Respiratory virus infection and airway hyperreactivity in children

A significant portion of infants and young children develop acute lower airway obstruction (wheezing) accompanying viral lower respiratory infections. These wheezing‐associated respiratory illnesses, particularly bronchiolitis, are often associated with evidence of prolonged airway obstruction and airway hyperreactivity following the initial infection. The factors that determine the type of clinical infection that initially develops and long‐term respiratory consequences of this infection are complex, including the genetic background of the child, the age at the time of infection, the respiratory virus involved, and environmental factors such as exposure to cigarette smoke. Several animal models have been developed to describe the pathophysiology of viral respiratory infection and have helped to define several potential mechanisms for the resulting airway obstruction. Data from these studies and studies in humans emphasize the importance of the immune system and of neural modulation of airway smooth muscle tone in determining the outcome of respiratory virus infection. These observations also help define the pivotal role of the airway epithelium in the regulation of airway smooth muscle tone and the host inflammatory response in the airway. Consequently, the interplay of host and environmental factors, as well as the relative importance of the different mechanisms potentially active in airway obstruction, determine the acute clinical outcome of infection for an individual child and the propensity of that child to develop recurrent episodes of wheezing (asthma).