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• Primary prevention: can the onset of asthma be prevented?
• Secondary prevention: can asthma be prevented in patients with other atopic disease?
• Tertiary prevention: can asthma be ‘cured'?
• References

Summary of practice points	Level of evidence
A smoke-free environment should be recommended for all children, and all pregnant and breastfeeding women should be advised not to smoke.	[√]
Avoidance of environmental tobacco smoke may reduce the risk of childhood asthma.	III-2
Breastfeeding may lower asthma risk during early childhood, but does not provide long-term asthma protection.	III-2
If breastfeeding is not possible, infant feeding with hydrolysed milk formulae may slightly lower the risk of childhood allergy, wheezing and asthma, compared with use of other formulae.	I
Infant feeding with soy formulae does not prevent asthma	I
Omega-3 fatty acid supplementation in at-risk infants does not appear to reduce the risk of childhood asthma or wheezing.	II
Probiotic supplementation of mothers during late pregnancy and lactation, or of non-breastfed infants, does not appear to prevent asthma.	II
Avoidance of commonly allergenic foods during pregnancy or lactation has no effect on the development of childhood asthma	I
Avoidance of commonly allergenic foods in infant diets does not reduce the risk of childhood asthma.	II
Measures to reduce exposure to dust mite do not appear to decrease the rates of asthma or wheeze in young children.	II
On current evidence, advising families to avoid exposure to pets is not warranted.	[√]
Multifaceted environmental controls that include allergen avoidance, undertaken during infancy, may reduce asthma symptoms in young children but have no effect on lung function or bronchial hyperresponsiveness.	II
Long-term treatment with antihistamines does not reduce the risk of asthma developing in children with atopic dermatitis, including those who are sensitive to house dust mite and/or grass pollen.<	II
Immunotherapy may reduced asthma risk in children with seasonal allergic rhinoconjunctivitis.	II

People consulting health professionals often ask about their risk or their children's risk of developing asthma, how their family history affects asthma risk, and whether they can do anything to prevent their child developing asthma.

The development of asthma involves a complex interaction of genetic factors and environmental influences. A family history of allergy and asthma can be used to identify children at increased risk of asthma.

There has been considerable interest in whether a child's likelihood of developing asthma may be reduced by changing the environment. Much of the evidence for benefits of proposed environmental modifications comes from epidemiological studies. Interventional studies have examined the effect of manipulating various environmental factors, but as yet there is little convincing evidence that specific interventions are highly effective in preventing the onset of asthma.

Practice tip For parents concerned about the possibility of their child developing asthma, keep the advice clear and simple: Breastfeed Avoid exposure to tobacco smoke Consider consulting a paediatric allergy specialist if the child is atopic.

Primary prevention: can the onset of asthma be prevented?

Advice for parents and carers is summarised in Table 1.

Table 1. What can health professionals advise families about preventing asthma in infants and unborn babies?

Intervention	Advice based on current evidence
Allergen avoidance in pregnancy	Dietary restrictions in pregnancy are not recommended. Aeroallergen avoidance in pregnancy has not been shown to reduce allergic disease, and is not recommended.
Breastfeeding	Breastfeeding should be recommended. Maternal dietary restrictions during breastfeeding are not recommended.
Infant formulae	The use of formulae is recommended only in infants who have a high risk for asthma and if breastfeeding is not possible. A partially or extensively hydrolysed formula may reduce the risk of developing asthma. Soy formulae and other formulae (e.g. goat's milk) are not recommended for the reduction of asthma risk.
Infant diet	There is no evidence that avoidance of commonly allergenic foods prevents the development of asthma. There is no evidence at present to suggest that supplementation with fish oil or probiotics prevents the development of asthma.
Avoidance of house dust mite exposure	There is no evidence to suggest that house dust mite reduction alone prevents the development of asthma in Australia. Multifaceted interventions that include house dust mite avoidance have been shown to reduce asthma symptoms, but have not influenced lung function or prevalence of bronchial hyperresponsiveness.
Pet exposure	On current evidence, advising families to avoid exposure to pets is not warranted.
Smoking and other irritants	Pregnant women should be advised not to smoke during pregnancy. Parents should be advised not to smoke, and to avoid children being exposed to other people's smoke.

Adapted from Presscott SL, Tang M, 2004²⁸

Does exposure to environmental tobacco smoke increase asthma risk?

Practice points
A smoke-free environment should be recommended for all children, and all pregnant and breastfeeding women should be advised not to smoke [√] Avoidance of environmental tobacco smoke may reduce the risk of childhood asthma (III-2).

A smoke-free environment should be recommended for all children, and all pregnant women should be advised not to smoke.

• The link between exposure to environmental tobacco smoke (ETS) during foetal development and early childhood, and increased risk of respiratory illnesses including asthma, has been well documented in epidemiological studies.¹
• Prenatal exposure may carry the greater risk.¹
• Genetic factors appear to influence the effects of ETS on the risk of developing asthma.²
• Exposure to ETS appears to be an independent risk factor for allergic sensitisation also.³

In the absence of randomised controlled trials assessing the effect of ETS avoidance on the risk of developing asthma, avoidance of ETS should be recommended, based on epidemiological evidence that it increases asthma risk, as well as other known detrimental effects.

For information on smoking-related risks and smoking cessation, see Healthy Living Fact Sheets.

Does infant feeding affect asthma risk?

Practice points

Breastfeeding may lower asthma risk during early childhood, but does not provide long-term asthma protection. (III-2) If breastfeeding is not possible, infant feeding with hydrolysed milk formulae may slightly lower the risk of childhood allergy, wheezing and asthma, compared with use of other formulae. (I) Infant feeding with soy formulae does not prevent asthma. (I) Omega-3 fatty acid supplementation in at-risk infants does not appear to reduce the risk of childhood asthma or wheezing. (II) Probiotic supplementation of mothers during late pregnancy and lactation, or of non-breastfed infants, does not appear to prevent asthma. (II)

Breastfeeding

While breastfeeding should be promoted for its many beneficial effects, the evidence that it may prevent the development of asthma is conflicting.

• Clinical studies have demonstrated that breastfeeding confers a small protective effect against asthma risk, especially in children with a family history of atopy.⁴
• A large Australian prospective cohort study in children aged 6 years found that the introduction of milk other than breast milk before age 4 months was a significant risk factor for asthma (odds ratio 1.25;CI 1.02-1.52), wheezing and atopy.⁵
• Breastfeeding appears to delay the onset of asthma and recurrent wheeze or actively protect children less than 24 months old against asthma and recurrent wheeze, and might reduce the prevalence of asthma and recurrent wheeze in children exposed to environmental tobacco smoke.⁶ However, some studies suggest that the protective effect of breastfeeding against wheezing is strongest in non-atopic children, and may be mainly due to prevention of wheezing during viral respiratory infections, rather than an effect on the development of asthma.^7,8
• There is no good evidence that the protective influence of breastfeeding seen in some studies in early childhood extends into later childhood or adult life. Some studies have suggested that breastfeeding, particularly in atopic children, may be associated with an increased risk of asthma development in later childhood and adulthood.^8,9

Infant feed formulae

• Hydrolysed milk formulae: there is some evidence that infant feeding with hydrolysed milk formulae is protective against allergy, wheezing and asthma,^10,11 but less protective than breastfeeding. The evidence is stronger for an effect on infant wheezing¹⁰ than on asthma.
• Soy formulae: infant feeding with soy formulae does not appear to prevent allergy or asthma.¹²

Fish oil supplementation

• Epidemiological evidence suggests that a diet rich in oily fish is associated with a decreased likelihood of developing asthma.
• However, a randomised controlled clinical trial observed no reduction in prevalences of childhood asthma or wheezing following supplementation with omega-3 fatty acids in at-risk infants. Omega-3 fatty acid supplementation was associated with a reduction in cough in atopic children at age 3 years and at age 5 years.^13,14

Probiotics

A randomised controlled clinical trial of probiotic supplementation in at-risk families (mothers during late pregnancy and lactation and non-breastfed infants during the first 6 months of life) demonstrated a reduction in eczema at 2 and 4 years, but no effect on allergen sensitisation or asthma.^15,16

Does allergen avoidance reduce asthma risk?

Practice points

Avoidance of commonly allergenic foods during pregnancy or lactation has no effect on the development of childhood asthma. (I) Avoidance of commonly allergenic foods in infant diets does not reduce the risk of childhood asthma. (II) Measures to reduce exposure to dust mite do not appear to decrease the rates of asthma or wheeze in young children. (II) On current evidence, advising families to avoid exposure to pets is not warranted. [√] Multifaceted environmental controls that include allergen avoidance, undertaken during infancy, may reduce asthma symptoms in young children but have no effect on lung function or bronchial hyperresponsiveness (II).

Maternal dietary allergen avoidance in pregnancy or lactation

Maternal dietary allergen avoidance during pregnancy or lactation appears to have no effect on the child's risk of developing of asthma.¹⁷

Dietary allergen avoidance in infancy

Combined maternal and infant food allergen avoidance in at-risk families does not appear to reduce rates of asthma or allergic rhinitis by age 7 years.¹⁸

Dust mite avoidance

To date, randomised controlled clinical trials assessing the effect of house dust mite avoidance in at-risk families have not demonstrated substantial reductions in asthma.

• An Australian study of dust mite avoidance has not demonstrated any significant reduction in asthma, wheeze or cough by 3 years of age or by 5 years of age, despite a modest reduction in sensitisation to house dust mite.^13,14
• A UK study of dust mite avoidance has demonstrated significantly better lung function at age 3 years but no significant difference in asthma or wheeze, and an increased risk of mite sensitisation.¹⁹ Two other European studies have not shown any reduction in mite sensitisation or the development of asthma in the first 2 years of life.^20,21

Pet allergen avoidance

Firm recommendations about exposure to pets cannot be made because of conflicting epidemiological data on the effect of early pet avoidance on asthma development.

• A systematic review concluded that exposure to pets increases the risk of asthma in children older than 6 years,²² but other studies have suggested that early pet exposure may protect against asthma development. ^23,24
• The issue is complex; any protective effects associated with animal exposure prior to the development of asthma may be complicated by sensitisation and triggering of symptoms in those who go on to develop asthma.

Multifaceted interventions including allergen avoidance

Rates of childhood asthma might be reduced by families avoiding exposure to multiple risk factors, including allergens.

• A UK randomised controlled trial of a combined dietary allergen and house dust mite avoidance intervention in at-risk infants in the first 9 months of life has demonstrated a significant reduction in asthma symptoms and atopic sensitisation at age 8 years, but no significant effect on lung function or the prevalence of bronchial hyperresponsiveness.²⁵
• A Canadian randomised controlled trial of a multifaceted intervention in at-risk infants in the first year of life (avoidance of house dust mite, pets, environmental tobacco smoke, encouragement of breast feeding and delayed introduction of solid feeds) demonstrated a reduction in asthma symptoms but no effect on bronchial hyperresponsiveness.²⁶

For more information on strategies to reduce allergen exposure, see Asthma and Allergy.

Other environmental factors

The observation among some communities that children living on farms show relatively low levels of allergic disease including asthma has led to the hypothesis that high exposure to bacterial endotoxins and lipopolysaccharides may reduce asthma risk.²⁷

Secondary prevention: can asthma be prevented in patients with other atopic disease?

Practice points
Long-term treatment with antihistamines does not reduce the risk of asthma developing in children with atopic dermatitis, including those who are sensitive to house dust mite and/or grass pollen. (II) Immunotherapy may reduce asthma risk in children with seasonal allergic rhinoconjunctivitis. (II)

Children

Avoidance of allergens and environmental tobacco smoke

No studies have assessed whether the avoidance of allergens or exposure to ETS is effective in reducing the risk of asthma developing in children with other manifestations of atopy.

Because of other known adverse effects of ETS, a smoke-free environment should be recommended for all children [√].

Immunotherapy

In children with seasonal allergic rhinoconjunctivitis, immunotherapy for house dust mite or pollen allergies may reduce their risk of developing asthma.^29,30 See Asthma and Allergy.

Drug therapy

• A 1-year randomised controlled clinical trial of ketotifen in children with atopic dermatitis but no history of asthma demonstrated a significant reduction in asthma symptoms during the year of therapy,³¹ but it is difficult to know whether this was a true preventive effect or reflected control of wheezing symptoms.
• Despite earlier findings that suggested cetirizine treatment may reduce risk of developing asthma among infants with atopic dermatitis and sensitivity to house dust mite or grass pollen,³² a recent randomised controlled clinical trial of cetirizine observed no reduction in asthma onset in this subgroup.³³

Adults

Smoking

Cigarette smoking increases the risk of developing asthma in response to frequent exposure to some sensitising agents associated with occupational asthma.³⁴

Occupational asthma

• Reducing airborne exposure to potential allergens in the workplace lowers workers' risk for becoming sensitised and developing occupational asthma.³⁴
• The correct use of respiratory protective equipment reduces the risk of occupational asthma but does not completely prevent it.³⁴
• See Occupational Asthma.