IJE Advance Access originally published online on June 3, 2005
International Journal of Epidemiology 2005 34(5):1018-1019; doi:10.1093/ije/dyi115
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Commentary |
Commentary: Asthma time trends—mission accomplished?
1 Centre for Public Health Research, Massey University Wellington Campus, Private Box 756, Wellington, New Zealand
2 Department of Biomedical Sciences and Human Oncology, University of Turin, 10126 Torino, Italy
* Corresponding author. E-mail: n.e.pearce{at}massey.ac.nz
Until recently most studies had reported that asthma prevalence has increased in recent decades and that the magnitude of the increase had, in some cases, been substantial.1 The best indication of what is now happening globally will be provided by the forthcoming findings of Phase III of the ISAAC study. However, some individual ISAAC centres in Western countries, as well as several studies in adults, have already reported either no increase, or even a decrease in asthma prevalence over the last ten years.2 Furthermore, Phase II of the European Respiratory Health Survey (ECRHS) found no increase in current or severe asthma symptoms, but a significant increase in diagnosed asthma.3 The most likely explanation for these patterns is that asthma prevalence has peaked or even begun to decline, whereas the observed increases in diagnosed asthma reflect changes in diagnostic labelling and/or medical treatment for mild and/or moderate asthma.2
In this context, the findings reported by Bollag et al.4 in this issue of the journal are of considerable interest. They examined time trends in consultations for asthma in primary care in Switzerland. Overall consultation rates for asthma increased from 1989 to 1994, then stabilised and have declined since 2000. There was also a small decline in first consultations for asthma since 1999, but subsequent consultations for asthma have been falling at least since 1994. The ratio of subsequent to first episodes of asthma fell in all age groups.
However, while it is tempting to agree with Bollag et al.4 that these patterns are due to improvements in asthma management, and to declare 'mission accomplished' in the fight for asthma control, such a declaration would be premature.
In particular, we need to distinguish between three different aspects of asthma occurrence: incidence, prevalence and severity.5 In parallel with these three measures of asthma occurrence, we have corresponding measures of contact with asthma health services. Incident cases may (or may not) be diagnosed, prevalent cases may (or may not) have a primary health care consultation during a specified period, and severe cases may (or may not) have frequent consultations. Thus, there are three important reservations that should be born in mind before we conclude that the observed patterns are due to improved asthma management.
First, an improvement in management cannot explain the decrease in first consultations for asthma that appears to have occurred since 1999.
Second, although an improvement in management could plausibly explain the decrease in severity of diagnosed asthma severity that has occurred since 1994, there are other equally plausible explanations for these patterns. As noted above, other studies have shown stabilisation or decreases in asthma prevalence, and perhaps asthma severity, since the early 1990s while diagnosed asthma has increased.2 This is most likely due to an increased awareness and willingness of physicians to diagnose asthma. Such a tendency would result in an increase in asthma diagnosis, but a decrease in average (diagnosed) asthma severity. When this is superimposed on an underlying trend for the population incidence (and prevalence) of asthma to decrease, this will produce the type of pattern observed by Bollag et al.4
Furthermore, although it is clear that the use of asthma self-management plans and inhaled corticosteroids can improve asthma control, and reduce severity, in individual patients, it is not clear that these factors explain the patterns at the population level. For example, Bollag et al.4 acknowledge that formal self-management plans with regular physician review are not widely used in Switzerland, as is also the case in other Western countries. It should also be noted that use of inhaled corticosteroids increased dramatically in New Zealand in the 1980s but mortality and hospital admissions continued to increase, whereas mortality fell by more than two-thirds immediately following the restriction of the available of the beta agonist fenoterol that had been responsible for an epidemic of asthma deaths during 1976–1989, and there was a similar but less sudden fall in hospital admissions during the same period.6 Thus, the New Zealand time trends are largely attributable to the removal of a hazardous medication, rather than other improvements in asthma management.
The observation that asthma incidence might be falling is in agreement with several other studies that showed similar time trends for asthma and hay fever,7–9 and is of major interest, not only because it may mark the end of the asthma epidemic, but also because it would have major implications in terms of our views with regard to the potential etiologic mechanisms underlying the asthma epidemic and asthma causation in general. It has been suggested that the increased cleanliness and subsequent decreased exposure to microbes in the past few decades may have resulted in Western populations becoming more susceptible to develop allergies and asthma. If true, then the current decline in asthma should be due to the environment becoming increasingly unhygienic.10 However, exposures to factors that have previously been identified as being 'protective' such as family size, endotoxin exposure, infectious diseases, pets, etc. are likely to have decreased in more recent times rather than increased. Also, there is no indication that exposures to suspected risk factors such as environmental tobacco smoke, house dust mites, air pollution, etc. have significantly decreased. Therefore, if the asthma epidemic is indeed receding, we may have to revise our current thinking about: (i) why there was an increase before the mid to late nineties (and a decrease after that period); and (ii) the etiological processes involved in asthma development in general. However, before abandoning the current paradigm we need further confirmation that the incidence of asthma is indeed falling, particularly since the observed effects were small and the observation period short. If asthma incidence on the other hand is 'just' levelling off as suggested by other studies2,3 then this would indicate that a saturation point might have been reached.
In summary, several studies have reported stabilisation or even decreased asthma prevalence over the last 5–10 years. These observations are unlikely to be completely attributable to improved asthma management, but may, in fact, be due to a decrease in incidence as demonstrated by Bollag et al.4 The reasons for this decrease are unclear and cannot easily be explained by the current 'hygiene hypothesis'. It will be of great interest to see whether the decrease in asthma incidence and prevalence will be confirmed in other studies, and if so, whether this trend is the beginning of the end of the asthma epidemic.
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Acknowledgments |
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The Centre for Public Health Research is supported by a Programme Grant from the Health Research Council of New Zealand. Jeroen Douwes is supported by a Sir Charles Hercus Fellowship from the Health Research Council of New Zealand. This work was also supported by the Lagrange Project, CRT-ISI Foundation.
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References |
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 Top References |
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1 Pearce N, Douwes J, Beasley R. Asthma. In: Tanaka H (ed). Oxford Textbook of Public Health. 4th edn. Oxford: Oxford University Press, 2002, pp. 1255–77.
2 Weiland SK, Pearce N. Asthma prevalence in adults: good news? Thorax 2004;59:637–38.
3 Chinn S, Jarvis D, Burney P et al. Increase in diagnosed asthma but not in symptoms in the European Community Respiratory Health Survey. Thorax 2004;59:646–51.
4 Bollag U, Capkun G, Caesar J, Low N. Trends in primary care consultations for asthma in Switzerland, 1989–2002. Int J Epidemiol 2005;34:1012–18.
5 Pearce N, Beasley R, Burgess C, Crane J. Asthma Epidemiology: Principles and Methods. New York: Oxford University Press, 1998.
6 Pearce N, Hensley MJ. Epidemiologic studies of beta agonists and asthma deaths. Epidemiol Rev 1998;20:173–86.
7 Anderson HR, Ruggles R, Strachan DP et al. Trends in prevalence of symptoms of asthma, hay fever, and eczema in 12–14 year olds in the British Isles, 1995–2002: questionnaire survey. BMJ 2004; 328:1052–53.
8 Fleming DM, Sunderland R, Cross KW, Ross AM. Declining incidence of episodes of asthma: a study of trends in new episodes presenting to general practitioners in the period 1989–98. Thorax 2000; 55:657–61.
9 Wong GW, Leung TF, Ko FW et al. Declining asthma prevalence in Hong Kong Chinese schoolchildren. Clin Exp Allergy 2004;34:1550–55.[CrossRef][Web of Science][Medline]
10 Douwes J, Pearce N. Asthma and the westernization 'package'. Int J Epidemiol 2002;31:1098–102.
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