Reversing the trend: Reducing the prevalence of asthma,☆☆

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Abstract

Background: We urgently need to take steps towards reducing the prevalence of asthma in countries where the prevalence has become unacceptably high in recent years. Because we do not have any good information about causes of the increased prevalence, we cannot act directly to reverse the trend. Therefore we need to take an indirect approach and use known information of etiologic factors to try to reduce asthma in the next generation, while acknowledging that we may be dealing with different factors from those responsible for the increased prevalence. Any successful strategies will also help to ensure that developing countries do not attain such high rates of asthma in their children in coming years. Objective: This article summarizes the roles of the risk factors that identify “high-risk” children, that provide insights into mechanisms, or that have potential for primary prevention. The factors with the most potential for primary prevention are allergen exposure, parental smoking, breast-feeding, and dietary fatty acids. Conclusion: In other health models, information about risk factors has been used in successful public health interventions. It is disappointing that the important risk factors for asthma are well documented but that there have been few trials of primary prevention and no changes in public health policies. (J Allergy Clin Immunol 1999;103:1-10.)

Section snippets

PREVALENCE

Studies of the worldwide prevalence of asthma by the International Study of Asthma and Allergy in Children (ISAAC)9 and the European Community Respiratory Health Survey (ECRHS)10 have been landmark events. Because both collaborations have used standardized study protocols and stringent quality control measures, they have overcome the limitations of numerous previous studies of asthma prevalence so that, for the first time, worldwide data can be reliably compared. Fig 1, which summarizes

MORBIDITY

Because of the large demand that asthma morbidity places on health care services, it is essential that countries in which prevalence is high work toward methods of primary prevention. The proportion of the population receiving treatment for asthma or being admitted to hospitals for asthma has been documented in many countries.1 In Australia, up to 1% of children are hospitalized for asthma annually, 10% to 12% have asthma with clinically important symptoms that affect lifestyle, and 25% to 30%

BETWEEN-COUNTRY COMPARISONS

It is encouraging that the findings of the ISAAC studies of 13 to 14 year old children verify the results of previous studies of younger children, which have been summarized elsewhere,16 and that the estimates are broadly consistent within countries. This consistency of evidence validates the beliefs that asthma is more prevalent in some countries than others, that an affluent lifestyle is associated with a higher prevalence, and that there are much greater between-country than within-country

RISK FACTORS

Many etiologic factors may be responsible for the differences in the prevalence of asthma between countries highlighted by the ISAAC studies (Fig 1), which range from 2% in Chinese and Indonesian children to 30% in children in the UK and Australia. However, it is important to recognize that the primary risk factors that determine why asthma develops in an individual may not be the same as those that determine why this disease develops in so many individuals in a population. Thus 2 possibilities

RISK FACTORS THAT IDENTIFY “HIGH-RISK” CHILDREN OR PROVIDE INSIGHT INTO MECHANISMS

Factors such as family history, gender, and ethnicity are important for identifying children who are at high risk for the development of asthma and who are most likely to benefit from the effects of public health interventions. Although genetic factors make a major contribution to the development of asthma and new techniques of gene mapping may improve treatment methods in the long term, it is unlikely that such techniques will have direct applicability for preventive strategies in the short

RISK FACTORS WITH POTENTIAL FOR PRIMARY INTERVENTION

Only the interventions that address the known effects of immediate etiologic exposures will be effective in reducing the prevalence of asthma in the short term. The risk factors that have the most potential in a primary preventive role are shown in Table I and are discussed in more detail below.

INDOOR ALLERGENS

The evidence for risk factors with a direct role in asthma causation is most complete for house dust mite allergens. There is clear biologic plausibility for their role in that the proteolytic activity of their enzymes can actively damage the airway epithelium.35 In addition, a dose-response effect and consistency of evidence have been clearly demonstrated.36 The role of allergen exposure is slightly more complicated than that of risk factors in that a 2-stage process is involved whereby

PARENTAL SMOKING

There is an undisputed consensus that parental smoking leads to increased respiratory infections and respiratory illnesses in children. A huge number of studies have demonstrated the effect of parental smoking on childhood illness, and the literature has been reviewed more often and with more rigor than for any other risk factor. Recently, a series of metaanalyses again confirmed that parental smoking increases the risk of children having many respiratory illnesses, including lower respiratory

BREAST-FEEDING

Breast-feeding plays a vital role in the development of the immune system.59 There is evidence that the antiinflammatory and antiinfective factors in breast milk contribute to the prevention of both allergic illness63 and a wide range of respiratory and gastrointestinal infections.64, 65, 66 In addition, breast-feeding reduces exposure to the food allergens contained in cow’s milk, whereas bottle-feeding provides regular exposure to many microbial products and foreign proteins that may cause

DIETARY FACTORS

There is both epidemiologic evidence and biologic plausibility that many dietary factors, including omega-3 fatty acids, antioxidants, and sodium, are associated with asthma and reduced lung function.79, 80, 81, 82, 83, 84 The evidence for an effect is most complete for the role of omega-3 fatty acids, which are abundant in fish and which prevent airway abnormalities through their antiinflammatory effects in the airway wall.85, 86 To date, 2 reports from the US87, 88 and our own studies in

ROLE OF RISK FACTORS IN INCREASING PREVALENCE

Because there was no simultaneous monitoring of environmental exposures that may have been responsible for the increased prevalence of asthma, the only data available are based on speculative or ecologic assumptions. We found some evidence that the numbers of house dust mites increased in 2 regional centers between 1982 and 1992,7 but no similar evidence has been reported from other places. In addition, the prevalence of smoking in young women increased in this time period. There is also

CONCLUSIONS

There is no evidence to suggest that asthma that develops in early life can be reversed by any treatment or environmental intervention. Thus it is important to test whether changes in exposure to the most important environmental risk factors can downregulate the development of allergic sensitization and airway inflammation and thus prevent the asthmatic process from “switching on” in early life. In the search for local exposures that can be manipulated to achieve this goal, it is important that

FUTURE DIRECTIONS

The identification of other potentially preventable risk factors and the confirmation of the roles of known or suspected risk factors in regions in which their effects are not established will continue to be a slow and painstaking task. This will be especially difficult if reliable tools or questionnaires that are suitable for measuring exposures in epidemiologic studies are not available. In addition, the roles of other risk factors for which exposures can now be measured, such as indoor and

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    Reprint requests: Jennifer K. Peat, PhD, Hospital Statistician, Clinical Epidemiology Unit, New Children’s Hospital, NSW 2145 Australia.

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