Stable asthma prevalence masks increase in allergic phenotype

By Joanna Lyford, Senior medwireNews Reporter

The prevalence of childhood asthma in Sweden appears to have stabilised, with no increase seen between 1996 and 2006, researchers report.

Analysis of the relative contribution of asthma risk factors to this trend revealed a reduction in environmental exposures, such as damp homes and maternal smoking, but an increase in non-environmental factors such as parental asthma and allergic sensitisation.

“This counter-balancing in risk factors may explain the level prevalence of current asthma”, write Anders Bjerg, from the University of Gothenburg, Sweden, and co-authors in Clinical and Experimental Allergy.

Bjerg and colleagues obtained data from the population-based International Study of Asthma and Allergy in Childhood, which assessed 7 to 8-year-old children in 1996 and 2006 using a comprehensive questionnaire. In all, 3430 children participated in 1996 and 2585 participated in 2006.

The prevalence of current wheeze was unchanged between the 1996 and 2006 cohorts, at 11.7% and 13.0%, respectively, as was the prevalence of current asthma, at 5.3% and 6.1%, respectively.

Interestingly, however, the prevalence of asthma risk factors changed significantly, with a reduction in known environmental risk factors and an increase in non-environmental risk factors.

For instance, the prevalence of respiratory infections decreased from 59.9% to 28.6%, maternal smoking fell from 31.5% to 15.8%, damp housing fell from 17.5% to 12.3% and breastfeeding for less than 3 months fell from 16.4% to 10.5%.

Meanwhile, the prevalence of male gender rose from 51.0% to 51.6%, parental asthma increased from 17.2% to 24.4% and allergic sensitisation rose from 20.6% to 29.9%.

For each risk factor, Bjerg et al calculated the adjusted population attributable fraction (aPAF) using the prevalence and multivariate odds ratio. They report that the aPAFs of non-environmental risk factors for current asthma increased, whereas the aPAFs of environmental risk factors decreased.

In particular, the aPAF of male gender increased from 20% to 25% whereas the aPAFs of damp housing and maternal smoking decreased to near zero.

“Expressed as aPAFs, the risk factor models accounted for ≥85% of current asthma and 78% of current wheeze both in 1996 and 2006”, Bjerg and co-authors remark.

They conclude: “These risk factor trends and the increase of the allergic asthma phenotype may seriously affect the future remission probability of asthma in this region… A level prevalence may in fact reflect counteracting trends in risk factors, which argues that risk factors should be monitored closely also in other areas.”

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