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Epidemiology and Pathophysiology

Pneumonia is the leading killer of children worldwide, with nearly 2 million deaths annually among children less than 5 years of age.1,2 Mortality is exceedingly rare in the developed world, occurring in less than 1% of hospitalized pneumonia cases;3-5 however, morbidity remains substantial with hospitalization rates estimated at 74 to 790 per 100,000 children in the United States, with the highest rates occurring among those less than five years of age.5 This translates to over 100,000 hospitalizations per year in the United States, making childhood pneumonia one of the most common conditions encountered in the hospital setting and also one of the most costly, with aggregate yearly hospitalization costs approaching $1 billion.6,7



Respiratory viruses lead to pneumonia by direct extension from the upper respiratory tract and progressive invasion of the respiratory epithelium, which may lead to diffuse lymphocytic proliferation and interstitial pneumonitis and/or alveolar destruction depending on the specific etiology.8 Viruses are detected in 60% to 80% of children with pneumonia in both ambulatory and hospitalized settings (Table 102-1).9-15 Viral pneumonias are most commonly encountered among young children, especially those less than 5 years of age in whom viral pathogens represent the predominant cause of pneumonia, either alone or in combination with bacteria. Respiratory syncytial virus, parainfluenza viruses, influenza viruses, and adenovirus have long been considered causative pneumonia pathogens.16-22 More recently, human metapneumovirus, human bocavirus, novel species of coronaviruses, and rhinoviruses have also been implicated; however, with the possible exception of human metapneumovirus, the precise contribution of these microorganisms is still debated.23-25

TABLE 102-1*Common Causes of Pneumonia by Age

Bacteria causing pneumonia often colonize the nasopharynx and gain access to the lower respiratory tract directly by inhalation, although this alone is not sufficient to cause disease as it likely happens many times per day. Only when bacteria overcome the typical pulmonary host defenses does pneumonia develop. Often this occurs in the setting of a viral upper respiratory tract infection or other period of impaired immunity.26,27 Bacteria replication in the lower airways leads to alveolar fluid accumulation and neutrophilic inflammation with rapid involvement of contiguous alveoli leading to lobar consolidation.28 Occasionally, pneumonia develops by hematogenous ...

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