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We have previously shown that the prevalence of asthma in Australian primary schoolchildren increased substantially between 1982 and 1992.1 Similar increases have been reported in studies of children of different ages and from various geographical regions, spanning periods up to the mid-1990s.2 It is not known whether this trend has continued during the late 1990s and early 2000s. We therefore conducted a third cross sectional study in the same population that was surveyed previously.1 We report here on prevalence trends over the latter 10 year period.
We conducted all studies during June and July in primary schools in and around Belmont, a coastal suburb some 150 km north of Sydney, Australia. We invited all children in years 3, 4, and 5 (ages 8-11 years) at selected schools to participate and studied only children who had parental consent. Parents completed a questionnaire about symptoms, diagnosis, and treatment of asthma and other allergic illnesses. We used a histamine challenge test to measure airway hyperresponsiveness and assessed atopy by skin prick tests to house dust, Dermatophagoides farinae, D pterronyssinus, ryegrass, cockroach, cat, Alternaria tenuis (Hollistier-Stier, Spokane, WA, USA). Questionnaires and tests were the same as in 1992.1 The data collected in 1982 are not directly comparable because only 8-10 year old children were included and some equipment was different1. Owing to a low initial response rate in 2002 a single page anonymous questionnaire was issued to parents who had not consented to their child's participation in the clinical tests. The limited data from this questionnaire have been included.
In 2002 we initially enrolled 627 children (292 (46.6%) boys), representing 51.3% of the eligible sample of 1222. A further 183 participants subsequently provided a questionnaire, yielding an overall sample of 810 children (399 (49.3%) boys), representing 66% of the eligible sample. The response rate in 2002 was lower than in previous surveys (table). Between 1992 and 2002 the prevalence of diagnosed asthma, recent wheeze, and use of asthma medication decreased significantly (table). However, the prevalence of hay fever, eczema, atopy, airway hyperresponsiveness, or current asthma (defined as recent wheeze plus airway hyperresponsiveness) did not change significantly. These trends contrast with the substantial rise in the prevalence of most of these indicators during the period 1982 to 1992.1
These results provide evidence that the trajectory of increasing prevalence of asthma has not continued. A potential limitation is the possibility of selection bias arising from a lower response rate in 2002 compared with 1992. In the 2002 survey the prevalence of asthma symptoms was higher in the initial responders than in the responders in the second phase (data not shown). It seems reasonable to assume that non-responders were more similar to the responders in the second phase than the initial responders,4 and some empirical evidence supports this.5 If this is the case then the prevalence estimates for 2002 are likely to be overestimates for the population. This direction of potential bias tends to strengthen our conclusion that the prevalence of asthma has not increased further during the period 1992 to 2002.
Although it is good news that the trajectory of increasing asthma prevalence has halted in the locality we studied, it remains to be seen how generalisable and sustained this new trend is. Uncertainty remains about the extent to which fluctuations in asthma prevalence over the past two decades can be attributed to changes in awareness of asthma. The explanation for the reduction in prevalence remains as elusive as the explanation for the initial increase.
Contributors: BGT collected data, conducted data analysis, and wrote the manuscript. KN coordinated the study, collected data and reviewed the manuscript. EB collected data, designed the database, checked accuracy of the data, and reviewed the manuscript. CMS participated in all three surveys, collected data, and reviewed the manuscript. GBM collected data, interpreted the data, and co-wrote the manuscript. JKP assisted with interpretation of the data and reviewed the manuscript.
Funding: Australia Health Management Group.
Competing interests: None declared.
Ethical approval: Human Ethics Committee of the University of Sydney.