West Sweden Asthma Study: Prevalence trends over the last 18 years argues no recent increase in asthma

In terms of prevalence and morbidity, asthma has increased in most parts of the world during the second half of the past century [1‐3]. The increase was first recognised in Australia, New Zealand and in areas of the United Kingdom (UK) and the USA, countries in which the mortality in asthma also increased at the time [1, 4, 5]. Less change in the prevalence, morbidity and mortality was seen in Continental [6, 7] and Eastern Europe [8]. In Eastern Europe, different diagnostic traditions compared to Western Europe partly explained a lower prevalence [9, 10]. During the last decades of the century a marked increase in asthma was also detected in developing countries [11], particularly in large cities [7, 11], while the prevalence did not change considerably in rural areas of Africa and China [12, 13]. Recent studies, including the ISAAC III [14], suggest that the increase in asthma among children and adolescents has leveled off in several westernized countries [15‐17]. However, in some of these countries, such as Germany and UK, studies also point out diverging and opposite trends [17‐19]. In contrast to children, there are no recent published studies of the change of prevalence in adult asthma and symptoms common in asthma.

In Sweden the asthma prevalence increased from 2-3% in the 1970s [20, 21], to approximately 5% in the 1980s [22, 23] and to 8-10% in the mid 1990s [24‐26]. The increase was first noticed in the north of Sweden [22, 27] and was, to some extent, explained by an increase in diagnostic activity [28]. Notably, there still seems to be a north-south gradient in the prevalence of asthma with a slightly higher prevalence in the north [24]. Population based data of asthma prevalence among adults in Sweden has not been published for the past ten years.

In 2008, a large study focusing on asthma and allergic diseases was initiated in West Sweden. The first part of the study was a postal questionnaire survey on respiratory symptoms and diseases, as well as allergic conditions. The initial aim was to update the prevalence of asthma, allergy and respiratory symptoms, which is presented here. Our hypothesis was that the increasing trend of adult asthma in Sweden has reached a plateau.

This study presents the most updated information on the current prevalence of asthma and respiratory symptoms in northern Europe. Furthermore, the study allows for analyses of change in prevalence over eighteen years, for which there is no published recent evaluation using similar methods. Importantly, the overall message of this study is that the previously demonstrated increase in prevalence of asthma has levelled off in the region. Furthermore, most respiratory symptoms have significantly decreased in prevalence.

The prevalence of physician-diagnosed asthma in this study was estimated to be 8.3% and was greater in women than men. The current questionnaire study used nearly identical questions to the ones used in studies performed in 1996 in Finland, Estonia and Sweden (the FinEsS Studies), as well as in other studies performed in three different regions of Sweden, the capital Stockholm, the county of Norrbotten and the city of Örebro [10, 24‐26]. In both Örebro and Stockholm, the prevalence of asthma was estimated to be 8%, while it was 10% in Norrbotten [10, 24‐26]. Thus, our study together with the previous studies strongly support the notion that the prevalence of asthma in Sweden is currently between 8-10%, with minor regional variation, with no further increase observed since the late 1990s.

When comparing our results with the Gothenburg part of the ECRHS performed eighteen years before our study, the reported prevalence of asthma had increased from 6% to 8% in the same area of Gothenburg, the Hisingen Island [31]. However, this comparison must be judged with some reservation, because the difference is modest and the questions about asthma were not exactly identical. An increase of a similar magnitude was also observed in the southern part of our study area, the former county of Southern Älvsborg [32], but in this case using identical questions about asthma. Whether this increase after early 1990s reflects a real increase in asthma prevalence cannot be firmly verified, as the symptoms of asthma in the currently investigated individuals have been reduced. Thus, our findings suggest that a greater proportion of mildly symptomatic asthmatics today are diagnosed as having asthma, since the prevalence of active asthma was only 6.9% in the current study, and about half of these individuals used either asthma medication or had only one or two symptoms common in asthma (Figure 2). Those diagnosed with asthma in the 1980s and 1990s had clearly more symptoms of asthma than found in the current study and other recent studies [23, 33]. In addition, a greater proportion of asthmatics utilised asthma medication in the 1980s and the early 1990s compared to the individuals with diagnosed asthma in our study. These two findings together argue that a greater proportion of patients with mild asthma and asthma like symptoms have received the diagnosis of asthma compared to the early 1990s. This conclusion is further supported by the studies of the incidence of asthma in northern Sweden from 1986 to 1996, which discovered that approximately half of the cases were as a result of better detection of asthma and of increased diagnostic activity within the medical community [28]. Therefore, the slight increase in doctor's diagnosis of asthma comparing our study with the two studies in 1990 and 1994 may be explained by an increased diagnosis of asthma rather than a true increase in prevalence.

In contrast to the decrease in prevalence of respiratory symptoms, our study demonstrated a clear increase in the prevalence of allergic rhinitis in the area of Hisingen compared to the 1990 ECRHS study results [31]. As allergic rhinitis is closely associated with allergic sensitisation, this marked increase might reflect an increase in allergy sensitisation in the area. While there is no recent data about the prevalence of allergic sensitisation in Sweden, ongoing clinical examinations of the current cohort will provide such information in the next few years. As rhinitis is a risk factor for the development of asthma, it cannot be excluded that the prevalence of asthma may again increase in Sweden in the future.

The argument that a plateau in asthma prevalence has been reached after the late 1990s is supported by the decrease or lack of increase of the prevalence of respiratory symptoms in different age groups in the current study. Comparing our results with the prevalence of symptoms in the 1990 ECRHS study in Gothenburg [31], almost all respiratory symptoms decreased significantly (Figure 3). Symptoms that may be related to smoking, such as any wheeze, sputum production and longstanding cough were reduced, but symptoms that are closely related to asthma including wheezing with breathlessness also decreased markedly. A clear decrease in the prevalence of wheezing was also observed in southern Älvsborg. No major changes were found regarding other respiratory symptoms, which may in part be explained by some differences in the questionnaires [32].

The decrease in prevalence of respiratory symptoms in the area of Hisingen in Gothenburg may have several explanations. Firstly, a major decrease in smoking prevalence was observed. Furthermore, a change in socio-economic status composition has been observed in parts of the area, from predominantly working class to middle class. In addition, this area contained pollution emitting industries until approximately the 1980s. Thus, changes in smoking habits, industrial structure and socio-economic status composition are all parallel with the decrease observed in respiratory symptoms. It should also be considered that the reduction in respiratory symptoms may partly be due to patients with airway diseases now having access to more efficient medications.

The demonstrated risk factor patterns for asthma and symptoms in the current study confirm findings from previous studies [23, 24, 26, 33]. Rhinitis was strongly associated with asthma, and the magnitude of the odds ratio was similar to that previously reported in asthma studies of Swedish adults [24]. As the study design was cross-sectional, the results only verify an association and cannot contribute to the discussion of either cause or consequence. A family history of asthma was significantly related to both asthma and respiratory symptoms, but tended to be somewhat less related to asthma than found previously [24, 33], a fact that may be explained by a broader labelling of the term asthma by the medical community, and the inclusion of patients with milder disease in this category. In agreement with previous studies, female sex and ex-smoking was closely associated with asthma, while current smoking was only associated with asthma with borderline significance [24, 26, 33].

Regarding age, the multivariate analysis verified that young adults are at highest risk of having a diagnosis of asthma. Interestingly, studies performed in the 1990s found asthma to be most common in adolescents and young adults [24], closely related to a high incidence in children and teenagers [34, 35]. In the current study, the prevalence was highest in the age group of 26-35 year olds (physician-diagnosed asthma 10.2%), while it was lower in the age group of 16-25 year olds, arguing against a further increase in prevalence of asthma in the lower age group. The current findings, together with findings from several studies presented in the last decades, argue that the continued increase in asthma prevalence that has been observed over half a century is now over.

All respiratory symptoms were significantly associated with smoking. The symptoms most closely related to smoking were any and recurrent wheeze, cough and sputum production. These findings are similar to reports from several Scandinavian studies [23, 26, 36]. Chronic respiratory symptoms increased with increasing age, a relationship that was confirmed by the multivariate analysis. However, it was observed that symptoms were less age-dependent compared with previous Swedish studies [21, 23, 27]. High age (61-75 years) was poorly related to most respiratory symptoms with odds ratios of 1.00 to 1.32, and most of these symptoms did not significantly relate to age.

The differences in prevalence in asthma and respiratory symptoms between the metropolitan area of Gothenburg and the non-metropolitan area were strikingly small. All asthma-associated variables were equally common in the two samples, while the prevalence of most symptoms was only slightly, but significantly, more common in the city of Gothenburg. Thus, in the multivariate analyses, the area of domicile, i.e. living in Gothenburg, was a significant risk factor only for sputum production. These results may reflect an improvement in the levels of outdoor air pollution in the metropolitan areas in Sweden [37].

This study provides conclusive results for several reasons. The large sample size and the use of well validated questionnaires contribute to high internal validity of the results. As identical questions have been used in several previous studies, many opportunities for comparisons were available, and can contribute to further analyses. Therefore, the external validity can be judged as high, partly because this study can be utilised for future comparisons. The response rate was slightly lower than in earlier Swedish and Nordic studies [23‐29]. However, in a study of non-responders, no important bias was detected between early and late responders [30]. The alternative of answering over the internet was utilised by surprisingly few individuals, but could still be a more efficient way of working in the future, especially with younger generations. Postal questionnaires always have one key weakness in that they can never provide evidence for any direct causal reasons or mechanistic information in any disease. Furthermore, the cross-sectional design itself makes discussion of cause or consequence, as well as detected associations, less convincing.

In conclusion, our study provides new and unique evidence that the previous increase in asthma prevalence over the last 10-15 years in West Sweden has now levelled off. Asthma is still highly prevalent, with 8.3% of the population being affected, which makes it one of the most common diseases in Sweden. Furthermore, asthma can still be lethal, and the incidence of asthma mortality in children and young adults has only partly decreased in the last decade [38].