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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 are 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 over a half billion dollars 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
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Respiratory syncytial virus (RSV) causes 50–70% of bronchiolitis-related illness (Table 32–1).6 Recently identified causes of bronchiolitis include human metapneumovirus (hMPV) and human bocavirus (HBoV), a human parvovirus. These viruses have been detected in 20–25% for hMPV7,8 and 5% for HBoV9 of children with bronchiolitis and negative direct fluorescent antibody testing of nasal aspirates for RSV, parainfluenza types 1 to 3, influenza A and B, and adenovirus.
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Bronchiolitis is more predominant in the winter months though 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. Between 1990 and 2000, the median onset of RSV season (defined as the first 2 consecutive weeks where more than 10% of nasal wash specimens tested at a regional reference laboratory were positive for RSV) was late November for the South and late December for the rest of the country.10 The median duration of RSV seasons during this 10-year time period was 16 weeks for states located in the South compared with 13 weeks for states in the Midwest.10 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.10 While the majority of bronchiolitis seasonality is caused by dramatic annual spikes in RSV prevalence, other viruses exhibit different seasonal outbreak patterns (Figure 32–1).11
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