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Definition
- Community-acquired pneumonia (CAP) is an infection of the alveoli, distal airways, and interstitium of the lungs that occurs outside the hospital setting.
- Characterized clinically by
- Fever, chills, cough, pleuritic chest pain, sputum production
- At least 1 opacity on chest radiography
- Manifests as 4 general patterns
- Lobar pneumonia: involvement of an entire lung lobe
- Bronchopneumonia: patchy consolidation in 1 or several lobes, usually in dependent lower or posterior portions centered around bronchi and bronchioles
- Interstitial pneumonia: inflammation of the interstitium, including the alveolar walls and connective tissue around the bronchovascular tree
- Miliary pneumonia: numerous discrete lesions due to hematogenous spread
Epidemiology
- Annual incidence: U.S.
- 800–1500 cases per 100,000 persons
- Affects 4 million adults
- ~20% require hospitalization.
- Mortality: 45,000 deaths
- Cost: $9–10 billion
- Age
- Incidence highest at extremes of age
- Overall: 12 cases per 1000 persons
- Children < 4 years of age: 12–18 cases per 1000
- Persons >60 years of age: 20 cases per 1000
- Sex
- Rate higher among men than among women
- Race
- More common among African Americans than among whites
- Seasonality
- More common during the winter months
Risk Factors
- Independent risk factors for CAP include:
- Alcoholism (relative risk [RR] 9)
- Asthma (RR 4.2)
- Immunosuppression (RR 1.9)
- Institutionalization
- Age > 70 years (RR 1.5 vs 60–69 years)
- Risk factors for pneumococcal pneumonia include:
- Dementia
- Seizures
- Congestive heart failure
- Cerebrovascular disease
- Tobacco smoking
- Alcoholism
- Chronic obstructive pulmonary disease (COPD)
- HIV infection
- Risk up to 40 times that in age-matched patients not infected with HIV
- Risk factors for invasive pneumococcal disease include:
- Male sex
- African-American race
- Chronic illness
- Current tobacco smoking (strongest independent predictor among immunocompetent young adults)
- Passive exposure to tobacco smoke
- Immunologic defects
- Multiple myeloma
- Nephrotic syndrome with low serum immune globulin levels
- Splenectomy
- HIV infection
- Others
- Risk factors for community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA)
- Native-American race
- Homeless youths
- Men who have sex with men
- Prison inmates
- Military recruits
- Children in day-care centers
- Athletes such as wrestlers
- Risk factors for Legionnaires’ disease include:
- Male sex
- Current tobacco smoking
- Diabetes
- Hematologic malignancy
- Cancer
- End-stage renal disease
- HIV infection
- Recent hotel stay or ship cruise
- Risk factors for gram-negative bacterial pneumonia (including that caused by Pseudomonas aeruginosa)
- Probable aspiration
- Previous hospital admission
- Previous antimicrobial treatment
- Bronchiectasis
- Neutropenia
- Comorbidities such as alcoholism, heart failure, or renal failure
- Alcohol use
- Heavy drinkers (i.e., those consuming > 100 g of ethanol per day for the preceding 2 years)
- Higher incidence of gram-negative bacterial pneumonia
- Worse clinical symptoms
- Require longer courses of IV antibiotic therapy than do nondrinkers
- More prolonged fever, slower resolution, and a higher rate of empyema have been noted in pneumococcal pneumonia patients with chronic alcoholism than in their nondrinking counterparts.
- The clinical entity designated ALPS—alcoholism, leukopenia, and pneumococcal sepsis—is associated with a mortality rate of 80%.
- Excessive alcohol use is an independent risk factor for the development of acute respiratory distress syndrome (ARDS).
- Epidemiologic risk factors suggesting possible causes of CAP
- Alcoholism
-
Streptococcus pneumoniae, oral anaerobes, Klebsiella pneumoniae, Acinetobacter spp., Mycobacterium tuberculosis
- COPD and/or smoking
-
Haemophilus influenzae, Pseudomonas aeruginosa, Legionella spp., S. pneumoniae, Moraxella catarrhalis, Chlamydophila pneumoniae
- Structural lung disease (e.g., bronchiectasis)
-
P. aeruginosa, Burkholderia cepacia, S. aureus
- Dementia, stroke, decreased level of consciousness
- Oral anaerobes, gram-negative enteric bacteria
- Lung abscess
- CA-MRSA, oral anaerobes, endemic fungi, M. tuberculosis, atypical mycobacteria
- Travel to Ohio or St. Lawrence river valleys
- Travel to southwestern U.S.
- Hantavirus, Coccidioides spp.
- Travel to Southeast Asia
-
Burkholderia pseudomallei, avian influenza virus
- Stay in hotel or on cruise ship in previous 2 weeks
- Local influenza activity
- Influenza virus, S. pneumoniae, S. aureus
- Exposure to bats or birds
- Exposure to birds
- Exposure to rabbits
- Exposure to sheep, goats, parturient cats
Etiology
- Most cases of CAP are caused by a few common respiratory pathogens, including:
-
S. pneumoniae
- Accounts for ~50% of all cases of CAP requiring hospital admission
-
H. influenzae
-
S. aureus
-
M. pneumoniae
-
C. pneumoniae
-
M. catarrhalis
-
Legionella
spp.
- Aerobic gram-negative bacteria
-
Influenza viruses
-
Adenoviruses
-
Respiratory syncytial virus (RSV)
- Other rare organisms
- Viral: hantavirus, Nipah virus, Hendra virus, metapneumovirus, severe acute respiratory syndrome (SARS) virus, coronavirus
- Nonviral:
Pneumocystis
,
Mycobacterium tuberculosis
, fungi, bioterrorism agents (e.g., those of Q fever, tularemia, anthrax, plague), etc.
- The relative frequency of these pathogens differs with the patient’s age and the severity of pneumonia.
- Data suggest that a virus may be responsible in up to 18% of cases of CAP that require admission to the hospital.
- ~10–15% of CAP cases are polymicrobial.
- Pathogenesis
- Microaspiration of oropharyngeal secretions colonized with pathogenic microorganisms (e.g.,
S. pneumoniae
,
H. influenzae
) is the most common route.
- Gross aspiration
- Central nervous system disorders that affect swallowing (e.g., stroke, seizures)
- Impaired consciousness (e.g., in alcoholism, IV drug use)
- Anesthesia or intubation
- Pathogens include anaerobic organisms and gram-negative bacilli.
- Anaerobes play a significant role only when an episode of aspiration has occurred days to weeks before presentation for pneumonia.
- Anaerobic pneumonias are often complicated by abscess formation and significant empyemas or parapneumonic effusions.
- Aerosolization (e.g., of M. tuberculosis, Legionella spp., viruses)
- Hematogenous spread (e.g., seeding of the lungs by S. aureus during endocarditis)
- Contiguous spread from another site
Associated Conditions
- Infections with encapsulated organisms such as S. pneumoniae, H. influenzae, and Neisseria meningitidis may suggest underlying immunodeficiency due to multiple myeloma, nephrotic syndrome, etc.
- Pneumococcal CAP is particularly common among patients with HIV infection.
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