Professor Ann Woolcock, AO - Director, Institute of Respiratory Medicine, Royal Prince Alfred Hospital

Understanding and awareness of asthma has come a long way in the past ten years yet prevalence continues to rise.

While we know that long-term compliance with an asthma management plan and avoiding trigger factors are proven ways of controlling asthma, attention must now be directed towards preventing the onset of the disease. Asthma is a complex disease which has yet to be fully understood. While genetic research is important, there are many other avenues to explore which may provide us with some insight into the prevention of asthma.

The asthma treatment paradigm has evolved considerably, particularly over the past twenty years or so, with the emphasis on symptom relief from short-acting beta agonists giving way to preventive treatment of underlying airway inflammation. The introduction of inhaled corticosteroids in the 1970s marked a major turning point in the battle against the long-term effects of asthma, as did the introduction of long-acting beta agonist symptom controllers in the early 1990s.

A review of the National Asthma Campaign's Six-Step Asthma Management Plan on evidence-based principles is currently underway with the aim of simplifying management, and some new drugs are undergoing trials. Leukotriene antagonists are the latest class of anti-asthma medications being introduced in Australia. Further clinical studies are required to determine the role of these medications and there is no diagnostic tool available to identify those patients whose symptoms are induced by leukotrienes. It is unlikely a dramatic alteration in therapy will occur in the near future to replace the current 'gold standard' therapy of inhaled corticosteroids (preventer), a reliever and a long-acting beta agonist for most moderate to severe asthma patients.

A disease of affluent populations

Mounting evidence points to the fact that asthma is a disease of affluent populations. The challenge for epidemiologists, clinicians and physiologists is to work together to try to define those aspects of modern lifestyle that are combining to cause the increase in childhood asthma, and to undertake formal intervention studies. A case can be made that the increase in asthma is due to changes in lifestyle leading to a loss of protective factors (bacterial infections early in life, healthy diet and lots of exercise) and a concomitant increase in risk factors (exposure to cigarette smoke and allergens).

Air pollution is often blamed for the increase in the prevalence of asthma, but the scientific evidence is for the reverse. It is time to stop blaming air pollution and to examine other possibilities. Indoor air pollution, on the other hand, is a risk factor. Regardless of the well known fact that tobacco smoke increases the risk of allergic sensitisation in children (especially infants) and increases the severity of symptoms in children with asthma, parents continue to smoke. Of even more concern is the number of pregnant women, as well as adults and teenagers with asthma, who continue to smoke.

The most common cause of allergy in Australia is the house dust mite. Reducing mite levels in houses is possibly one of the easier ways to reduce the risk of asthma. To be effective, however, exposure reduction needs to be both large-scale and prolonged. The warm and humid Australian climate is particularly suited to dust mites and changes in housing during the past few decades are one possible factor in the rise of asthma. Most new homes are built on concrete slabs with carpet laid on top, as opposed to the bare boards that provide under-floor ventilation in older homes. Frequently, both parents work, so homes also tend to be closed during the day. Our main approach to eliminate dust mite should be to reduce the reservoirs of dust in houses by encasing mattresses, washing bedding regularly in hot water and choosing non-dust-collecting furnishings and floor coverings.

Investigating new risk factors

With the lack of certainty about asthma, research needs to widen from the established risk factors such as dust mites, animals, mould and pollen, to examine the role of other factors such as diet, breast milk and viruses. It is intriguing that fresh fish is in some way protective, suggesting that changes to diet in affluent societies worldwide need to be explored. Possible changes to the diet which may predispose to asthma include lower intake of omega-3 fatty acids, lower antioxidant intake and higher salt intake.¹

Breastfeeding versus bottle feeding is a much debated issue, yet numerous studies show breastfeeding may reduce the risk of asthma in the first years of life. A study undertaken at the Institute of Child Health Research at Princess Margaret Hospital in Perth, found that bottle-fed infants have a 30% greater chance of developing asthma. It is recommended that mothers should breastfeed their babies, whenever possible, for at least the first three to six months of a baby's life.

Infections may be another important protective factor. With a hygienic Western lifestyle, infants have few bacterial infections, multiple immunisations and frequent courses of antibiotics. Theoretically, in the first months of life, if the T cells are stimulated to produce TH₁ rather than TH₂ cytokines, the child may be protected from producing the excessive amounts of TH₂ cytokines that could result in the onset of asthma.² Methods to induce a TH₁ response in early life, including vaccination³, are now being considered to prevent the development of asthma.

Interestingly, another possible factor in the rise of asthma is the advent of television. In societies with television, children exercise far less and are inside their houses for long periods, with exposure to allergens. Reduced exercise may result in less stretching of the airways and a greater tendency for the smooth muscle to contract abnormally in the presence of small quantities of inflammatory mediators that would normally not cause symptoms.

Once important factors associated with affluent and hygienic lifestyles are identified, interventions (such as changes to indoor environment, diet and exercise) should be possible. Since it would be difficult to encourage a less hygienic lifestyle so that infants would have more infections, we should consider methods to induce a TH ₁ response in early life, including vaccination.³ Such interventions could lead to a decrease in prevalence, as has occurred with cardiovascular disease following lifestyle interventions in such areas as diet and exercise.

² Robinson DS, Hamid O, Yin S, et al. Predominant TH₁-like bronchoalveolar T-lymphocyte population in atopic asthma. N Engl J Med 1992; 326: 298-304.

³ Holt PG. A potential vaccine strategy for asthma and allied atopic diseases during early childhood. Lancet 1994; 344: 456-458.

Content Updated July, 2001