Respiratory infections in children, particularly community-acquired bacterial pneumonias, are among the most important worldwide public health issues. Pneumonia is the most common cause of death in children <5 years of age and is particularly important in developing countries. It is estimated that 4-5 million children die annually of bacterial pneumonias, with the large majority of those deaths occurring in developing countries. In the United States, about 6 in 1,000 children over the age of 9 years develop pneumonia, while the incidence in children <5 years old is more than 5 times this rate. In less affluent countries, the incidence of pneumonia is an order of greater magnitude, primarily owing to other factors including malnutrition, environmental and indoor air pollution, immunization status, crowding, and coinfection with HIV or measles. Although bacterial pneumonias are an important cause of morbidity and mortality throughout the world, nonbacterial pneumonias and viral lower respiratory tract infections (such as bronchiolitis) are more prevalent in newborns and children. Terms such as viral pneumonia, atypical pneumonia, and interstitial pneumonia are used to describe the clinical presentations and radiographic images of patients presenting with these latter illnesses. This chapter addresses respiratory infections—both “atypical” and bacterial—in newborns and children.

Etiology and Age-Specific Considerations

In determining the possible etiology of pneumonia in a specific child, it is useful to appreciate that certain infectious causes are more or less unique to particular age groups. The most important of these by age cohort are shown in Table 40.1. In the first month of life, most cases of pneumonia are caused by either group B Streptococcus or gram-negative pathogens such as E. coli. This chapter will not specifically review pneumonias in neonates, but instead will focus on children beyond the newborn period. Viruses as a group are the most common causes of pneumonia in younger children. The specific viral pathogens causing such pneumonia, as well as their relative frequencies and usual severities, are depicted in Table 40.2.

Infections with bacterial pathogens become more prevalent in school-aged children and adolescents (though bacterial infections can occur at any age). Organisms such as C. pneumoniae and M. pneumoniae occur more frequently in adolescents and, to a lesser extent, among children in their elementary school years. It is worth emphasizing that the most common bacterial pathogen causing pneumonia in all children is Streptococcus pneumoniae. This is true despite universal administration of the pneumococcal conjugate vaccine which protects against seven serotypes. There is considerable evidence, however, that a number of serotypes not contained in the 7-valent vaccine can cause pneumococcal disease in children. It is unclear at this time as to the impact of the newest pneumococcal conjugate vaccine, which protects against 13 serotypes, on the incidence of bacterial pneumonia due to S. pneumoniae.

The lower respiratory tract is generally sterile as a result of an abundant set of pulmonary host defense mechanisms including mucociliary transport, cough reflexes, components of the complement system, immunoglobulins (particularly secretory IgA), and alveolar macrophages (Chap. 10). Most viral pathogens that cause pneumonia first inoculate and proliferate in the upper respiratory tract. In a subset of patients, viral infection spreads directly to involve more distal portions of the respiratory tract, leading to illnesses such as bronchiolitis and pneumonia. In the course of the illness, the infected respiratory epithelial cells lose their cilia and are sloughed into the respiratory passages resulting in stasis of mucus and accumulation of cellular debris. An inflammatory response at the sites of tissue damage results in mononuclear cell infiltration, and causes swelling of the submucosa and interstitial structures that further contributes to narrowing of the airways. These pathologic changes are particularly problematic in younger children whose airways are already of a small caliber. Various degrees of obstruction of the airways may lead to areas of atelectasis, interstitial edema, and hyperinflation. The resulting imbalance in V̇_A/ Q̇ further contributes to underlying alveolar hypoxia. A direct consequence of these multiple airway abnormalities is an enhanced risk of superinfection with bacterial pathogens.

The pathogenesis of bacterial infection in the lungs often varies according to the etiological agent. For example, S. pneumoniae produces local airway edema allowing proliferation and spread of the organism into adjacent areas of the pulmonary parenchyma. This leads to the typical lobar involvement observed in pneumococcal pneumonia. On the other hand, M. pneumoniae organisms adhere to the ciliated epithelium and paralyze ciliary function, leading to an intense inflammatory response and cellular necrosis. Much like viral respiratory infections, Mycoplasma infection spreads along the bronchial tree. Staphylococcus aureus pneumonia is often a unilateral process that is characterized by areas of bronchopneumonia, hemorrhagic necrosis, pneumatocele formation, and complicated pleural effusions.

In children with either viral or bacterial pneumonia, nonspecific upper respiratory tract findings such as rhinitis or cough may be initial symptoms. Tachypnea is the most common feature of pneumonia at all ages; this finding along with cough, fever, respiratory distress, and an abnormal chest exam (crackles or wheezes on auscultation) are often present in children with pneumonia. In infants, severe viral or bacterial infections may be accompanied by cyanosis and signs of impending respiratory failure. In older children and adolescents with bacterial pneumonia, symptoms may include shaking chills, high fever, cough, chest pain, and splinting (defined here as reduced inspiratory effort due to pleuritic pain). Some children with bacterial pneumonia may present only with fever and chest or abdominal pain, but with cough either absent or a minor feature. Fever is usually present in children with viral pneumonias, although it is often lower than in children with bacterial pneumonia. In reality, there are no sensitive clinical, radiographic, or laboratory findings that reliably distinguish between viral and bacterial pneumonias. In general, x-rays in viral pneumonia show bilateral interstitial infiltrates, hyperinflation, and peribronchial thickening (Fig. 40.1

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