Viruses in asthma

Abstract

Current evidence suggests that the overall load of infectious agents, including respiratory viruses, encountered early in life is an important factor influencing maturation of the immune system from a type 2 bias at birth towards predominantly type 1 responses, thus avoiding atopic diseases. The 'hygiene hypothesis' proposes that the relatively sterile environment present in industrialised Western countries has contributed to the recent epidemic of asthma and atopy. Whether specific infections are of greater or lesser protective value is an important question if strategies are to be derived to mimic the beneficial effects of childhood infection whilst avoiding morbidity and potential mortality of the natural pathogens. Infection by respiratory viruses is a major trigger of wheezing in infants and of exacerbations of asthma in older children. Viruses are detected in up to 85% of such episodes. Rhinovirus is common in all age groups; respiratory syncytial virus (RSV) is most important in infants and young children. Knowledge of the immunopathogenetic mechanisms of virus infection in the asthmatic airway will lead to the development of new treatments for virus-induced asthma.

Fig 1.

Prevalence of frequent wheezing among children who had two or more older siblings or who attended day care during the first 6 months of life and among children with less exposure to other children. Children with less exposure to other children were those who had one or no older siblings and who did not attend day care during the first 6 months of life. P values are for the comparisons between the two groups of children. [Figure 2 from Ball et al., N Engl J Med 2000; 343: 538–43⁶ © 2000 Massachusetts Medical Society. All rights reserved.]