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Chapter 2 : Respiratory Infections
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This chapter describes the clinical syndromes of respiratory virus infection, the spectrum of viruses associated with these syndromes, and the pathophysiology of these illnesses. Many of the viruses associated with acute respiratory disease display a significant seasonal variation in incidence, especially in temperate climates. The majority of common colds are associated with infection with rhinoviruses or other picornaviruses, particularly when very sensitive techniques, such as polymerase chain reaction (PCR), are used for diagnosis. Viral pathogens associated with acute pharyngitis are summarized in this chapter. Pharyngitis is a typical component of acute influenza in which individuals experience the sudden onset of systemic symptoms of fever, myalgias, and malaise accompanied by upper respiratory signs and symptoms, including pharyngitis. The majority of cases of inspiratory stridor in children are caused by viral croup. In addition to causing croup and bronchiolitis, viral infection of the trachea and bronchi may cause tracheitis or tracheobronchitis. Antiviral therapy is available for some of the viruses responsible for bronchiolitis, but the use of such therapy for the most common etiology, respiratory syncytial virus (RSV), remains controversial. Viral pneumonia is an important cause of morbidity and mortality in individuals with compromised immune systems, with a broader spectrum of viral agents than seen in immunologically intact individuals. The general features of primary viral pneumonia are discussed using influenza as a model. Pathogenesis of other forms of viral pneumonia is also discussed.
Many of the viruses which affect the respiratory tract exhibit a seasonal variation in prevalence. In this figure, numbers of virus isolates from children seen in private pediatric practices in Rochester, NY, are plotted by month of isolation. Data represent the 6-year average from 1990 to 1995 and are expressed as the percentage of all isolates of each virus which occurred in the given month.
Posteroanterior roentgenogram of the neck of a child with viral croup that shows the characteristic narrowing of the air shadow of the trachea in the subglottic area. (Courtesy of Carolyn B. Hall, University of Rochester.)
Lateral neck radiograph of the neck of a child with epiglottitis demonstrates the characteristic thickening of the epiglottis in this disease and may be helpful in distinguishing this illness from croup or retropharyngeal abscess. (Courtesy of Carolyn B. Hall, University of Rochester.)
Pathophysiology of croup. Both mechanical obstruction of airflow and ventilation-perfusion mismatching due to parenchymal infection of the lung are responsible for the hypoxia and respiratory distress of croup. (Modified from reference 60 with permission.)
The chest radiograph in bronchiolitis characteristically shows hyperinflation due to obstruction of airflow. A variety of other findings may be present, including interstitial infiltrates or lobar consolidation. (Courtesy of Carolyn B. Hall, University of Rochester.)
Pathophysiology of bronchiolitis. Viral infection of the lower respiratory tract results in inflammation and increased mucus production. Both airway obstruction and ventilation-perfusion mismatching are responsible for the clinical findings of hypoxia, hyperinflation, and hypoventilation. If uncorrected, these defects can lead to apnea or sudden death. FRC, functional residual capacity; RV, residual volume; A-P, anterior-posterior. (Modified from reference 141 with permission.)
Estimated frequency with which individual viral respiratory syndromes are caused by specific viral pathogens
Therapies of potential benefit in viral pneumonia