Pneumonia is an infection of the lower respiratory tract that involves the airways and parenchyma with consolidation of the alveolar spaces. The term lower respiratory tract infection is often used to encompass bronchitis, bronchiolitis (see Chapter 109), pneumonia, or any combination of the three. Pneumonitis is a general term for lung inflammation that may or may not be associated with consolidation. Lobar pneumonia describes pneumonia localized to one or more lobes of the lung. Atypical pneumonia describes patterns typically more diffuse or interstitial than lobar pneumonia. Bronchopneumonia refers to inflammation of the lung that is centered in the bronchioles and leads to the production of a mucopurulent exudate that obstructs some of these small airways and causes patchy consolidation of the adjacent lobules. Interstitial pneumonitis refers to inflammation of the interstitium, which is composed of the walls of the alveoli, the alveolar sacs and ducts, and the bronchioles. Interstitial pneumonitis is characteristic of acute viral infections but also may be a chronic inflammatory or fibrosing process.
Defects in host defenses increase the risk of pneumonia. Lower airways and secretions are sterile as a result of a multifactorial system. Airway contaminants are caught in the mucus secreted by the goblet cells. Cilia on epithelial surfaces, composing the ciliary elevator system, beat synchronously to move particles upward toward central airways and into the throat, where they are swallowed or expectorated. Polymorphonuclear neutrophils from the blood and tissue macrophages ingest and kill microorganisms. IgA secreted into the upper airway fluid protects against invasive infections and facilitates viral neutralization.
Infectious agents that commonly cause community-acquired pneumonia vary by age (Table 110-1). Most common causes are respiratory syncytial virus (RSV) in infants (see Chapter 109), other respiratory viruses (parainfluenza viruses, influenza viruses, human metapneumovirus, adenoviruses) in children younger than 5 years old, and Mycoplasma pneumoniae in children older than age 5 years. Streptococcus pneumoniae is the most common bacterial cause of lobar pneumonia, and occurs in children of any age outside the neonatal period.
M. pneumoniae and Chlamydophila pneumoniae are principal causes of atypical pneumonia.
Chlamydia trachomatis and less commonly Mycoplasma hominis, Ureaplasma urealyticum, and cytomegalovirus (CMV) cause a similar respiratory syndrome in infants 1 to 3 months of age with subacute onset of an afebrile pneumonia; cough and hyperinflation are the predominant signs. These infections are difficult to diagnose and to distinguish from each other. In adults these organisms are carried primarily as part of the genital mucosal flora. Women who harbor these agents may transmit them perinatally to newborns.
Additional agents occasionally cause pneumonia. Severe acute respiratory syndrome (SARS) is due to SARS-associated coronavirus (SARS-CoV). Avian influenza (bird flu) is a highly contagious viral disease of poultry and other birds caused by influenza A (H5N1). There were outbreaks among humans in South East Asia in 1997 and 2003 to 2004 with high mortality rates. A novel influenza A (H1N1) of swine origin began circulating in 2009. Other etiologic agents to consider, based on specific exposure history, include Mycobacterium tuberculosis, Francisella tularensis, Brucella spp., Coxiella burnetii, Chlamydophila psittaci, Legionella pneumophila, hantavirus, Histoplasma capsulatum, Coccidioides immitis, and Blastomyces dermatitidis.