Changes in prevalence of asthma and allergy in Swedish school children over almost three decades and factors reducing risk of allergy

Abstract: ABSTRACT Background: Asthma and allergy are one of the most common chronic diseases among children. The prevalence of allergic diseases increased dramatically during the last decades of the twentieth century. The reason behind this increase is still not fully understood. Aim: The aim of the thesis was to investigate changes over time in the prevalence of asthma and allergy in two Swedish towns, and to identify protective and risk factors for allergy development. Methods: Data were obtained from two studies: a population-based cross-sectional study that was used in all four papers, and in paper III results from a birth cohort study was added. The cross-sectional study (N=1029) was performed in 2007 and prevalence data were compared with two previous cross-sectional studies conducted in 1979 and 1991. All three studies were based on questionnaires with the same set of questions on asthma and allergy, and the children were of the same age and from the same two Swedish towns: Mölndal/Gothenburg and Kiruna. Questions on dietary habits and food allergy were added in 2007 study. The birth cohort study (N=249), used in paper III, recruited children from Västra Götaland county. The children were recruited at birth, between 1998 and 2005, and they were clinically assessed at the age of 8-9 years. Results: Paper I: The prevalence of asthma were 2.5% (1979), 5.7% (1991) and 7.1% (2007), allergic rhino-conjunctivitis: 5.5% (1979), 8.1% (1991) and 11.1% (2007), eczema 7.1% (1979), 18.3% (1991) and 19.7% (2007) respectively. Allergic rhino-conjunctivitis continued to increase from 1979 to 2007, while asthma and eczema levelled off between 1991 and 2007. Having both parents born abroad was a protective factor for developing allergy. Paper II: Hand-dishwashing decreased the risk of allergy (odds ratio 0.57; 95% confidence interval 0.37-0.85). The risk was reduced in a dose-response pattern if the child was also served fermented food, and if the family bought food directly from a farm. Paper III: Keeping cats and dogs during the first year of life was associated with a decreased risk of allergy in a dose-dependent manner. Sensitisation to animals and pollen also decreased with an increasing number of cats and/or dogs kept indoors. Paper IV: The total cumulative incidence of self-reported food allergy was 19.6%, and it was significantly higher in Kiruna (28.5%) than in Mölndal (15.7%). Introducing complementary food from 7 months of age or later, and a mother’s history of allergy, were both independent risk factors for developing food allergy. Complementary food was introduced at a later age in Kiruna. Conclusion and implications: In the latter years of the previous century, the rising trend of allergic diseases in children seemed to level off, with the exception of allergic rhino-conjunctivitis. Our results does not give any specific explanation for this break in the trend line for asthma and eczema, but one may speculate that climate change with longer pollen periods may have prevented a similar course for rhino-conjunctivitis. However, some important protective factors were found, such as lifestyle factors that were associated with a reduced risk of allergy development. Both dishwashing by hand, eating fermented food and buying food directly from farms were protective factors, as was the keeping of indoor pets during the first year of life. With pets, the risk of allergy was reduced in a dose-dependent pattern. Of the identified risk factors, introducing complementary food late to an infant’s diet increased the risk of food allergy, as did a maternal history of allergy. Time trends describing changes in the prevalence of a disease is always important, not only for the organisation of the health care system, but also for our understanding of the underlying mechanisms. Of similar importance is the finding of factors that can protect children from allergy development, especially if they could be adopted to daily life and lifestyle habits. One such factor is when to introduce complementary food to a child’s diet, and our results indicate that it should not be delayed. Pet keeping and dishwashing by hand may also be possible to implement by some families, but the main importance of their protective effects are mainly of mechanistic significance, with results supporting the hygiene hypothesis.