Pneumonia is an inflammation of the lung affecting the alveoli. We consider whether pneumonia is community acquired versus hospital acquired to help us determine the spectrum of potential pathogens that differs based on setting. More importantly, because empiric therapy is often given in pneumonia, therapeutic interventions differ based on the different populations.
The focus in this section will be on community-acquired pneumonia, which is a leading cause of death both in the United States and worldwide. Hospital-acquired pneumonia, also known as nosocomial pneumonia, is pneumonia that occurs 48 hours or more after admission to the hospital and was not present at the time of admission. The term “healthcare-associated” pneumonia is also used.
The alveoli of the lungs are continually exposed to microbes from the environment via the upper respiratory tract. Our host defenses usually keep these potential pathogens in check. However, disease can occur when there is a particularly virulent organism, when there is a large burden of organisms inhaled from the environment or aspirated from the oropharynx, or when there is a defect in host immunity.
Predisposing factors to pneumonia include the extremes of age (the very young and very old), chronic obstructive pulmonary disease (COPD) and chronic bronchitis, diabetes mellitus, cystic fibrosis, and congestive heart failure. Injection drug users who overdose, alcoholics, and those with seizure disorders have a high risk of pneumonia because they can aspirate organisms into the lung when unconscious, resulting in aspiration pneumonia. People exposed to water aerosols, especially from air conditioners, are at risk for pneumonia caused by Legionella. Hospitalized patients in the intensive care unit are at risk for ventilator-associated pneumonia caused by gram-negative rods such as Escherichia coli, Pseudomonas, and Acinetobacter.
Symptoms include cough that may be productive of sputum, fever, chills, chest pain, and shortness of breath. “Rusty” sputum is a well-known finding in pneumococcal pneumonia. Sputum that has a “currant jelly” appearance can occur in pneumonia caused by Klebsiella because the organism is heavily encapsulated. Physical examination findings include tachypnea, rales, and rhonchi. If the lung is consolidated, dullness to percussion may be detected.
Patients who are intubated and who acquire a nosocomial pneumonia may only have fever as a presenting sign, which may be accompanied by increased respiratory secretions or increased oxygen requirements. Pneumonia may be complicated by an infected pleural effusion or a pleural empyema. A pleural empyema is a walled-off collection of pus in the pleural space.
Streptococcus pneumoniae is the most common cause of community-acquired pneumonia. Approximately 15% to 30% of isolates of S. pneumoniae are resistant to one or more commonly used antibiotics.
Other common bacterial pathogens include Klebsiella pneumoniae and Haemophilus influenzae. Note that it is the nontypeable strains of H. influenzae rather than the type B strain that cause pneumonia in elderly patients with COPD. Community-acquired, methicillin-resistant Staphylococcus aureus (CA-MRSA) is increasingly found.
Mycoplasma pneumoniae, Legionella species, and Chlamydophila pneumoniae are other pathogens commonly involved. Infection with Mycobacterium tuberculosis can also manifest as a pneumonia. In approximately 30% of adults with community-acquired pneumonia, no pathogen, neither bacteria nor virus, is isolated.
Table 76–1 shows the important causes of community-acquired pneumonia as a function of age. Note that the causes of pneumonia in a neonate are those acquired during passage through the birth canal. The main cause of pneumonia in an infant, Chlamydia trachomatis, is also acquired during passage through the birth canal but is a less aggressive pathogen so its onset in delayed. Note that M. pneumoniae is the most common cause in young adults.
TABLE 76–1Important Bacterial and Viral Causes of Community-Acquired Pneumonia by Age (Listed in Order of Frequency) ||Download (.pdf) TABLE 76–1 Important Bacterial and Viral Causes of Community-Acquired Pneumonia by Age (Listed in Order of Frequency)
|Age ||Bacteria ||Viruses |
|Neonates || |
Group B streptococci
|Respiratory syncytial virus (RSV) |
|Infants || |
|Children || |
|Young adults || |
|Various respiratory viruses (e.g., adenovirus) |
|Older adults || |
|Influenza virus |
Table 76–2 shows the typical causes of community-acquired pneumonia as a function of various predisposing factors. In certain patient populations, Pseudomonas aeruginosa and other gram-negative organisms and S. aureus may be important pathogens causing pneumonia. For example, P. aeruginosa, Stenotrophomonas, and Burkholderia cause pneumonia in cystic fibrosis patients, and S. aureus is a well-recognized cause of pneumonia in patients with influenza. Oral anaerobes are often involved in aspiration pneumonia.
TABLE 76–2Predisposing Factors Associated with Typical Pathogens Causing Community-Acquired Pneumonia ||Download (.pdf) TABLE 76–2 Predisposing Factors Associated with Typical Pathogens Causing Community-Acquired Pneumonia
|Predisposing Factors ||Typical Pathogens |
|Alcoholism ||Klebsiella pneumoniae, oral anaerobes |
|Bird exposure, especially psittacine birds such as parrots (psittacosis) ||Chlamydophila psittaci |
|Chronic obstructive pulmonary disease (COPD), including smoking related ||Haemophilus influenzae |
|Cystic fibrosis ||Pseudomonas aeruginosa |
|Imported wool, spores in wool (woolsorter’s diseases) ||Bacillus anthracis |
|Influenza virus infection ||Staphylococcus aureus |
|Intubation, postsurgery, and intensive care unit (ICU) ||Coliforms,1 P. aeruginosa, S. aureus |
|Mouse droppings exposure, especially in southwestern states ||Hantavirus |
|Sheep exposure, especially placental tissue (Q fever) ||Coxiella burnetii |
|Travel to or reside in Central Valley of California, Arizona, or New Mexico ||Coccidioides immitis |
|Travel to or reside in Ohio or Mississippi river valleys ||Histoplasma capsulatum |
|Ventilator associated, especially in ICU ||Acinetobacter species |
|Water aerosols, especially from air conditioners ||Legionella pneumophila |
People exposed to certain animals have an increased risk of pneumonia; for example, those exposed to psittacine birds such as parrots are at risk for psittacosis caused by Chlamydophila psittaci and those exposed to the placentas of pregnant sheep are at risk for Q fever caused by Coxiella burnetii. People exposed to the spores of the anthrax bacillus in sheep wool may get “woolsorter’s disease,” a pneumonia caused by Bacillus anthracis.
Common pathogens for hospital-acquired pneumonia include gram-negative rods such as E. coli, K. pneumoniae, P. aeruginosa, Enterobacter species, Serratia marcescens, Acinetobacter species, and gram-positive cocci, especially S. aureus.
The most common viral cause of pneumonia is influenza virus. However, other viral pathogens such as RSV, parainfluenza virus, adenovirus, human metapneumovirus, and severe acute respiratory syndrome (SARS) coronavirus can also cause pneumonia. In patients with reduced cell-mediated immunity, herpesviruses, such as herpes simplex virus, varicella-zoster virus, and cytomegalovirus, can cause life-threatening pneumonia. In certain geographical areas, such as the rural southwestern part of the United States, outbreaks of pneumonia caused by hantavirus have occurred.
Fungi such as Coccidioides and Histoplasma also cause pneumonia. Pneumocystis jiroveci causes pneumonia, especially in patients with acquired immunodeficiency syndrome (AIDS) with low CD4 counts.
The “gold standard” for a diagnosis of pneumonia is an infiltrate on a plain chest radiograph (Figure 76–1). Clinical data may help, but ultimately the chest radiograph is the most important diagnostic tool. Sputum analysis for Gram stain and culture and blood cultures may be helpful in the hospitalized patient but are only optional in an outpatient setting because therapy is largely empiric for community-acquired pneumonia. If sputum cultures and blood cultures are indicated, then these specimens should be obtained before antibiotics are started.
Lobar pneumonia caused by Streptococcus pneumoniae. Arrow points to area of consolidation in right lung. (Reproduced with permission from McKean SC, Ross JJ, Dressler DD, et al. Principles and Practice of Hospital Medicine. New York, NY: McGraw-Hill Education; 2012.)
In pneumonia caused by one of the encapsulated pyogenic bacteria, such as S. pneumoniae, the white blood cell count is frequently elevated and the number of neutrophils is often increased. The urinary antigen test for pneumococcal polysaccharide is also useful.
It is important that sputum (not saliva) be sent to the lab for Gram stain and culture. If the specimen contains many neutrophils and few epithelial cells, then the specimen is likely to be sputum and will be analyzed. If, however, the specimen contains many epithelial cells and few neutrophils, then the specimen is saliva and will be rejected by the lab. An “induced sputum” sample produced by using nebulized hypertonic saline increases the probability of obtaining a good sputum sample.
Pneumonia caused by M. tuberculosis is diagnosed by acid-fast stain of sputum and culture on mycobacterial medium. A PCR assay done directly on sputum is available also. Pneumonia caused by Legionella pneumophila is often diagnosed by urinary antigen or PCR. PCR tests for various respiratory pathogens such as M. pneumoniae, C. pneumoniae, influenza virus, and RSV are useful in special circumstances. The cold agglutinin test is no longer recommended for the diagnosis of pneumonia caused by Mycoplasma.
Treatment for community-acquired pneumonia is largely empiric because microbiologic diagnostic strategies are generally insensitive. Outpatients are typically treated with a macrolide such as azithromycin, a tetracycline such as doxycycline, or a respiratory quinolone such as levofloxacin. Amoxicillin-clavulanate with or without a macrolide can also be used. Lefamuin can also be used.
Hospitalized patients with community-acquired pneumonia are often treated with ceftriaxone plus a macrolide or respiratory quinolone monotherapy. If aspiration pneumonia is suspected, metronidazole or clindamycin can be added.
If influenza is occurring in the community, then oseltamivir should be prescribed. Note that influenza can predispose to bacterial pneumonia, especially caused by S. aureus and S. pneumoniae, so antibiotics may be needed even in a person with established influenza.
Patients with suspected hospital-acquired pneumonia may be given broader spectrum agents such as a carbapenem (e.g., ertapenem), depending on the local epidemiology, given that many hospital-acquired infections are multidrug resistant. A combination of ceftriaxone and azithromycin can also be used. Prompt initiation of antibiotics is important because morbidity and mortality increase after a delay of more than 8 hours. Drainage of an empyema or infected pleural fluid should be performed.
The influenza vaccine is effective in decreasing the likelihood of pneumonia. The pneumococcal protein-conjugate vaccine is generally recommended for infants and children, and the pneumococcal polysaccharide vaccine for adults at intermediate risk of pneumococcal disease (e.g., cigarette smokers). The pneumococcal polysaccharide and protein-conjugate vaccines are both recommended for older and select high-risk adults (e.g., immunocompromised patients). The conjugate vaccine against H. influenzae type B is not an important source of protection, because it is the nontypeable strains of H. influenzae, rather than the type B strain, that are the most common cause of pneumonia. Smoking cessation and treatment of alcohol abuse may also decrease pneumonia risk.