Overview Of Pneumonia  Health Article

In excess of five million cases of infectious pneumonia are estimated to occur annually in the United States, resulting in more than one million hospitalizations. Pneumonia is the sixth leading cause of death and the most common lethal infectious disease. The mortality of community-acquired pneumonia ranges from less than 5% in mildly ill outpatients to somewhat greater than 12% overall in patients who are admitted to a hospital. Mortality is even greater in patients who have severe invasive disease, which often is associated bacteremia, and in elderly nursing home patients. Mortality from pneumonia can exceed 40% in patients who require management in the intensive care unit.

This chapter discusses the common clinical features, diagnosis, prevention, and initial management of infectious pneumonia. The term pneumonia itself, however, includes other causes of inflammation of the lower respiratory air spaces, particularly the alveoli, such as acute or chronic eosinophilic pneumonia, bronchiolitis obliterans with organizing pneumonia, and usual interstitial pneumonia, all of which are presented in more detail elsewhere.

Mechanisms Of Infection

The most common mechanism by which the lung is inoculated with pathogenic organisms is through microaspiration of oropharyngeal contents, a process that occurs in otherwise healthy individuals during sleep. Colonization of the oral pharynx with pathogenic organisms, such as Streptococcus pneumoniae , can thereby lead to delivery of sufficient quantities of organisms to infect the lung. In contrast, gross aspiration normally occurs only in individuals with an altered sensorium, depressed consciousness, abnormalities of protective cough or gag reflexes, or substantial gastroesophageal reflux. Gross aspiration, which also can deliver large numbers of anaerobic bacteria to the lower respiratory tract, is a major contributing factor to anaerobic lung infection and abscess formation.

The second most frequent mechanism of lung infection is the inhalation of small, suspended aerosolized droplets, which range in size from 0.5 to 1 micron and which may contain microorganisms. In view of the limited number of organisms delivered in such a manner, only relatively aggressive pathogenic organisms such as Mycobacterium tuberculosis , Legionella pneumophila , Yersinia pestis (plague;), Bacillus anthracis (anthrax;), and some viral infections can be transmitted in this manner.

Less commonly, the lung may become infected as a consequence of a bloodstream infection. Blood-borne pneumonia is seen especially in staphylococcal sepsis or right-sided endocarditis, which are more common in intravenous drug users, and in gram-negative bacteremias, particularly in the immunocompromised host. The lung also may be rarely inoculated directly by penetrating chest trauma or by local spread from a nearby infected organ (paragonimiasis or amebic liver abscess;) or a contiguous soft tissue infection.

Fortunately, the lung is well equipped to defend against inoculation with most microbes. When large droplets of infected material reach the airways, they are removed by the mucociliary escalator, which sweeps entrapped contents up to the oropharynx, where they are swallowed or expectorated. Smaller particles, in the range of 0.5 to 2.0 microns, are deposited in the alveoli, where alveolar macrophages phagocytize and destroy most pathogens. These macrophages are further activated to release potent cytokines and chemokines, including tumor necrosis factor-α, interleukin-8, and leukotriene B 4 , which help recruit neutrophils from the bloodstream into the alveolar spaces, where they participate in the uptake and degradation of microorganisms. For many microorganisms, such as S. pneumoniae , clearance of infection is greatly facilitated by the development of specific immunoglobulin (Ig) G, which binds the surface of the organisms or its polysaccharide capsule. These specific antibodies, which act as immune opsonins, greatly augment the ability of neutrophils and macrophages to phagocytize and destroy the bacteria. In addition, pattern recognition receptors and other nonimmune opsonins, including surfactant proteins A and D, fibronectin, and vitronectin, also bind to specific epitopes on the surface of organisms that reach the lower respiratory tract and assist in their recognition and elimination. Only when organisms overwhelm or evade these multiple host defense systems does inoculation of the lung result in clinically significant pneumonia.