Pneumonia Health Article

More than 2 million cases of community-acquired pneumonia (CAP) occur each year in the United States, resulting in approximately 10 million physician visits, more than 500,000 hospitalizations, and approximately 50,000 deaths. Over time the number of microorganisms identified as pathogens has increased, along with new broad-spectrum antibiotics available for treatment. At the same time, common pathogens have become increasingly resistant to frequently used antibiotics, complicating the management of CAP and prompting the development of management guidelines.

Epidemiology

The actual incidence of pneumonia in ambulatory patients is difficult to estimate because the etiologic agent is rarely identified except in clinical trials, and CAP is not currently considered a reportable disease. Each year in the United States there are 2 to 3 million cases of CAP. The incidence of hospitalization is estimated at 260 cases per 100,000 population but is about fourfold higher in those over age 65. CAP results in about 500,000 hospitalizations annually, with approximately 45,000 deaths; pneumonia is the sixth most common cause of death in the United States. Between 1979 and 1994, pneumonia and influenza–related death rates have increased because of the increasing number of patients over 65 and patients with underlying illnesses. Studies of patients with CAP report mortality rates of 5.1% to 36.5%, averaging about 14%. ⁴ An analysis of 1993 hospital discharge data from Washington, Illinois, and Florida revealed death rates of 7.0%, 8.1%, and 9.7%, respectively. ⁶ Risk factors for mortality include age, alcoholism, bacteremia, and multilobar involvement on radiographs. Contributing factors include underlying malignancy, immunosuppression, neurologic disease, congestive heart failure, and diabetes. Aspiration, postobstructive, gram-negative, and Staphylococcus aureus forms of pneumonia are also associated with higher mortality risk.

Pathophysiology

Traditionally thought to be responsible for 60% to 70% of pneumonias, the prevalence of Streptococcus pneumoniae has decreased with the identification of other agents (Table 73-1). The likelihood of each of these agents causing disease in a given patient is not certain, although certain host factors and geographic location may predispose to certain infections (Table 73-2). Travel to southwestern United States, including California and Texas, and contiguous areas of Mexico suggests Coccidioides immitis. Histoplasma capsulatum is endemic in states bordering the Mississippi and Ohio rivers. Blastomyces dermatitidis is endemic in southeastern United States but also in Wisconsin, Minnesota, and neighboring Canadian provinces. Exposure to birds necessitates the addition of psittacosis to the differential diagnosis, and exposure to parturient cats, cattle, or sheep suggests Q fever (Coxiella burnetii).

The respiratory tract is a unique system in that it is open to the external environment and therefore continuously exposed to microorganisms, particulate matter, and fumes. In addition to all the organisms that are coughed or sneezed into the environment by others, humans regularly aspirate nasopharyngeal flora during sleep. Multiple defense mechanisms counteract these continuous exposures, including mechanical, anatomic, and immunologic barriers. The cough reflex, the mucociliary transport mechanism, and secretory immunoglobulins remove and neutralize microbes in the upper and central airways. In the alveoli the alveolar macrophages, immunoglobulins, and complement combine to clear organisms from the distal lung. Alterations in mental status may reduce the cough reflex; mucous production and ciliary function can be overcome by viral illness or tobacco smoke; and the immune response can be blunted by many illnesses or medications. Loss of these defenses in the setting of a large inoculum or particularly virulent organism can produce significant infection. Whether or not colonization of the upper airway is necessary before the development of pneumonia is unclear. In the outpatient population the carrier rate for S. pneumoniae is quite high but the incidence of pneumonia quite low. In hospitalized patients, however, colonization by gram-negative organisms probably occurs before development of pneumonia. In a few patients, pneumonia may result from hematogenously spread infection.

Annual vaccination against influenza should reduce its incidence and that of secondary bacterial pneumonias. Vaccination against pneumococcal infection is recommended for patients 65 years and older and for younger persons at increased risk, that is, with anatomic or functional asplenia (including sickle cell disease), cardiovascular disease, pulmonary disease, diabetes mellitus, alcoholism, cirrhosis, and cerebrospinal leaks. The current vaccine is a 23-valent preparation that provides coverage against approximately 90% of the most frequently reported capsular types. Routine revaccination of adults is not currently recommended unless the patient is at high risk for pneumococcal infection (asplenic) and originally received the 14-valent vaccine. Revaccination should also be considered for persons 65 or older who received the vaccine 5 or more years earlier and were under age 65 at the time of primary vaccination. Although evidence is lacking, single revaccination should also be considered in immunocompromised patients if 5 or more years have elapsed since initial vaccination.