Latest Statistical Trends

Professor Adrian Bauman - School of Community Medicine, University of New South Wales

The impact of asthma in our community is well documented with facts and figures showing who is affected where they live, what they experience, and, when asthma occurs. The real benefit of this research, however, is the ability to identify trends and factors which may help us determine why.

There are two main themes emerging from recent research. Firstly, worldwide studies demonstrate a wide variation in the incidence of asthma. Secondly, studies that have examined the incidence of asthma in the same community over a period of time, have shown clearly that the incidence, particularly in children, is increasing.

International comparisons

While the reasons for wide variations in asthma prevalence around the world are not known, it is clear that incidence is on the rise, with Australia having a higher prevalence than almost all other countries (see Figure I). The international Study of Asthma and Allergies in Childhood (ISAAC), currently being conducted in 155 centres around the globe, is providing a valuable international comparison of the prevalence and characteristics of asthma.¹ One of the most interesting findings from the ISAAC study is that the international pattern of prevalence cannot be completely explained by our current knowledge of recognised risk factors for the development of asthma.

Contrary to popular belief, the global pattern of asthma prevalence provides evidence that air pollution is not a major risk factor for the development of asthma, rather, it is merely a minor trigger in some individuals. For example, some regions in China and Eastern Europe with high levels of air pollution have generally low rates of asthma prevalence. Conversely, some regions with the lowest rates of air pollution such as parts of New Zealand have high rates of asthma. Overall, it would appear that outdoor air pollution is more likely to be a trigger factor in older people with respiratory conditions other than asthma.

The reasons for wide variations in asthma prevalence may also point to other environmental factors. These could include changes in housing which allow greater proliferation of house dust mites; indoor pollutants (e.g. passive smoking); or perhaps changes in diet. One risk factor which may cause asthma is mothers smoking during pregnancy. These children are twice as likely to develop asthma.

While there is sufficient prevalence data about asthma, and it is well known that a common gene predisposes people to asthma and allergies, more prevention studies need to be done. Future research needs to look at the effect of removing exposure to environmental triggers such as maternal smoking and house dust mite, as well as the issue of diet. While it's still too early to make dietary recommendations, and it is likely to be a complex and costly exercise to change housing design, it should theoretically be easier to act now to prevent smoking in pregnancy and determine the potential benefits of this action.

Figure 1. Worldwide asthma prevalences

The highest 12-month prevalences of asthma symptoms were found in regions in the UK, New Zealand, Australia and the Republic of Ireland (between 28 and 36%). The lowest prevalences of asthma symptoms (less than 5 0/4) were reported in several Eastern European countries, Indonesia, Greece, China and India. Source: ISAAC study

The Australian picture

Since the early 1980s, prevalence data shows an increase in asthma of about 0.5% a year. Asthma was prevalent in less than 10 % of children in the early 1980s while it is now close to 20 %. Over the same period however, asthma in adults has increased less significantly from approximately 6 to 7% of the population to 12%. The lower rate of adults with asthma indicates that there is either a reduced incidence of onset of asthma in adult life or a decreased rate of expression of asthma symptoms in adulthood.

While there is evidence of some differences in the geographical distribution of asthma within Australia, there is a lack of significant evidence of large discrepancies between or within the states and territories. One study however, conducted in several centres in New South Wales, found some regional variations in the prevalence and severity of childhood asthma.² The authors suggested that the variations observed could relate to different levels of allergic sensitisation - for example, sensitisation to house dust mites was higher in coastal than inland regions, while sensitisation to alternaria (a mould common in agricultural areas) was higher in inland regions.

Asthma and children

In Australia, one in four primary school children and one in seven teenagers currently suffer from asthma. Asthma is the most common cause of hospital admissions for children between the ages of five and fourteen and is a major cause of school absenteeism. In childhood, prevalence is higher in boys than in girls but by adult life this becomes relatively even.

While 'city kids' wheeze in ever-increasing numbers, the incidence of asthma in many remote Aboriginal communities is almost non-existent. In contrast, the incidence of respiratory infections in children in remote communities is generally very high, prompting some researchers to believe that early childhood infections play a protective role by stimulating the immune system. More research is needed in this area.

Similarly, this difference between people of Aboriginal and Caucasian origin points to the need for further research contrasting prevalence of asthma in people of different races. This may provide us with further valuable information in determining reasons for the differing rates of asthma.

According to the ISAAC study which involved 463,801 children aged 13 to 14 years, from 56 countries, Australia has the world's third highest prevalence of current wheeze in 13 to 14-year-olds. There was also evidence from throughout Australia of continuing lack of effective treatment of asthma. Among 13 to 14-year-olds who had more than 12 episodes of wheeze per year, only 43 % were taking regular preventive medication. The appropriate use of preventive therapy needs to be defined for children, however those with frequent disabling asthma require regular asthma preventers.

Comparison with results from a previous study of Melbourne school children showed that the prevalence has increased from 23.1% in 1990 to 27.2% in 1993. This rate of increase (1.24% per annum) is higher than reported from European countries (0.1 to 0.4% per annum).

Mortality rates

Asthma mortality rates have dropped substantially since 1992, especially in the 5 to 34 year-old age group. This represents a decline of over 50% since the mid 1980s, declining from about 1.5 deaths in 100,000 to less than 0.8 deaths in 100,000 in 1995.³ This drop in mortality can be attributed to the net sum of all asthma management, education and treatment changes, including: better use of preventive medicine; people having better asthma plans; better reviews by GPs; better management by doctors and emergency departments and greater awareness of asthma, meaning that people are getting help earlier.

Having said this, Australia still has the second highest asthma mortality rate in the world. Along with hospital admissions from asthma, half to two thirds of deaths should be preventable.

References:

1. Beasley R et al. Worldwide in variation in prevalence of symptoms of asthma, allergenic rhinoconjunctivitis and atopic eczema: ISAAC. Lancet 1998; 351:1225- 32.

2. Peat JK, Toelle BG, Gray EJ, et al. Prevalence and severity of childhood asthma and allergic sensitisation in seven climatic regions of New South Wales. Med/Aust 1195;163:22-26

3. Comino EJ, Bauman A. Trends in asthma mortality in Australia, 1960-1996. MJA Vol 168 May 1998.