Community-acquired pneumonia develops in people with limited or no contact with medical institutions or settings. The most commonly identified pathogens are Streptococcus pneumoniae, Haemophilus influenzae, and atypical organisms (ie, Chlamydia pneumoniae, Mycoplasma pneumoniae, Legionella sp). Symptoms and signs are fever, cough, pleuritic chest pain, dyspnea, tachypnea, and tachycardia. Diagnosis is based on clinical presentation and chest x-ray. Treatment is with empirically chosen antibiotics. Prognosis is excellent for relatively young or healthy patients, but many pneumonias, especially when caused by S. pneumoniae or influenza virus, are fatal in older, sicker patients.

Etiology

Many organisms cause community-acquired pneumonia, including bacteria, viruses, and fungi. Pathogens vary by patient age and other factors (see Table 1: Pneumonia: Community-Acquired Pneumonia in Children and Table 2: Pneumonia: Community-Acquired Pneumonia in Adults ), but the relative importance of each as a cause of community-acquired pneumonia is uncertain, because most patients do not undergo thorough testing, and because even with testing, specific agents are identified in < 50% of cases.

Table 1
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Community-Acquired Pneumonia in Children Age Organisms Treatment Birth to 3 wk Group B streptococci, Listeria monocytogenes, gram-negative bacilli, cytomegalovirus Ampicillin (or nafcillin) and Gentamicin (or cefotaxime)* 3 wk to 3 mo Streptococcus pneumoniae, viruses (RSV, parainfluenza viruses, metapneumovirus), Bordetella pertussis, Staphylococcus aureus, Chlamydia trachomatis (transnatal exposure) Outpatient: Erythromycin 10 mg/kg IV q 6 h for 10−14 days Inpatient non-ICU: Cefuroxime 50 mg/kg IV q 8−12 h Inpatient ICU: Cefotaxime 66 mg/kg IV tid and Cloxacillin 50 mg/kg IV q 6 h 4 mo to 4 yr S. pneumoniae, viruses (RSV, parainfluenza viruses, influenza viruses, adenovirus, rhinovirus, metapneumovirus), Mycoplasma pneumoniae (in older children), group A streptococci Outpatient: Erythromycin 10 mg/kg po qid Inpatient: Erythromycin 10 mg/kg po qid and Cefuroxime 50 mg/kg IV q 8 h 5 to 15 yr S. pneumoniae, M. pneumoniae, Chlamydia pneumoniae Outpatient: Clarithromycin 500 mg po bid Inpatient: Ceftriaxone 50 mg/kg once/day IV (maximum 2 g) and Azithromycin 10 mg/kg once/day (maximum 500 mg) *For doses and discussion of neonatal pneumonia, see Infections in Neonates: Neonatal Pneumonia. RSV = respiratory syncytial virus. Data from McIntosh K: Community-acquired pneumonia in children. The New England Journal of Medicine 346:429–437, 2002.

Table 2
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Community-Acquired Pneumonia in Adults  Group Likely Organisms Empiric Treatment I. Outpatients—no modifying factors present† Streptococcus pneumoniae, Mycoplasma pneumoniae, Chlamydia pneumoniae, Haemophilus influenzae, respiratory viruses, miscellaneous (Legionella sp, Mycobacterium tuberculosis, endemic fungi) Macrolide (azithromycin 500 mg po once, then 250 mg once/day; clarithromycin 250 to 500 mg po bid; or extended-release clarithromycin 1 g once/day) or Doxycycline 100 mg po bid (if allergic to macrolide) II. Outpatients—modifying factors present† S. pneumoniae, including drug-resistant forms; M. pneumoniae; C. pneumoniae; mixed infection (bacteria + atypical pathogen or virus); H. influenzae; enteric gram-negative organisms; respiratory viruses; miscellaneous (Moraxella catarrhalis, Legionella sp, anaerobes [aspiration], M. tuberculosis, endemic fungi) β-Lactam (cefpodoxime 200 mg po q 12 h; cefuroxime 500 mg po q 12 h; amoxicillin 1 g q 8 h; amoxicillin/clavulanate 875/125 mg q 12 h) plus A macrolide po or Antipneumococcal fluoroquinolone‡ po or IV (alone) III. Inpatient—not in ICU S. pneumoniae, H. influenzae; M. pneumoniae; C. pneumoniae; mixed infection (bacteria + atypical pathogen or virus); respiratory viruses; Legionella sp, miscellaneous (M. tuberculosis, endemic fungi, Pneumocystis jirovecii) Azithromycin 500 mg IV q 24 h plus β-Lactam IV (cefotaxime 1 to 2 g q 8 to 12 h; ceftriaxone 1 g q 24 h) or Antipneumococcal fluoroquinolone‡ po or IV (alone) IVA. ICU patient—no Pseudomonas risk factors S. pneumoniae, including drug-resistant forms, Legionella sp, H. influenzae, enteric gram-negative organisms, Staphylococcus aureus, M. pneumoniae, respiratory viruses, miscellaneous (C. pneumoniae, M. tuberculosis, endemic fungi) β-Lactam IV (cefotaxime 1 to 2 g IV q 8 to 12 h; ceftriaxone 1 g IV q 24 h) plus either Antipneumococcal fluoroquinolone‡ IV or Azithromycin 500 mg IV q 24 h IVB. ICU patient—Pseudomonas risk factors present Same as previous plus Pseudomonas sp Antipseudomonal β-lactam§ or aztreonam (if allergic to or intolerant of β-lactams) 1 to 2 g q 8 h plus either Ciprofloxacin 400 mg IV q 12 h or levofloxacin 750 mg po or IV q 24 h Alternatively: Antipseudomonal β-lactam§ plus An aminoglycoside plus either Ciprofloxacin 400 mg IV q 12 h or levofloxacin 750 mg po or IV q 24 h *These guidelines do not apply to patients with immunosuppression, influenza, aspiration pneumonia, or health care–associated pneumonia. †Modifying factors: Increased risk of drug-resistant organisms: Age > 65, alcoholism, antibiotic within 3 mo, exposure to child in day care center, multiple coexisting illnesses. Increased risk of enteric gram-negative organisms: Antibiotic use within 3 mo, cardiopulmonary disease (including COPD and heart failure), multiple coexisting illnesses. Increased risk of Pseudomonas aeruginosa: Broad-spectrum antibiotics > 7 days in past month, corticosteroid use, undernutrition, structural pulmonary disease. ‡Antipneumococcal fluoroquinolones = levofloxacin 750 mg po or IV q 24 h or moxifloxacin 400 mg po or IV q 24 h. §Antipseudomonal β-lactams = cefepime 1 to 2 g IV q 12 h, imipenem 500 mg IV q 6 h, meropenem 500 mg to 1 g IV q 8 h, piperacillin/tazobactam 3.375 g IV q 4 h. Data from Mandell A, Wunderink R, Azueto A, et al: Infectious Disease Society of America and American Thoracic Society Guidelines for the management of adults with community-acquired pneumonia. Clinical Infectious Diseases 44:S27–S72, 2007.

S. pneumoniae, H. influenzae, C. pneumoniae, and M. pneumoniae are the most common bacterial causes. Pneumonia caused by chlamydia and mycoplasma are often clinically indistinguishable from other pneumonias. Common viral agents include respiratory syncytial virus (RSV), adenovirus, influenza viruses, metapneumovirus, and parainfluenza viruses. Bacterial superinfection can make distinguishing viral from bacterial infection difficult.

C. pneumoniae accounts for 2 to 5% of community-acquired pneumonia and is the 2nd most common cause of lung infections in healthy people aged 5 to 35 yr. C. pneumoniae is commonly responsible for outbreaks of respiratory infection within families, in college dormitories, and in military training camps. It causes a relatively benign form of pneumonia that infrequently requires hospitalization. Chlamydia psittaci pneumonia (psittacosis) is rare and occurs in patients who own or are often exposed to birds.

A host of other organisms causes lung infection in immunocompetent patients, although the term community-acquired pneumonia is usually reserved for the more common bacterial and viral etiologies.

Q fever, tularemia, anthrax, and plague are uncommon bacterial syndromes in which pneumonia may be a prominent feature; the latter three should raise the suspicion of bioterrorism.

Adenovirus, Epstein-Barr virus, and coxsackievirus are common viruses that rarely cause pneumonia. Varicella virus and hantavirus cause lung infection as part of adult chickenpox and hantavirus pulmonary syndrome; a coronavirus causes severe acute respiratory syndrome (SARS—see Respiratory Viruses: Coronaviruses and Severe Acute Respiratory Syndrome (SARS)).

Common fungal pathogens include Histoplasma capsulatum (histoplasmosis) and Coccidioides immitis (coccidioidomycosis). Less common fungi include Blastomyces dermatitidis (blastomycosis) and Paracoccidioides braziliensis (paracoccidioidomycosis). Pneumocystis jirovecii commonly causes pneumonia in patients who have HIV infection or are immunosuppressed.

Parasites causing lung infection in developed countries include Toxocara canis or T. catis (visceral larva migrans), Dirofilaria immitis (dirofilariasis), and Paragonimus westermani (paragonimiasis). (For a discussion of pulmonary TB or of specific microorganisms, see Mycobacteria.)

Symptoms and Signs

Symptoms include malaise, cough, dyspnea, and chest pain. Cough typically is productive in older children and adults and dry in infants, young children, and the elderly. Dyspnea usually is mild and exertional and is rarely present at rest. Chest pain is pleuritic and is adjacent to the infected area. Pneumonia may manifest as upper abdominal pain when lower lobe infection irritates the diaphragm. Symptoms become variable at the extremes of age. Infection in infants may manifest as nonspecific irritability and restlessness; in the elderly, as confusion and obtundation.

Signs include fever, tachypnea, tachycardia, crackles, bronchial breath sounds, egophony, and dullness to percussion. Signs of pleural effusion may also be present (see Mediastinal and Pleural Disorders: Symptoms and Signs). Nasal flaring, use of accessory muscles, and cyanosis are common among infants. Fever is frequently absent in the elderly.

Symptoms and signs were previously thought to differ by type of pathogen, but presentations overlap considerably. In addition, no single symptom or sign is sensitive or specific enough to predict the organism. Symptoms are even similar for noninfective lung diseases such as pulmonary embolism, pulmonary cancer, and other inflammatory lung diseases.

Diagnosis

• Chest x-ray
• Consideration of pulmonary embolism
• Sometimes identification of pathogen

Diagnosis is suspected on the basis of clinical presentation and is confirmed by chest x-ray (see Table 3: Pneumonia: Probability of Pneumonia Given Chest X-ray Infiltrate). The most serious condition misdiagnosed as pneumonia is pulmonary embolism, which may be more likely in patients with minimal sputum production, no accompanying URI or systemic symptoms, and risk factors for thromboembolism (see Table 1: Pulmonary Embolism: Risk Factors for Deep Venous Thrombosis and Pulmonary Embolism).

Table 3
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Probability of Pneumonia Given Chest X-ray Infiltrate Assign 1 point each for Temperature > 37.8°C Heart rate > 100 beats/min Crackles on auscultation Decreased breath sounds Absence of asthma Score Likelihood Ratio Probability of Pneumonia* 0–1 0.3 ≤ 1% 2–3 — 3–10% 4–5 8.2 25–50% *Based on baseline prevalence (pre-test probability) of 5%. Data from Heckerling PS, Tape TG, Wigton RS, et al: Clinical prediction rule for pulmonary infiltrates. Annals of Internal Medicine 113: 664–670, 1990.

Chest x-ray almost always shows some degree of infiltrate; rarely, an infiltrate is absent in the first 24 to 48 h of illness. In general, no specific findings distinguish one type of infection from another, although multilobar infiltrates suggest S. pneumoniae or Legionella pneumophila infection and interstitial pneumonia suggests viral or mycoplasmal etiology.

Hospitalized patients (see Pneumonia: Hospital-Acquired Pneumonia) should undergo WBC count and electrolytes, BUN, and creatinine testing to classify risk and hydration status. Two sets of blood cultures are often obtained to detect pneumococcal bacteremia and sepsis, because about 12% of all patients hospitalized with pneumonia have bacteremia; S. pneumoniae accounts for two thirds of these cases. Whether the results of blood cultures alter therapy commonly enough to warrant the expense is under study. Pulse oximetry or ABG should also be done.

Pathogens

Attempts to identify a pathogen are not routinely indicated; exceptions may be made for critically ill patients, patients in whom a drug-resistant or unusual organism is suspected (eg, TB, P. jirovecii), and patients who are deteriorating or not responding to treatment within 72 h.

The use of Gram stain and culture of sputum for diagnosis is of uncertain benefit, because specimens often are contaminated and because overall diagnostic yield is low. Samples can be obtained noninvasively by simple expectoration or after hypertonic saline nebulization for those unable to produce sputum. Alternatively, patients can undergo bronchoscopy or endotracheal suctioning, either of which can be easily done through an endotracheal tube in mechanically ventilated patients. Testing should include mycobacterial and fungal stains and cultures in patients whose condition is deteriorating and in those unresponsive to broad-spectrum antibiotics.

Left Lower Lobe Infiltrate

Multilobar Pneumonia

Interstitial Opacities

Additional tests are indicated in some circumstances. Patients at risk of Legionella pneumonia (eg, patients who smoke, have chronic pulmonary disease, are > 40, receive chemotherapy, or take immunosuppressants for organ transplantation) should undergo testing for urinary Legionella antigen, which remains present long after treatment is initiated, but the test detects only L. pneumophila serogroup 1 (70% of cases). A 4-fold rise in antibody titers to ≥ 1:128 (or a single titer of ≥ 1:256 in a convalescent patient) is also considered diagnostic. These tests are specific (95 to 100%) but are not very sensitive (40 to 60%); thus, a positive test indicates infection, but a negative test does not exclude it.

Infants and young children with possible RSV infection should undergo rapid antigen testing of specimens obtained with nasal or throat swabs. No other tests for viral pneumonias are done; viral culture and serologic tests are rarely clinically warranted.

PCR testing for mycoplasma and chlamydia species, although not widely available, holds promise as a highly sensitive and specific rapid diagnostic test and is likely to play a greater role as PCR technologies are refined.

Prognosis

Candidates for outpatient treatment usually improve over 24 to 72 h. Hospitalized patients may improve or deteriorate depending on comorbidities. Aspiration is a major risk factor for death, as are older age and number and type of comorbidities. Pneumonia caused by certain organisms may also increase the risk of death. Death may be caused by pneumonia itself, progression to sepsis syndrome affecting other organs, or exacerbation of comorbidities.

Pneumococcal infection accounts for about two thirds of fatal cases of community-acquired pneumonia in which an etiologic agent is known. The overall mortality rate in hospitalized patients is about 12%. Poor prognostic factors include age < 1 or > 60 yr; involvement of > 1 lobe; peripheral WBC count < 5000/μL; comorbidities (eg, heart failure, alcoholism, hepatic or renal insufficiency), immunosuppression (eg, agammaglobulinemia, anatomic or functional asplenia), infection with serotypes 3 and 8; and hematogenous spread with either positive blood cultures or extrapulmonary complications (usually arthritis, meningitis, endocarditis). Infants and children are at special risk of pneumococcal otitis media, bacteremia, and meningitis.

Mortality in Legionella infection is 10 to 20% among community-acquired cases and is higher among immunosuppressed or hospitalized patients. Patients who respond do so slowly, and x-ray abnormalities usually persist for ≥ 1 mo. Most patients require hospitalization, many require ventilator support, and 10 to 20% die despite appropriate antibiotic therapy.

Prognosis in mycoplasma pneumonia is excellent; nearly all patients recover.

Chlamydophila pneumoniae responds more slowly to treatment than mycoplasma pneumonia and tends to recur if therapy is stopped prematurely. Young adults with C. pneumoniae usually do well, but the elderly have a mortality rate of 5 to 10%.

Treatment

• Risk stratification
• Antibiotics
• Antivirals for influenza or varicella
• Supportive measures

A prediction rule may be used to estimate mortality risk. The rule has been used to identify those patients who can be safely treated as outpatients and those who require hospitalization because of high risk of complications (see Table 4: Pneumonia: Risk Stratification for Community-Acquired Pneumonia). However, the rule was not developed to determine site of care. Thus, the rule should supplement, not replace, clinical judgment, because many unrepresented factors, such as likelihood of adherence, ability to care for self, and wishes to avoid hospitalization, should also influence triage decisions.Also, certain criteria that extend across a continuum of severity have dichotomous cutoffs; eg, a heart rate of 124 beats/min may indicate distress, but points are not assigned unless heart rate is ≥ 125 beats/min. ICU admission is required for patients who need mechanical ventilation and for those with hypotension (systolic BP < 90 mm Hg) that is unresponsive to volume resuscitation. Other criteria that mandate consideration for ICU admission include respiratory rate > 30/min, Pao₂/fraction of inspired O₂ (Fio₂) < 250, multilobar pneumonia, diastolic BP < 60 mm Hg, confusion, and BUN > 19.6 mg/dL.

Clinical Calculator

Appropriate treatment involves starting antibiotics as soon as possible, preferably ≤ 8 h after presentation. Supportive care includes fluids, antipyretics, analgesics, and, for patients with hypoxemia, O₂.

Table 4
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Risk Stratification for Community-Acquired Pneumonia Factor Points Age Men Women Age (in yr) Age(in yr) − 10 Nursing home resident 10 Coexisting illness Cancer 30 Liver disease 20 Heart failure 10 Cerebrovascular disease 10 Renal disease 10 Physical examination Altered mental status 20 Respiratory rate ≥ 30 20 Systolic BP < 90 mm Hg 20 Temperature ≥ 40° or < 35°C 15 Heart rate ≥ 125 beats/min 10 Test results Arterial pH < 7.35 30 BUN ≥ 30 mg/dL (11 mmol/L) 20 Na < 130 mmol/L 20 Glucose ≥ 250 mg/dL (14 mmol/L) 10 Hct < 30% 10 Pao2 < 60 mm Hg or O2sat < 90%* 10 Pleural effusion 10 Points Mortality Recommendation ≤ 70 < 1% Outpatient treatment† 71−90 < 5% Outpatient treatment† 91−130 5−15% Admit > 130 > 15% Admit *Many consider hypoxemia an absolute indication for admission. †Acute care admission, subacute care admission, observation period, home IV antibiotics, or home nursing visits should be considered for patients who are frail, isolated, or living in unstable environments. Adapted from Pneumonia: New prediction model proves promising (AHCPR Publication No. 97-R031). Risk Stratification for Community-Acquired Pneumonia Factor Points Age Men Women Age (in yr) Age(in yr) − 10 Nursing home resident 10 Coexisting illness Cancer 30 Liver disease 20 Heart failure 10 Cerebrovascular disease 10 Renal disease 10 Physical examination Altered mental status 20 Respiratory rate ≥ 30 20 Systolic BP < 90 mm Hg 20 Temperature ≥ 40° or < 35°C 15 Heart rate ≥ 125 beats/min 10 Test results Arterial pH < 7.35 30 BUN ≥ 30 mg/dL (11 mmol/L) 20 Na < 130 mmol/L 20 Glucose ≥ 250 mg/dL (14 mmol/L) 10 Hct < 30% 10 Pao2 < 60 mm Hg or O2sat < 90%* 10 Pleural effusion 10 Points Mortality Recommendation ≤ 70 < 1% Outpatient treatment† 71−90 < 5% Outpatient treatment† 91−130 5−15% Admit > 130 > 15% Admit *Many consider hypoxemia an absolute indication for admission. †Acute care admission, subacute care admission, observation period, home IV antibiotics, or home nursing visits should be considered for patients who are frail, isolated, or living in unstable environments. Adapted from Pneumonia: New prediction model proves promising (AHCPR Publication No. 97-R031).

Because organisms are difficult to identify, antibiotics are selected based on likely pathogens and severity of illness. Consensus guidelines have been developed by many professional organizations; one widely used set is detailed in Table 2: Pneumonia: Community-Acquired Pneumonia in Adults . Guidelines should be adapted to local susceptibility patterns, drug formularies, and individual patient circumstances. Importantly, none provide recommendations for treatment of viral pneumonia.

Ribavirin and RSV Ig have been used alone and in combination for RSV bronchiolitis in children, but their effectiveness is controversial, and neither is standard practice. Ribavirin is not used in adults with RSV infection. Oseltamivir 75 mg po bid or zanamivir 10 mg inhaled bid started within 48 h of symptom onset and given for 5 days reduces the duration and severity of symptoms in patients who develop influenza infection. Acyclovir 5 to 10 mg/kg IV q 8 h for adults or 250 to 500 mg/m² body surface area IV q 8 h for children is recommended for varicella lung infections. Some patients with viral pneumonia, especially those with influenza, develop superimposed bacterial infections and require antibiotics directed against S. pneumoniae, H. influenzae, and Staphylococcus aureus.

With empiric treatment, 90% of patients with bacterial pneumonia improve. Improvement is manifested by decreased cough and dyspnea, defervescence, relief of chest pain, and decline in WBC count. Failure to improve should trigger suspicion of an unusual organism, resistance to the antimicrobial used for treatment, empyema, coinfection or superinfection with a 2nd infectious agent, an obstructive endobronchial lesion, immunosuppression, metastatic focus of infection with reseeding (in the case of pneumococcal infection), or nonadherence to treatment (in the case of outpatients). If none of these can be proved, treatment failure is likely due to inadequate host defenses.

Most viral pneumonias resolve without specific treatment.

Chest physical therapy can be used to treat pneumonia; however, there is no clear evidence for its efficacy. Follow-up x-rays should be obtained 6 wk after treatment in patients > 35; persistence of an infiltrate at ≥ 6 wk raises suspicions of an underlying, possibly malignant endobronchial lesion or of TB.

Prevention

Some forms of community-acquired pneumonia are preventable with pneumococcal conjugate vaccine (for patients < 2 yr), H. influenzae type b (Hib) vaccine (for patients < 2 yr), pneumococcal pneumonia vaccine (for patients at high risk, such as those with underlying heart, lung, or immune system disorders), varicella vaccine (for patients < 18 mo and a later booster vaccine), and influenza vaccine (for patients age ≥ 65 and those at high risk—see Immunization: Common Vaccinations for Adults and see Table 12: Approach to the Care of Normal Infants and Children: Recommended Immunization Schedule for Ages 0–6 yr). Oseltamivir 75 mg po once/day or zanamivir 10 mg once/day can be given for 2 wk to prevent influenza (although resistance has recently been described for oseltamivir) for household contacts of patients with influenza and to high-risk patients not vaccinated against influenza during influenza epidemics. Pneumococcal pneumonia vaccination is recommended for all patients ≥ 65 (see Immunization: Pneumococcal Disease).

Last full review/revision May 2008 by John G. Bartlett, MD