In recent decades, the prevalence of asthma and allergic diseases has increased substantially, and today asthma is the most frequent chronic disease among children in developed countries . This imposes a considerable burden of disease on patients, healthcare systems and society. Whereas, some studies still demonstrate an increasing prevalence of asthma and allergic disease [2-4], other reports indicate a levelling off or even a decrease in the prevalence [5-7]. Results from the ISAAC phase III study (2000–2003) indicate that the difference in asthma symptom prevalence between developed and developing countries has fallen . In spite of this change, the prevalence of asthma, allergic rhinoconjunctivitis (AR) and eczema shows wide global variation . The diverse global trends make regional repeated investigations important to assess time trends. Local surveys provide information about geoclimatic variables and topographical factors that may affect disease prevalence . In the northern part of Norway, a questionnaire-based, cross-sectional survey of asthma and allergic disease was performed in 1985  and repeated 10 years later: The lifetime prevalence of asthma and allergic diseases increased over this period . Thus, the objective of the current study was to explore whether or not the prevalence of asthma, AR and eczema continued to increase in a population of children in a subarctic area.
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- Subjects and Methods
- Conflict of interests
The main findings in the present study were an increasing prevalence of asthma ever and AR ever, while the prevalence of eczema ever, after increasing between 1985 and 1995, stayed unchanged in the last time period. The prevalence rates found in 2008 are similar to those shown in the Norwegian birth cohort study, Oslo , but somewhat higher compared to results in the OLIN studies (Obstructive Lung Disease in northern Sweden) [16, 17]. In spite of an increasing prevalence of asthma ever, data from the last time period indicated a peak in asthma symptoms. The divergent prevalence of asthma symptoms and asthma diagnosis are consistent with data from other Nordic reports [7, 18]. In contrast to several prevalence studies in comparable populations [2, 9, 19], we demonstrated a substantial increase in the proportion of children reporting current diseases in the last time period.
Atopic disease in the family is an important risk factor for developing asthma, AR and/or eczema [16, 20, 21]. Increased atopic burden among family members is evident. Two-thirds of the children in the 2008 survey had parents and/or siblings with atopic diseases, along with a doubling of parental asthma and allergy between 1985 and 2008. The proportion of children suffering from asthma, AR and/or eczema confirming atopic disease in the family more than doubled during the study period. Asthma, AR and eczema are closely related [2, 22]. Still, we found a levelling off in the comorbidity of asthma and AR, while the comorbidity of asthma and eczema increased. This pattern is in line with the findings in a recent report from the ISAAC study .
Male gender is a risk factor for asthma and allergy among children [7, 24]. We detected a male predominance, which is in line with the results found by Anthracopoulos et al. . Whereas the results for asthma and AR revealed a male dominance through the study period, the results for eczema demonstrated a higher proportion of girls in the last two surveys. A study from Larsson et al.  supports these findings. Thus, the unchanged prevalence of eczema ever in the last time period might be due to an unchanged prevalence in girls.
Local environmental factors might be important contributors to different disease prevalence . Passive smoke exposure decreased sizeable from 1985 to 2008. Because second-hand smoke exposure is associated with both the development of asthma and more severe disease, the decrease might be expected to lower the prevalence of asthma . Most of the study population lives in a cold, coastal climate. During wintertime, the amount of indoor time is higher in a subarctic population. Weiland and colleges have proposed a negative effect of the annual variation of temperature and relative humidity outdoors on asthma and eczema symptoms .
The use of identical study design and questionnaires in three surveys has allowed us to assess a valid estimate of time trends for self-reported atopic diseases. The reliability of the results of studies based on questionnaires can be questioned. However, self-reported symptom history conducts the necessary basis for defining asthma in epidemiological studies [10, 28]. Repeated surveys in the same population are as close a proxy, as it was possible to attain of longitudinal data. In contrast to the ISAAC studies, we compared data from the same population during three decades, which formed the basis of more valid time trends. Still, the time interval between the last two surveys might have been too long to detect a plateau in disease prevalence. Thus, the need of follow-up surveys in due time is evident. Questions about current symptoms are more reliable than questions about symptoms ever due to less recall bias and might give a better estimate of time trends. The proportions of children reporting use of asthma medication ever and last year are not different from the prevalence of asthma ever and current asthma, which strengthen the results.
Allergic diseases are given significant public health and media attention in Western societies. The impact of increased general awareness has been proposed in several papers to explain the increasing trend in the prevalence of allergic diseases . Increased awareness among health professionals and parents might have influenced the time trends in our study, and the increased atopic burden might represent a selection bias. Parents suffering from these diseases are expected to be more positive to participate and to be more aware of symptoms and diseases. Thus, it is a possibility that the increase in heredity and disease prevalence might partly be due to this selection bias. The response rate of 64% in 2008 is lower than in the previous surveys, and might entail a potential selection bias. In 2008, we were not allowed to give personal reminders to the participants. We believe this represent the main cause of the lower response rate compared with the earlier surveys. It was not possible to perform any analysis of the nonresponders. However, a large postal survey in Sweden by Rönmark et al.  concluded that nonresponders did not differ significantly in the prevalence of airway diseases or symptoms compared with responders. Thus, response bias is unlikely to have seriously affected the results. As we included a large, representative fraction of the population from randomly selected schools in Nordland, we believe the external validity of our study to be high.
In conclusion, the prevalence of asthma ever and AR ever in a subarctic children population increased substantially between 1985 and 2008, while the prevalence of eczema ever reached a plateau. The doubling and tripling of current asthma, eczema and AR in the latest study period raises questions why and provide the basis of further investigation.