Viruses in asthma exacerbations

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Abstract
Respiratory viruses are well recognized as major triggers of acute exacerbations of asthma in children and adults, resulting in frequent outpatients visits and hospitalizations. Clinical and epidemiologic evidence supports this association. The application of molecular diagnostic methods has improved understanding of viral epidemiology and the pathophysiological mechanisms involved in viral induced acute asthma. This article reviews publications since October 2002 for an update of the role of viruses in exacerbations of asthma. Respiratory viruses are present in most patients hospitalized for life-threatening asthma and acute non life-threatening asthma. Rhinovirus is the most common, but coinfection with other viruses may be important. Patients with asthma are not more susceptible to upper respiratory tract rhinovirus infections than healthy people but suffer from more severe consequences of the lower respiratory tract infection. Recent epidemiologic studies suggest that viruses provoke asthma attacks by additive or synergistic interactions with allergen exposure or with air pollution. An impaired antiviral immunity to rhinovirus may lead to impaired viral clearance and hence prolonged symptoms. Respiratory viral infections cause asthmatic exacerbations by triggering recruitment of Th2-type cells into the lungs. There is no specific antiviral strategy for prevention of respiratory-triggered asthma exacerbations, although clinical trials of potential antiviral agents are ongoing. Indirect prevention strategies focus on the reduction of overall airway inflammation to reduce the severity of the host response to respiratory viral infections. Respiratory viral infections are a major cause of morbidity and mortality in asthma. There is a lack of specific antiviral strategies in the prevention or reduction of viral-triggered asthma exacerbations. Recent advances in understanding of the epidemiology and immunopathogenesis of respiratory viral infection in asthma provide opportunities or identification of specific targets for antiviral agents and strategies for management and prevention.