Estimating the number of people in the community who have asthma is fundamentally important in assessing the impact of asthma at a population level. It is relevant to estimating resource needs and priorities both now and in the future. Examining differences among population subgroups in the prevalence of asthma provides insights into possible causative factors and also assists in targeting resources to areas of need. Finally, examination of changes over time in the number of people who have asthma contributes to the evaluation of population-based efforts to prevent the disease and, if a rising trend is observed, may stimulate the search for an environmental or lifestyle-related cause for that rise.
In this chapter we present data on the prevalence of asthma in Australia gathered from a wide range of sources. Data on time trends, differences among population groups, and international comparisons are reported.
In interpreting the information presented in this chapter, it is important to be aware of difficulties in measuring asthma and reporting its prevalence. There is no universally applied definition for asthma. The prevalence of asthma has been estimated using a wide range of subjective, or self-reported, and objective measures, alone or in combination, in both clinical and population-based settings. Self-reported measures include doctor diagnosis of asthma—self or parent-reported (Robertson et al. 1991; Ruffin et al. 2001); symptoms, such as wheeze (Grant et al. 1999; ISAAC 1995; Robertson et al. 1991), shortness of breath (particularly at night) (Burney et al. 1996; Woods et al. 2001), cough at night (Grant et al. 2000), and wheezing with exercise (Grant et al. 2000; Jones 1994; Ponsonby et al. 1996); and taking treatment for asthma (Burney et al. 1996).
Objective measures include measuring the twitchiness of the airways in response to inhaled stimuli (known as ‘bronchial provocation challenge test’) or measuring the extent to which airway narrowing can be reversed by inhaled medication (known as ‘bronchodilator reversibility test’) (Toelle et al. 2004); and measurement of day-to-day variability in airway narrowing (‘peak flow variability’) (Parameswaran et al. 1999).
This broad range of measures, all of which are relevant to asthma, has led to considerable controversy about exactly how best to identify this disease in population studies and, hence, how best to quantify the prevalence of the disease. As will be seen in this chapter, the observed variation in the prevalence of asthma owes more to the differences in definitions, than to real variation.
Over the last decade the prevalence of asthma in Australia has been measured in a range of population health surveys, including the Australian Bureau of Statistics’ (ABS) National Health Survey and state and territory health surveillance programs. However, there are limited time series data available from these survey programs. Many surveys have been conducted only once, or, where there are repeated measures, the definition used to identify people with asthma has changed. There are some international studies involving Australia (Abramson et al. 1996; ISAAC 1998; Robertson et al. 1998), and there are results from studies of local populations (Haby et al. 2001; Peat et al. 1994; Toelle et al. 2004).
It has been estimated that 3,864,987 Australians have ever been diagnosed with asthma by a doctor (ABS 2002a). Of these, 2,199,411, or 11.6% of the population, stated that they still had asthma in 2001 (i.e. had current asthma). These estimates are based on data from the National Health Survey 2001, which is the only nationally representative, household survey in which the prevalence of asthma has been measured. In this survey 13.8% of children aged 0 to 17 years and 10.8% of adults aged 18 years and over reported current asthma.
In addition to the nationwide National Health Survey, a number of state, territory and locally-based surveys of the prevalence of asthma have been conducted (Tables 3.1, 3.2 and 3.3). There is some variation in the survey methods used, the age ranges surveyed, the sample sizes, and, most importantly, the way in which asthma was measured (see Appendix 1, Section A1.2, for a further description of asthma prevalence questions used in Australian health surveys). Hence, the data from these surveys cannot be used to compare prevalence rates among states or other population subgroups. Nevertheless, an examination of the range of values obtained in these surveys gives an idea of likely true prevalence of asthma in the population.
Among adults, the prevalence of reporting ever having been diagnosed with asthma ranges from 17% to 25%, with most estimates between 19% and 21% (Table 3.1; see also Appendix 2, Table A2.5). The prevalence of current asthma among adults has ranged from 9% to 15%, with most estimates falling between 10% and 12% (Table 3.2; see also Appendix 2, Table A2.6).In four surveys conducted among children, estimates of the number who had ever been diagnosed with asthma ranged from 20% to 26%. Most estimates of the proportion of children with current asthma ranged between 14% and 16%, based on self-report (Table 3.3). Additional studies providing estimates of the prevalence of asthma among children have been included in Table 2.1 and Table 2.2 in Chapter 2.
Time trends in the prevalence of current asthma
There are widespread reports that asthma has become more common in the last 20 years, particularly in Western nations (Burney 2002; Peat et al. 1994; Robertson et al. 1991). Some recent studies, however, suggest this trend may be levelling or decreasing (Anderson et al. 2004; Braun-Fahrlander et al. 2004; Devenny et al. 2004; Mommers et al. 2005; Robertson et al. 2004; Wong et al. 2004). However, the interpretation of these reports is complex since small differences in study methodology and definitions may confound comparisons between surveys. Furthermore, most surveys are based on self-reports of diagnosed asthma and these may be subject to changes in the tendency of doctors to apply the diagnostic label ‘asthma’. Confident conclusions about time trends in the prevalence of asthma can only be made if the following criteria are met:
The finding of a consistent trend in one age group, for example children, does not necessarily imply that the same trend exists in other age groups.
There are some surveys that have been conducted using the same methodology in the same populations, although few have used a broad range measures of asthma and even fewer have used objective measures.
The available data on trends in the prevalence of asthma in Australia are shown in Figure 3.1 for adults and Figure 3.2 for children. In these figures, each line represents a series of surveys conducted in a single population using the same methodology, including the same measure of asthma. In some instances two measures of asthma from the same series of surveys are presented, each represented by a different line. It is important to point out that these lines should be interpreted as independent trends. The relation between the positions of these lines is difficult to interpret because it reflects methodological differences between the surveys, including the way in which asthma was measured.
There is evidence of a rising trend in the prevalence of asthma among adults since the early 1990s (Figure 3.1). This is most evident in the long series of surveys conducted in South Australia (Wilson et al. 2002). Over the more recent period, since the late 1990s, when several series are available, the prevalence of asthma appears to be stable in adults. There is also consistent evidence of a rise in the prevalence of asthma among children during the 1980s and into the early 1990s (Figure 3.2). More recent data suggest that this rising trend may have peaked.
In assessing the burden of asthma in Australia it is useful to be able to place the prevalence of the disease in this country in an international context. The difficulties in comparing local data derived using various methods, definitions and settings are magnified substantially when attempting to make international comparisons of the prevalence of asthma. Fortunately, two large international studies, one conducted in adults (Burney 2002) and the other in children (ISAAC 1995), have applied standardised methods and definitions in an attempt to overcome these problems.
The European Community Respiratory Health Survey (ECRHS) was conducted among adults aged 20 to 44 years in 35 centres in 16 countries (Chinn et al. 1997). Melbourne was the Australian centre in this study (Abramson et al. 1996). Figure 3.3 shows the prevalence of self-reported wheeze among adults using data from the ECRHS and other comparable studies from countries not participating in ECRHS. The diagram shows that Australia had one of the highest prevalence rates of reported wheeze in the last 12 months among the 41 countries studied. The prevalence of self-reported wheeze among 13 to 14 year old children was also high in Australia compared with most other countries participating in ISAAC (see Figure 2.1).
Differentials in the prevalence of current asthma
In this section we report on population differentials in the prevalence of asthma in Australia, using data from the ABS National Health Survey 2001. Subjects were classified as having current asthma if they reported ever being diagnosed with asthma by a doctor and still getting asthma.
Age and sex
The prevalence of current asthma in males was highest in the 5 to 9 years age group, whereas in females it was highest in 15 to 24 year olds (Figure 3.4). The prevalence of asthma was also high in both males and females aged 10 to 14 years and males 15 to 24 years.
The prevalence of asthma was higher in males than females among persons aged less than 15 years, especially in those aged 5 to 9 years. In contrast, the prevalence of asthma was higher in females than in males among persons aged 15 years and over, particularly among those aged 25 to 64 years.
States and territories
The prevalence of asthma did not differ significantly from the national average in any of the states or territories (Figure 3.5).
Urban, rural and remote areas
Overall, the prevalence of asthma did not differ substantially between major cities, inner regional areas and outer regional and remote areas (Figure 3.6). The excess prevalence of asthma among females was greater in inner regional and outer regional and remote areas than in major cities.
Aboriginal and Torres Strait Islander Australians
Data from the National Health Survey and the Indigenous National Health Survey 2001 (see Appendix 1, Section A1.6.2) show that the prevalence of current asthma was significantly higher among adult Aboriginal and Torres Strait Islander women than among other Australian women (p<0.001; Figure 3.7). However, the prevalence of asthma was similar among Aboriginal and Torres Strait Islander men and other Australian men also and among children of both groups. The prevalence of ever having doctor-diagnosed asthma and the prevalence of wheeze in the last 12 months were also higher in the Indigenous Australian population (data not shown). Among Aboriginal and Torres Strait Islander adults, the prevalence of asthma was much higher among females than males. In fact, among Aboriginal and Torres Strait Islander women, the prevalence was higher in older adults than in children, an age distribution of asthma that was quite unlike the age distribution in other Australian women.
Among children and young adults, the prevalence of asthma was higher in Aboriginal and Torres Strait Islander people living in non-remote areas than in those living in remote areas (Figure 3.8). This trend was reversed among older adults, with a higher prevalence among Aboriginal and Torres Strait Islander people living in remote areas. However, this estimate may have included a broader range of conditions. Due to linguistic differences, the questionnaire administered to Aboriginal and Torres Strait Islander people living in sparsely-populated remote areas did not distinguish between ‘asthma’ and ‘breathing problems’ (see Appendix 1, Section A1.6.2).
There have also been several locally-based surveys that measured the prevalence of asthma among Aboriginal and Torres Strait Islander Australians (Table 3.4). There is substantial variation among the prevalence estimates. In part, this reflects the range of measures of asthma that have been used and also the means of identifying Indigenous status. However, some real variation in the prevalence of asthma within the Aboriginal and Torres Strait Islander population is likely, in particular relating to the remoteness or other characteristics of the setting. Additional studies reporting the prevalence of asthma among Indigenous Australian children have been included in Table 2.3 of this report.
Culturally and linguistically diverse background
Data from the National Health Survey 2001 demonstrated that the prevalence of current asthma was lower in people from non-English-speaking backgrounds (Figure 3.9). The prevalence of ever having asthma was also lower among people from non-English-speaking backgrounds (data not shown).
This is consistent with previous observations that the prevalence of asthma is higher in children and adults born in Australia than among those who were born overseas and subsequently migrated to Australia (Leung et al. 1994; Peat et al. 1992). The prevalence of asthma has been shown to increase among migrant populations with the duration of residence (Leung et al. 1994).
People living in the most socioeconomically disadvantaged localities (classified using SEIFA; see Appendix 1, Section A1.12.3) did not have a substantially higher (or lower) prevalence of asthma compared with those in less disadvantaged areas (Figure 3.10). This finding contrasts with observations in some other countries. For example, in the USA there is a higher prevalence of asthma in children from lower income families (Miller 2000) and among families eligible for subsidised school lunches (Yawn et al. 2002).
Recent data suggest that 10 to 12% of adults and 14 to 16% of children report a diagnosis of asthma that remains a current problem. International comparative studies have shown a high prevalence of asthma in Australia, compared with many other countries. During the 1980s and early 1990s there is some evidence of a small increase in the prevalence of asthma among adults.
Asthma is more common in boys than girls before teenage years and, thereafter, it is more common among females than males. The highest reported prevalence is among 5 to 9 year old boys. In contrast to some overseas studies, there is no convincing evidence that people living in rural and remote areas and in socioeconomically disadvantaged areas in Australia have a higher risk of having asthma. Indigenous Australian women more commonly report asthma than other Australian women and this difference increases with increasing age. Persons of English-speaking backgrounds have a higher prevalence of asthma than other populations within the community.
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