The recent substantial increase in the reported prevalence of asthma worldwide () has led to numerous studies of the prevalence and characteristics of this condition.2
Foremost among these are 2 major international initiatives that have collected data using validated questionnaires, one among children, the International Study of Asthma and Allergies in Childhood,3
and the other among young adults, the European Community Respiratory Health Survey.4
Follow-up investigations for both of these studies5,6
have examined temporal trends within and across populations. During a mean of 7 years following phase I of the International Study of Asthma and Allergies in Childhood, which in most participating countries was conducted between 1991 and 1993, the prevalence of asthma was stable or decreased in some areas of the world but increased substantially in many other areas, especially among children 13–14 years of age ().5
Figure 1 Changes in prevalence of diagnosed asthma (A) and asthma symptoms (B) over time among children and young adults. Reproduced, with permission, from Eder W, Ege MJ, von Mutius E. The asthma epidemic. N Engl J Med 2006;355:2226–35. Copyright 2006 (more ...)
Figure 2 Annual changes in worldwide prevalence of asthma symptoms among children 6–7 years old and 13–14 years old, over a mean of 7 years following phase I of the International Study of Asthma and Allergies in Childhood (which in most participating (more ...)
Cross-sectional population-based studies such as these are highly dependent on recognition of symptoms, so they do not necessarily reflect the true heterogeneity of asthma. However, a wide variation in prevalence rates has been documented: studies of both children and adults have revealed low prevalence rates (2%–4%) in Asian countries (especially China and India) and high rates (15%–20%) in the United Kingdom, Canada, Australia, New Zealand and other developed countries.3–6
Observations of migrating populations7
and of Germany after reunification8
have strongly supported the role of local environmental factors, including allergens but likely many lifestyle factors as well, in determining the degree of expression of asthma within genetically similar populations. A recent analysis of data from the International Study of Asthma and Allergies in Childhood, comparing data from Vancouver, Canada, with data from centres in China, showed significant differences in prevalence rates between children of similar genetic ancestry living in different environments, with evidence for an effect of duration of residence in the new environment.9
Prevalence rates for asthma among children 13–14 years old were lowest for Chinese children born and studied in China, intermediate for Chinese children who had migrated during their lifetime to Canada and highest for Chinese children who had been born in Canada. In addition, the prevalence rate for the third of these groups was still lower than among non-Chinese children in the same environment. Together, these results strongly suggested gene-by-environment interactions.
Local and national studies have also provided insights into the epidemiology of exacerbations of asthma. For example, epidemics of asthma exacerbations in Barcelona, Spain, were eventually linked to exposure to atmospheric soybean dust released during cargo handling at the local port.10
The highly predictable annual epidemic of asthma exacerbations in school-age children in the northern hemisphere every September, peaking some 17 days after the return to school, appears to be predominantly driven by seasonal rhinovirus infection, probably compounded by other risk factors for asthma exacerbations, including reduction in use of asthma controller therapy over the summer months, exposure to seasonal allergens and possibly the stress of returning to school.11,12
Complementing these cross-sectional studies are longitudinal epidemiologic studies in a variety of populations and countries, which have allowed examination of risk factors predicting the development, persistence, remission or relapse of childhood asthma. One such population-based birth cohort study in Dunedin, New Zealand, which had a high retention rate, examined outcomes of childhood asthma at age 26 years.13
Female sex, airway hyperresponsiveness in mid and later childhood, and sensitization to house dust mites were all significantly and independently related to the likelihood of persistence of childhood asthma to early adulthood. Early age of onset of wheezing symptoms was predictive of relapse after remission, as were airway hyperresponsiveness and allergy to house dust mites. That study and others have clearly demonstrated the tracking of characteristics of asthma from childhood to adulthood, including severity and impairment of lung function.