Role of allergens in the development of asthma
The role of allergens in the development of asthma is complex and not fully understood.

Measures to prevent asthma may be targeted at the prevention of allergic sensitisation (i.e. the development of atopy) or the prevention of asthma development in sensitised persons.

Tobacco exposure in utero and after birth is associated with a greater risk of developing wheezing illness in childhood. Pregnant women and parents of young children should be advised not to smoke. Other than preventing tobacco exposure both in utero and after birth, there are no proven and widely accepted interventions that can prevent the development of asthma (primary prevention).

Exclusive breast-feeding during the first months after birth is associated with lower asthma rates during childhood. Breast milk is associated with a lower incidence of wheezing illness in childhood compared to cow’s milk or soy protein.

Sensitisation and exposure to house-dust mite and Alternaria mold are important in the development of asthma in children. Exposure to cockroach allergen, a major allergen found in inner city dwellings, is an important cause of allergic sensitisation, a risk factor for the development of asthma.

Allergens and asthma symptoms and exacerbations
Immediate hypersensitivity to indoor allergens is known to be associated with allergic asthma. Amongst asthmatics, exposure to the allergens to which they are sensitive has been shown to increase asthma symptoms and precipitate asthma exacerbations. Reducing exposure to these allergens improves the control of asthma and reduces medication needs.

The important allergens are those that are inhaled. Food allergens are not a common precipitant of asthma symptoms.

Prevalence and distribution of allergens and sensitisation to allergens in Singapore
Local studies indicate that although public places are contaminated with common indoor allergens, the home constitutes a major reservoir of these allergens. As such, homes should be the target of allergen avoidance measures. Major cat and dog allergens were found to be well distributed and not confined to homes with pets. This passively transferred allergen may become airborne and cause symptoms.

Blomia tropicalis is the most prevalent species of house-dust mite in Singapore. Their densities were found to be highest in living room carpets and mattresses in the bedrooms.

There was no significant seasonal variation in dust-mite allergen levels in the homes over a 1-year period.

The sensitisation rates among patients with asthma and/or allergic rhinitis in Singapore (in the order of importance) were found to be as follows: Blomia tropicalis dust mite (96.2%), D. pteronyssinus dust mite (93.4%), D. farinae dust mite (92.3%), 3 other species of dust mites (78.2%, 71.6%, 71.3%), bird’s feathers (69.9%), 2 species of cockroach (59.5%, 56.4%), mosquito (46.4%), dog dander (34.3%), cat hair (29.1%) and 3 species of indoor fungi (20.8%,18%, 9.3%).

The allergenic extracts of the local mite fauna should therefore be included in the diagnostic panel for the evaluation of allergic disorders in our local practice.

Evaluation
The clinician should evaluate the potential role of allergens, particularly indoor inhalant allergens. The patient’s medical history often helps in the identification of allergen exposures that may worsen his asthma. Sensitivity to seasonal allergens should be assessed from the patient’s history. Working asthmatics should be queried about the possible occupational exposures. The early identification of occupational sensitisers and the removal of sensitised patients from any further exposure are important aspects in the management of occupational asthma. When occupational asthma is suspected, a referral to the specialist is indicated.

Skin testing or in vitro testing (specific IgE antibodies to allergen) is used to determine sensitivity to perennial indoor inhalant allergens to which the patient is exposed. A positive alone does not determine whether the specific IgE is responsible for the patient’s symptoms. Hence, patients should be tested only for sensitivity to the allergens to which they may be exposed. It is important to determine the clinical significance and relevance of positive tests in the context of the patient’s medical history.

Allergen avoidance in asthma
The asthmatic should be given advice to reduce exposure to the relevant indoor or outdoor allergens to which he is sensitive. Effective allergen avoidance requires a multifaceted, comprehensive approach. Single allergen avoidance steps are generally ineffective in reducing the allergen load sufficiently to lead to clinical improvement.

Clinical benefit usually required three to six months of sustained interventions in clinical studies. Patients should be encouraged to control exposures in their houses as part of a long-term management plan.

Avoidance measures for house-dust mite should focus mainly on the bedroom. Recommended measures to control house-dust mite allergen include the following: encase mattress and pillow in allergen-impermeable covers, wash pillow, bed sheets and blanket in hot water (55-60ºC). A temperature of >55ºC is necessary for killing house-dust mites.

The most effective measure to control animal dander allergens is to persuade patients not to keep animals in the home. Patients who are allergic to cat or dog allergens should be informed about the relevance of passively transferred allergen.

Amongst asthmatics, exposure to tobacco smoke is associated with increased symptoms, decreased lung function and greater use of health services. Asthmatics should be advised not to smoke or be exposed to environmental tobacco smoke.

Adults with asthma, nasal polyps and a history of sensitivity to aspirin or NSAIDs should be counselled regarding the risk of severe and fatal exacerbations from using these drugs.

References:
1. Global initiative for asthma. Global strategy for asthma management and prevention (revised 2006).
2. Expert Panel Report 3: Guidelines for the Diagnosis and Management of Asthma. National Asthma Education and Prevention Program. National Heart, Lung, and Blood Institute. US Department of Health and Human Services. 2007
3. The effectiveness of measures to change the indoor environment in the treatment of allergic rhinitis and asthma: ARIA update (in collaboration with GA2LEN). A Custovic, RG van Wijk. Allergy 2005: 60: 1112-5.
4. Prevalence and distribution of indoor allergens in Singapore. L Zhang, FT Chew, SY Goh et al. Clinical and Experimental Allergy 1997 :27:876-85.
5. House dust mite fauna of tropical Singapore. FT Chew, L Zhang, TM Ho et al. Clinical and Experimental Allergy 1999: 29: 201-206
6. Sensitisation to local dust-mite fauna in Singapore. FT Chew, SH Lim, DYT Goh et al. Allergy 1999: 54: 1150-1159