
– Edema: Proteinaceous exudates are present in the alveoli.
– Red hepatization: Erythrocytes and neutrophils are present in the intraalveolar exudate.
– Gray hepatization: Neutrophils and fibrin deposition are abundant.
– Resolution: Macrophages are the dominant cell type.
In VAP, respiratory bronchiolitis can precede a radiologically apparent infiltrate.
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COMMUNITY-ACQUIRED PNEUMONIA
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Although many bacteria, viruses, fungi, and protozoa can cause CAP, most cases are caused by relatively few pathogens. In >50% of cases, a specific etiology is never determined.
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Typical bacterial pathogens include S. pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and gram-negative bacteria such as Klebsiella pneumoniae and Pseudomonas aeruginosa.
Atypical organisms include Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella spp., and respiratory viruses (e.g., influenza viruses, adenoviruses, human metapneumovirus, respiratory syncytial viruses).
– A virus may be responsible for a large proportion of CAP cases that require hospital admission, even in adults.
– 10–15% of CAP cases are polymicrobial and involve a combination of typical and atypical organisms.
Involvement of anaerobes, which play a significant role in CAP only when aspiration precedes presentation by days or weeks, often results in significant empyemas.
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CAP affects >5 million adults each year in the United States, 80% of whom are treated on an outpatient basis. CAP causes >55,000 deaths annually and is associated with an overall yearly cost of ~$12 billion.
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Incidence rates of CAP are highest at the extremes of age (i.e., <4 and >60 years).
Risk factors for CAP include alcoholism, asthma, immunosuppression, institutionalization, and an age of ≥70 years (vs. 60–69 years).
Many factors—e.g., tobacco smoking, chronic obstructive pulmonary disease, colonization with methicillin-resistant S. aureus (MRSA), recent hospitalization or antibiotic therapy—influence the types of pathogens that should be considered in the ...