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DEFINITIONS AND EPIDEMIOLOGY

Bronchiolitis, a common communicable respiratory illness manifesting with signs of small airways inflammation, primarily affects children younger than 2 years. Over one in five children will have bronchiolitis-associated wheezing in the first year of life.1 Bronchiolitis is the most common cause of hospitalization of children in the United States. Between 1% and 3% of all children are hospitalized as a result of bronchiolitis; one in four infants hospitalized for bronchiolitis will be younger than 6 weeks of age, and over half are younger than 6 months of age. The mean length of hospital stay for children is 3 days (interquartile range 2–5 days), and the cost for care is estimated at >$0.5 billion annually.2–4 Despite the high prevalence of the disease and high rates of hospitalization, bronchiolitis is responsible for only about 100 pediatric deaths each year in the United States.5

Respiratory syncytial virus (RSV) causes the majority of bronchiolitis-related illness (Table 35-1).6 One recent study of both inpatient and outpatient patients with bronchiolitis found that 76% had RSV, 18% human rhinovirus (HRV), 10% influenza, 2% coronavirus, 3% human metapnemovirus (hMPV), and 1% parainfluenza virus, with some patients testing positive for multiple viruses.7

TABLE 35-1Pathogens Commonly Identified in Patients with Bronchiolitis

Bronchiolitis is more common in the winter months, although the timing of disease onset within a particular season and the duration of the respiratory virus season vary by geographic location. For example, RSV season in the southern United States begins earlier and lasts longer than the RSV season in the rest of the nation.8 There is also significant season-to-season variation in RSV, with the period of peak prevalence varying by as much as 7 weeks (ranging from early January to late February) between seasons.9 While the majority of bronchiolitis seasonality is caused by dramatic annual spikes in RSV prevalence, other viruses exhibit different seasonal outbreak patterns (Figure 35-1).10

FIGURE 35-1.

Seasonal variation of selected viruses that cause bronchiolitis.

Several factors place children at increased risk of severe bronchiolitis. Shortly after recognition of hMPV as a causative agent in bronchiolitis, a group in England noted that 70% of RSV-positive children who required admission to the intensive care unit were coinfected with hMPV, suggesting that coinfection with these two viruses was a risk factor for a more severe disease course.11 In another study, hMPV was detected in 50 (8.5%) of 589 children with respiratory tract infections; 15 ...

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