(See also Overview of Pneumonia.)
Many organisms cause community-acquired pneumonia, including bacteria, viruses, and fungi. Pathogens vary by patient age and other factors (see table Community-Acquired Pneumonia in Adults), but the relative importance of each as a cause of community-acquired pneumonia is uncertainbecause most patients do not undergo thorough testing, and because even with testing, specific agents are identified in < 50% of cases.
The most common bacterial causes are
Pneumonias caused by chlamydia and mycoplasma are often clinically indistinguishable from other pneumonias.
Common viral causes include
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 years. 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 psittacine birds (ie, parrots, parakeets, macaws).
Since the year 2000, the incidence of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) skin infections has increased markedly. This pathogen can rarely cause severe, cavitating pneumonia and tends to affect young adults.
P. aeruginosa is an especially common cause of pneumonia in patients with cystic fibrosis, neutropenia, advanced acquired immunodeficiency syndrome (AIDS), and/or bronchiectasis.
A host of other organisms causes lung infection in immunocompetent patients. In patients with pneumonia, a thorough history of exposures, travel, pets, hobbies, and other exposures is essential to raise suspicion of less common organisms.
Adenovirus, Epstein-Barr virus, and coxsackievirus are common viruses that rarely cause pneumonia. Seasonal influenza can rarely cause a direct viral pneumonia but often predisposes to the development of a serious secondary bacterial 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) and the Middle East respiratory syndrome (MERS).
Common fungal pathogens include Histoplasma capsulatum (histoplasmosis) and Coccidioides immitis (coccidioidomycosis). Less common fungal pathogens include Blastomyces dermatitidis (blastomycosis) and Paracoccidioides braziliensis (paracoccidioidomycosis). Pneumocystis jirovecii commonly causes pneumonia in patients who have human immunodeficiency virus (HIV) infection or are immunosuppressed (see Pneumonia in Immunocompromised Patients).
S. pneumoniae and MRSA can cause necrotizing pneumonia.
In children, the most common causes of pneumonia depend on age:
Pneumonia in neonates is discussed elsewhere.
Symptoms include malaise, chills, rigor, fever, 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. GI symptoms (nausea, vomiting, diarrhea) are also common. Symptoms become variable at the extremes of age. Infection in infants may manifest as nonspecific irritability and restlessness; in the elderly, manifestation may be as confusion and obtundation.
Signs include fever, tachypnea, tachycardia, crackles, bronchial breath sounds, egophony (E to A change—said to occur when, during auscultation, a patient says the letter “E” and the examiner hears the letter “A”), and dullness to percussion. Signs of pleural effusion may also be present. 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. For example, factors thought to suggest viral pneumonia included gradual onset, preceding symptoms of an upper respiratory infection (URI), diffuse findings on auscultation, and absence of a toxic appearance. Atypical pathogens were considered more likely when onset was less acute and are more likely during known community outbreaks. However, manifestations in patients with typical and atypical pathogens overlap considerably. In addition, no single symptom or sign is sensitive or specific enough to predict the organism. Symptoms and signs are even similar for other noninfective inflammatory lung diseases such as hypersensitivity pneumonitis and cryptogenic organizing pneumonia.
Diagnosis is suspected on the basis of clinical presentation and infiltrate seen on chest x-ray. When there is high clinical suspicion of pneumonia and the chest x-ray does not reveal an infiltrate, doing computed tomography (CT) or repeating the chest x-ray in 24 to 48 hours is recommended.
Differential diagnosis in patients presenting with pneumonia-like symptoms includes acute bronchitis and exacerbation of chronic obstructive pulmonary disease (COPD), which can be distinguished from pneumonia by the absence of infiltrates on chest x-ray. Other disorders should be considered, particularly when findings are inconsistent or not typical, such as heart failure, organizing pneumonia, and hypersensitivity pneumonitis. The most serious common misdiagnosis is pulmonary embolism, which may be more likely in patients with acute onset of dyspnea, minimal sputum production, no accompanying URI or systemic symptoms, and risk factors for thromboembolism (see table Risk Factors for Deep Venous Thrombosis); thus, testing for pulmonary embolism should be considered in patients with such symptoms and risk factors.
Quantitative cultures of bronchoscopic or suctioned specimens, if they are obtained before antibiotic administration, can help distinguish between bacterial colonization (ie, presence of microorganisms at levels that provoke neither symptoms nor an immune response) and infection. However, bronchoscopy is usually done only in patients receiving mechanical ventilation or for those with other risk factors for unusual microorganisms or complicated pneumonia (eg, immunocompromise, failure of empiric therapy).
Distinguishing between bacterial and viral pneumonias is challenging. Many studies have investigated the utility of clinical, imaging, and routine blood tests, but no test is reliable enough to make this differentiation.
In outpatients with mild pneumonia, no further diagnostic testing is needed (see table Risk Stratification for Community-Acquired Pneumonia). In patients with moderate or severe pneumonia, a white blood cell count and measurement of electrolytes, blood urea nitrogen (BUN), and creatinine are useful to classify risk and hydration status. Pulse oximetry or arterial blood gas (ABG) testing should also be done to assess oxygenation. For patients with moderate or severe pneumonia who require hospitalization, 2 sets of blood cultures are obtained to assess for bacteremia and sepsis. The Infectious Diseases Society of America (IDSA) provides a guide to recommended testing based on patient demographic and risk factors (Infectious Diseases Society of America Clinical Guidelines on Community-Acquired Pneumonia).
Identification of the pathogen can be useful to direct therapy and verify bacterial susceptibilities to antibiotics. However, because of the limitations of current diagnostic tests and the success of empiric antibiotic treatment, experts recommend limiting attempts at microbiologic identification (eg, cultures, specific antigen testing) unless patients are at high risk or have complications (eg, severe pneumonia, immunocompromise, asplenia, failure to respond to empiric therapy). In general, the milder the pneumonia, the less such diagnostic testing is required. Critically ill patients require the most intensive testing, as do patients in whom a antibiotic-resistant or unusual organism is suspected (eg, Mycobacterium tuberculosis, P. jirovecii) and patients whose condition is deteriorating or who are not responding to treatment within 72 hours.
Chest x-ray findings generally cannot distinguish one type of infection from another, although the following findings are suggestive:
Multilobar infiltrates suggest S. pneumoniae or Legionella pneumophila infection.
Interstitial pneumonia (on chest x-ray, appearing as increased interstitial markings and subpleural reticular opacities that increase from the apex to the bases of the lungs) suggests viral or mycoplasmal etiology.
Cavitating pneumonia suggests S. aureus or a fungal or mycobacterial etiology.
Blood cultures, which are often obtained in patients hospitalized for pneumonia, can identify causative bacterial pathogens if bacteremia is present. About 12% of all patients hospitalized with pneumonia have bacteremia; S. pneumoniae accounts for two thirds of these cases.
Sputum testing can include Gram stain and culture for identification of the pathogen, but the value of these tests is uncertain because specimens often are contaminated with oral flora and overall diagnostic yield is low. Regardless, identification of a bacterial pathogen in sputum cultures allows for susceptibility testing. Obtaining sputum samples also allows for testing for viral pathogens via direct fluorescence antibody testing or polymerase chain reaction (PCR), but caution needs to be exercised in interpretation because 15% of healthy adults carry a respiratory virus or potential bacterial pathogen. In patients whose condition is deteriorating and in those unresponsive to broad-spectrum antibiotics, sputum should be tested with mycobacterial and fungal stains and cultures.
Sputum samples can be obtained noninvasively by simple expectoration or after hypertonic saline nebulization (induced sputum) for patients 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. Otherwise, bronchoscopic sampling is usually done only for patients with other risk factors (eg, immunocompromise, failure of empiric therapy).
Urine testing for Legionella antigen and pneumococcal antigen is now widely available. These tests are simple and rapid and have higher sensitivity and specificity than sputum Gram stain and culture for these pathogens. Patients at risk of Legionella pneumonia (eg, severe illness, failure of outpatient antibiotic treatment, presence of pleural effusion, active alcohol abuse, recent travel) 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).
The pneumococcal antigen test is recommended for patients who are severely ill; have had unsuccessful outpatient antibiotic treatment; or who have pleural effusion, active alcohol abuse, severe liver disease, or asplenia. This test is especially useful if adequate sputum samples or blood cultures were not obtained before initiation of antibiotic therapy. A positive test can be used to tailor antibiotic therapy, though it does not provide antimicrobial susceptibility.
Short-term mortality is related to severity of illness. Mortality is < 1% in patients who are candidates for outpatient treatment. Mortality in hospitalized patients is 8%. Death may be caused by pneumonia itself, progression to sepsis syndrome, or exacerbation of coexisting conditions. In patients hospitalized for pneumonia, risk of death is increased during the year after hospital discharge.
Mortality varies to some extent by pathogen. Mortality rates are highest with gram-negative bacteria and CA-MRSA. However, because these pathogens are relatively infrequent causes of community-acquired pneumonia, S. pneumoniae remains the most common cause of death in patients with community-acquired pneumonia. Atypical pathogens such as Mycoplasma have a good prognosis. Mortality is higher in patients who do not respond to initial empiric antibiotics and in those whose treatment regimen does not conform with guidelines.
Risk stratification via risk prediction rules may be used to estimate mortality risk and can help guide decisions regarding hospitalization. These rules have been used to identify patients who can be safely treated as outpatients and those who require hospitalization because of high risk of complications (see table Risk Stratification for Community-Acquired Pneumonia). However, these rules should supplement, not replace, clinical judgment because many unrepresented factors, such as likelihood of adherence, ability to care for self, and wish to avoid hospitalization, should also influence triage decisions. An intensive care unit (ICU) admission is required for patients who
Other criteria that mandate consideration of ICU admission include
The Pneumonia Severity Index (PSI) is the most studied and validated prediction rule. However, because the PSI is complex and requires several laboratory assessments, simpler rules such as CURB-65 are usually recommended for clinical use. Use of these prediction rules has led to a reduction in unnecessary hospitalizations for patients who have milder illness.
In CURB-65, 1 point is allotted for each of the following risk factors:
Scores can be used as follows:
Risk Stratification for Community-Acquired Pneumonia (the Pneumonia Severity Index)
The SMART-COP score can be used to assess the risk that ventilatory or vasopressor support will be needed (1).
Antibiotic therapy is the mainstay of treatment for community-acquired pneumonia. Appropriate treatment involves starting empiric antibiotics as soon as possible, preferably ≤8 hours after presentation. Because organisms are difficult to identify, the empiric antibiotic regimen is 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 the table Community-Acquired Pneumonia in Adults (see also Infectious Diseases Society of America Clinical Guideline on Community-Acquired Pneumonia). Guidelines should be adapted to local susceptibility patterns, drug formularies, and individual patient circumstances. If a pathogen is subsequently identified, the results of antibiotic susceptibility testing can help guide any changes in antibiotic therapy.
For children, treatment depends on age, previous vaccinations, and whether treatment is outpatient or inpatient. For outpatient treatment, treatments are dictated by age:
< 5 years: Amoxicillin or amoxicillin/clavulanate is usually the drug of choice. If epidemiology suggests an atypical pathogen as the cause and clinical findings are compatible, a macrolide (eg, azithromycin, clarithromycin) can be used instead. Some experts suggest not using antibiotics if clinical features strongly suggest viral pneumonia.
≥ 5 years: Amoxicillin or (particularly if an atypical pathogen cannot be excluded) amoxicillin plus a macrolide. Amoxicillin/clavulanate is an alternative. If the cause appears to be an atypical pathogen, a macrolide alone can be used.
For children treated as inpatients, antibiotic therapy tends to be more broad-spectrum and depends on the child's previous vaccinations:
Fully immunized (against S. pneumoniae and H. influenzaetype b): Ampicillin or penicillin G (alternatives are ceftriaxone or cefotaxime). If MRSA is suspected, vancomycin or clindamycin is added. If an atypical pathogen cannot be excluded, a macrolide is added.
Not fully immunized: Ceftriaxone or cefotaxime (alternative is levofloxacin). If MRSA is suspected, vancomycin or clindamycin is added. If an atypical pathogen cannot be excluded, a macrolide is added.
Full details are described in the Clinical Practice Guidelines by the Pediatric Infectious Diseases Society and the Infectious Diseases Society of America.
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 white blood cell count. Failure to improve should trigger suspicion of
An unusual organism
Resistance to the antimicrobial used for treatment
Coinfection or superinfection with a 2nd infectious agent
An obstructive endobronchial lesion
Metastatic focus of infection with reseeding (in the case of pneumococcal infection)
Nonadherence to treatment (in the case of outpatients)
If none of these conditions can be proved, treatment failure is likely due to inadequate host defenses. When therapy has failed, consultation with a pulmonary and/or infectious disease specialist is indicated.
Antiviral therapy may be indicated for select viral pneumonias. Ribavirin is not used routinely for RSV pneumonia in children or adults but may be used occasionally in high-risk children age < 24 months.
Oseltamivir 75 mg orally twice a day or zanamivir 10 mg inhaled twice a day started within 48 hours of symptom onset and given for 5 days reduces the duration and severity of symptoms in patients who develop influenza infection. In patients hospitalized with confirmed influenza infection, observational studies suggest benefit even 48 hours after symptom onset.
Acyclovir 5 to 10 mg/kg IV every 8 hours for adults or 250 to 500 mg/m2 body surface area IV every 8 hours for children is recommended for varicella lung infections. Though pure viral pneumonia does occur, superimposed bacterial infections are common and require antibiotics directed against S. pneumoniae, H. influenzae, and S. aureus.
Follow-up x-rays should be obtained 6 weeks after treatment in patients > 35; persistence of an infiltrate at ≥ 6 weeks raises suspicions of tuberculosis or an underlying, possibly malignant endobronchial lesion.
Community-Acquired Pneumonia in Adults
Supportive care includes fluids, antipyretics, analgesics, and, for patients with hypoxemia, oxygen. Prophylaxis against thromboembolic disease and early mobilization improve outcomes for patients hospitalized with pneumonia. Cessation counseling should also be done for smokers.
1. Charles PG, Wolfe R, Whitby M, et al: SMART-COP: A tool for predicting the need for intensive respiratory or vasopressor support in community-acquired pneumonia. Clin Infect Dis 47(3):375-384, 2008. doi: 10.1086/589754
The category of health care-associated pneumonia was removed as a separate category of pneumonia in the 2016 Infectious Diseases Society of America guidelines for hospital-acquired pneumonia. Health care-associated pneumonia includes community based patients who have had recent contact with the health care system, such as those who reside in nursing homes or other long-term care facilities or visit dialysis centers and infusion centers. This category was created to help identify patients at increased risk for antibiotic-resistant bacteria. However, the 2016 IDSA guidelines found increasing evidence that many patients with health care-associated pneumonia were not at high risk for antibiotic-resistant bacteria. Rather, the risk for antibiotic-resistant bacteria in these patients can be based on validated risk factors described for patients with community-acquired pneumonia.
Some forms of community-acquired pneumonia are preventable with vaccination. Pneumococcal conjugate vaccine (PCV13) is recommended for children age 2 months to 2 years and for adults ≥ 19 years with certain comorbid (including immunocompromising) conditions. Pneumococcal polysaccharide vaccine (PPSV23) is given to all adults ≥ 65 years and to any patient ≥ 2 years who has risk factors for pneumococcal infections, including but not limited to those with underlying heart, lung, or immune system disorders and those who smoke (see table Vaccine Administration Guidelines for Adults). The full list of indications for both pneumococcal vaccines can be seen at the CDC website. Recommendations for other vaccines, such as H. influenzae type b (Hib) vaccine (for patients < 2 years), varicella vaccine (for patients < 18 months and a later booster vaccine), and influenza vaccine (annually for everyone ≥ 6 months and especially for those at higher risk of developing serious flu-related complications), can also be found at the CDC website. This higher risk group includes people ≥ 65 years and people of any age with certain chronic medical conditions (such as diabetes, asthma, or heart disease), pregnant women, and young children (see also Childhood Vaccination Schedule).
In high-risk patients who are not vaccinated against influenza and household contacts of patients with influenza, oseltamivir 75 mg orally once/day or zanamivir 10 orally mg once/day can be given for 2 weeks. If started within 48 hours of exposure, these antivirals may prevent influenza (although resistance has been described for oseltamivir).
Smoking cessation can reduce the risk of developing pneumonia.
Community-acquired pneumonia is a leading cause of death in the United States and around the world.
Common symptoms and signs include cough, fever, chills, fatigue, dyspnea, rigors, sputum production, and pleuritic chest pain.
Treat patients with mild or moderate risk pneumonia with empiric antibiotics without testing designed to identify the underlying pathogen.
Hospitalize patients with multiple risk factors, as delineated by risk assessment tools.
Consider alternate diagnoses, including pulmonary embolism, particularly if pneumonia-like signs and symptoms are not typical.