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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, 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 and/or healthy patients, but many pneumonias, especially when caused by S. pneumoniae and 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
Community-Acquired Pneumonia in Children Age Organisms Treatment Birth to 3 wk Group B streptococci, Listeria monocytogenes, gram-negative bacilli, cytomegalovirus Ampicillin Some Trade Names OMNIPEN PRINCIPEN Click for Drug Monograph (or nafcillin Some Trade Names UNIPEN Click for Drug Monograph ) and Gentamicin Some Trade Names GARAMYCIN Click for Drug Monograph (or cefotaxime Some Trade Names CLAFORAN Click for Drug Monograph )* 3 wk to 3 mo Streptococcus pneumoniae, viral infection (RSV, parainfluenza, metapneumovirus), Bordetella pertussis, Staphylococcus aureus, Chlamydia trachomatis (transnatal exposure) Outpatient: Erythromycin Some Trade Names ERY-TAB ERYTHROCIN Click for Drug Monograph 10 mg/kg IV q 6 h for 10−14 days Inpatient non-ICU: Cefuroxime Some Trade Names CEFTIN ZINACEF Click for Drug Monograph 50 mg/kg IV q 8−12 h Inpatient ICU: Cefotaxime Some Trade Names CLAFORAN Click for Drug Monograph 66 mg/kg IV tid and Cloxacillin Some Trade Names No US trade name Click for Drug Monograph 50 mg/kg IV q 6 h 4 mo to 4 yr S. pneumoniae, viral infection (RSV, parainfluenza, influenza, adenovirus, rhinovirus, metapneumovirus), Mycoplasma pneumoniae (in older children), group A streptococci Outpatient Erythromycin Some Trade Names ERY-TAB ERYTHROCIN Click for Drug Monograph 10 mg/kg po qid Inpatient: Erythromycin Some Trade Names ERY-TAB ERYTHROCIN Click for Drug Monograph 10 mg/kg po qid and Cefuroxime Some Trade Names CEFTIN ZINACEF Click for Drug Monograph 50 mg/kg IV q 8 h 5 to 15 yr S. pneumoniae, M. pneumoniae, Chlamydia pneumoniae Outpatient: Clarithromycin Some Trade Names BIAXIN Click for Drug Monograph 500 mg po bid Inpatient: Ceftriaxone Some Trade Names ROCEPHIN Click for Drug Monograph 50 mg/kg once/day IV (maximum 2 g) and Azithromycin Some Trade Names ZITHROMAX Click for Drug Monograph 10 mg/kg once/day (maximum 500 mg) RSV = respiratory syncytial virus. *For doses and discussion of neonatal pneumonia, see Infections in Neonates: Neonatal Pneumonia. Data from McIntosh K: Community-acquired pneumonia in children. The New England Journal of Medicine 346:429–437, 2002.

Table 2
Community-Acquired Pneumonia in Adults  This table is presented as a PDF and requires the free Adobe PDF reader. Get Adobe Reader

S. pneumoniae , H. influenzae , C. pneumoniae, and M. pneumoniae are the most common bacterial causes. Chlamydia and mycoplasma are clinically indistinguishable from other causes. Common viral agents include respiratory syncytial virus (RSV), adenovirus, influenza, metapneumovirus, and para-influenza virus in children and influenza in the elderly. Bacterial superinfection can make distinguishing viral from bacterial infection difficult.

C. pneumoniae accounts for 5 to 10% 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) occurs in patients who own birds.

A host of other organisms cause 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 novel coronavirus causes severe acute respiratory syndrome (SARS—see Respiratory Viruses: Severe Acute Respiratory Syndrome (SARS)).

The most common fungal pathogens are Histoplasma capsulatum (histoplasmosis) and Coccidioides immitis (coccidioidomycosis). Less common fungi include Blastomyces dermatitidis (blastomycosis) and Paracoccidioides braziliensis (paracoccidioidomycosis).

Parasites causing lung infection in developed countries include Plasmodium sp (malaria), Toxocara canis or 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 in infants.

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 malignancy, and other inflammatory lung diseases.

Diagnosis

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 (PE): Risk Factors for Deep Venous Thrombosis and Pulmonary Embolism).

Table 3
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 (pretest 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 demonstrates 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, though multilobar infiltrates suggest S. pneumoniae or Legionella pneumophila infection, and interstitial pneumonia suggests viral or mycoplasma 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 ²⁄₃ of these cases. Whether the results of blood cultures actually alter therapy commonly enough to warrant the expense is under study. Pulse oximetry or ABG should also be performed.

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), and patients who are deteriorating or not responding to treatment within 72 h. Sputum Gram stain and culture are controversial, 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 performed through an endotracheal tube in mechanically ventilated patients. Testing should include mycobacterial and fungal stains and cultures in deteriorating patients 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 urinary Legionella antigen testing, which stays positive long after treatment is initiated but detects only L. pneumophila serogroup 1 (70% of cases). A 4-fold rise in antibody titers to ≥ 1 : 128 (or a single convalescent serum of ≥ 1 : 256) 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 nasal or throat swabs. No other tests for viral pneumonias exist; 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.

A test for SARS-associated coronavirus exists, but its role in clinical practice is unknown, and its application is limited outside of known outbreaks (see Respiratory Viruses: Severe Acute Respiratory Syndrome (SARS)). In rare situations, anthrax should be considered (see Gram-Positive Bacilli: Anthrax).

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 is older age, number and type of comorbidities, and certain infectious agents. Death may be caused by pneumonia itself, progression to sepsis syndrome affecting other organs, or exacerbation of underlying comorbidities.

Pneumococcal infection still accounts for about 66% of all 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 more than one lobe; peripheral WBC count < 5000/μL; comorbidities (heart failure, alcoholism, hepatic and renal insufficiency), immunosuppression (agammaglobulinemia, anatomic or functional asplenia), infection with serotypes 3 and 8; and hematogenous spread with either positive blood cultures or extrapulmonary complications (arthritis, meningitis, or 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. Chla- mydial pneumoniae responds slower to treatment than mycoplasma and tends to recur if therapy is stopped prematurely. Young adults usually do well, but the elderly have a mortality rate of 5 to 10%.

Treatment

A prediction rule may be 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). 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. ICU admission is required for patients who need mechanical ventilation and for those with hypotension (systolic BP < 90 mm Hg). Other criteria for ICU admission include respiratory rate > 30/min, Pao ₂/inspired O₂ (Fio ₂) < 250, multilobar pneumonia, diastolic BP < 60 mm Hg, confusion, and BUN > 19.6 mg/dL. Appropriate treatment involves starting antibiotics as soon as possible, preferably ≤ 8 h after presentation. Supportive care includes fluids, antipyretics, analgesics, and O₂ for patients with hypoxemia.

Table 4
Risk Stratification for Community-Acquired Pneumonia Factor Points Age   Men Women Age Age−10 Nursing home resident 10 Coexisting illness   Malignancy 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 Hematocrit < 30% 10 Pao 2 < 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. †Consider acute care admission, subacute care admission, observation period, home IV antibiotics, or home nursing visits 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 Some Trade Names
VIRAZOLE
Click for Drug Monograph
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 Some Trade Names
VIRAZOLE
Click for Drug Monograph
is not used in adults with RSV infection. Amantadine Some Trade Names
SYMMETREL
Click for Drug Monograph
or rimantadine Some Trade Names
FLUMADINE
Click for Drug Monograph
200 mg po once/day started within 48 h of symptom onset reduces the duration and severity of symptoms in patients who develop presumed influenza A infection as part of an outbreak, but efficacy for preventing adverse outcomes of influenza pneumonia is unknown. Zanamivir (10 mg inhaled bid) and oseltamivir Some Trade Names
TAMIFLU
Click for Drug Monograph
(75 mg po bid) are similarly effective in reducing the duration of symptoms caused by influenza A or B if started within 48 h of symptom onset, though zanamivir may be contraindicated in patients with asthma. Acyclovir Some Trade Names
ZOVIRAX
Click for Drug Monograph
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 advocated 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, 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, antibiotic resistance with inadequate coverage, co-infection 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 proven, treatment failure is likely due to inadequate host defenses.

Most viral pneumonias resolve without specific treatment.

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 B (HIB) vaccine (for patients < 2 yr), and influenza vaccine (for patients > 65 yr)—see Immunization and see Fig. 3: Approach to the Care of Normal Infants and Children: Recommended childhood and adolescence immunization schedule.. Pneumococcus , HIB, and influenza vaccines are also recommended for high-risk patients. High-risk patients not vaccinated against influenza may be given amantadine Some Trade Names
SYMMETREL
Click for Drug Monograph
, rimantadine Some Trade Names
FLUMADINE
Click for Drug Monograph
, or oseltamivir Some Trade Names
TAMIFLU
Click for Drug Monograph
during influenza epidemics.

Last full review/revision November 2005

Content last modified November 2005