Respiratory viral infections are the most important and frequent causes of asthma exacerbation, unfortunately their proper pathophysiological mechanisms remain unclear. This article discusses the epidemiological evidence to suggest viral infections exacerbate asthma that is available to date. An understanding of the probable pathophysiological mechanisms of certain viruses will hopefully provide a theoretical basis for controlling and preventing virus induced asthma exacerbation.

Respiratory Viruses

Viruses cause many respiratory disease, from the common cold to pneumonia, depending on the site and dose of virus inoculation and the degree of host immunity.

Human rhinoviruses represent a large genus-containing more than 100 antigenically distinct serotypes within the class of picornaviruses. Consequently, their detection in clinical samples and effective vaccination against them is difficult. Rhinoviruses are the most common viruses affecting older children and adults. They cause approximately 60% of acute respiratory illness and asthma exacerbation. Respiratory syncythial viruses (RSVs) are most commonly responsible for viral infections in infants in the hospital setting. These cause approximately 50% of all wheezing illness and 80% of bronchiolitis. Influenza viruses usually occur in epidemics due to major and minor antigenic drift.

Responses to influenza viruses can vary from mild upper respiratory infection to severe lung infection. Parainfluenza viruses are specifically related to croup in young children, and corona viruses cause approximately 10 to 15% of all primary respiratory infections. Adenoviruses can cause colds, but are also associated with severe lower respiratory tract infections. All of these viruses are capable of exacerbating asthma to varying degrees.

Asthma Exacerbation and Viruses

Since the reports of asthma exacerbation during the influenza epidemics to 1957, there have been many observations of asthma exacerbated associated with viral infection. The advent of polymerase chain reaction (PCR) assay has been important in improving our ability to detect the viruses involved.

In 32 asthmatic children aged one to five years, Mclntosh et al found that 139 asthma attacks were related to viral infection which were confirmed by virus cultures or increased serum viral antibody titres, but none were found in association with bacterial infections. In several community-based studies, it has been demonstrated that 85% of asthma attacks in children, and 44% to 80% of those in adults, are associated with respiratory viral infections. The virus that most commonly causes asthma like symptoms and bronchiolitis in infections is RSV; rhinoviruses are the most common viral cause of asthma exacerbations in older children and adults.

Johnston et al reported that viral infections precipitate asthma exacerbation leading to hospital admission. In a time-trend analysis, the seasonal patterns of respiratory infection were found to correlate strongly with hospital admissions for asthma for both children and adults. In children both viral infections and asthma admissions were seen to peak at the beginning of school terms.

In a community based prospective study conducted over one year, a close relationship was also found between viral infection and asthma exacerbation in 9 to 11 year-old children. The children suffered four episodes of lower respiratory symptoms per year and upper respiratory symptoms symptoms preceded lower respiratory symptoms and fall in peak flow by 1 or 2 days. These data suggest that viral infection is an important cause of asthma exacerbation, including asthma attacks sufficient to require hospital admission in both children and adults.