There is a high prevalence of IgE-mediated sensitization to specific allergens in patients with rhinitis and/or asthma. In addition, the presence of IgE antibodies to dust mite, cockroach, cat, and pollen allergens (grasses, Ambrosia, Betula, Parietaria, Olea, Chenopodium, Cryptomeria, cupressaceae, Platanus, and others) has been associated with an increased risk of exacerbations of rhinitis and/or asthma requiring emergency care in both children and adults.
It is important for clinicians to identify environmental triggers, both allergenic and non-allergenic, that may exacerbate asthma so that steps can be taken to control or eliminate them. The five main allergenic triggers of asthma are dust mites, cockroaches, animal epithelials, pollens and fungi. Initially, only outdoor allergens, including pollens and fungi, could be identified and quantified by light microscopy and a correlation could be established between the presence of these two types of pneumoallergens and asthma morbidity. With the development of immunoassay techniques and monoclonal antibodies to detect indoor allergens (such as dust mites, cockroaches, and cat allergens), the researchers were also able to identify and quantify these important antigens. This allowed them to examine and define potential risk factors for the asthmatic population exposed to different levels of these potent allergens and to assess the effectiveness of environmental control measures.
The allergens are mostly water-soluble proteins with MW between 10,000 and 50,000 kD, many of which have enzymatic activity (e.g., the major dust mite antigen is a protease produced in the gastrointestinal tract and excreted in feces, the major cockroach antigen is also a protease with digestive function).
Individuals allergic to these proteins exhibit an immediate hypersensitivity reaction to them, including increased production of IgE antibodies and TH2 phenotype T cells. Laboratory bronchial challenges with these proteins (usually only about two minutes of inhalation) result in patients with allergic asthma (80% of the asthmatic population) in immediate bronchospasm (within 30 minutes) followed by an eosinophil-rich inflammatory airway response that can last up to 7 days and results in increased bronchial hyperresponsiveness.
In other words, allergens are capable of producing in susceptible patients:
- Bronchial inflammation,
- Bronchial hyperreactivity,
which are the three outcomes that characterize bronchial asthma (Figure).
House dust mites
The major allergen in house dust remained obscure until 1967, when it was suggested that the most important source of house dust allergy was mites belonging to the genus Dermatophagoides (Figure). Subsequent studies identified two members of the Dermatophagoides family, D. pteronyssinus and D. farinae, as the two most important mite allergens in Europe and North America, although other species have also been described as important in rural areas (Lepidoglyphus destructor) and in some geographical areas (Blomia tropicalis). Subsequent studies in Denmark and the United States have corroborated these observations, with exposure to dust mites recognized as a highly significant risk factor for bronchial asthma (Figure). An epidemic of asthma in highland villages in New Guinea has been linked to the use of mite-infested blankets.
The concentration of dust mite allergens (an indirect assessment of exposure) is measured in micrograms per gram (mcg/g) of dust from samples obtained by vacuuming a defined area of a carpet or mattress. This test can be performed at home for a small fee (AED test), at the request of your physician (consult your allergist at the clinic). The proposed threshold concentration of allergen exposure necessary to become sensitized to dust mites is 2 mcg of group 1 allergens (Der p 1 and Der f 1) per gram of dust. Levels of 10 mcg/g of dust induce allergic symptoms or asthma in sensitized individuals, but lower levels may also cause symptoms in some patients.
Dust mite levels vary with climate, season and type of furniture, but humidity is by far the most important factor influencing dust mite growth. Asthmatic patients with dust mite allergies who live in homes with receptacles that are conducive to dust mite growth (e.g., upholstered furniture, bedding, carpets, and especially rugs) are at increased risk, especially if they live in humid climates, because dust mites thrive in bedding and carpets, especially if the humidity in the home is greater than 55%. A lower incidence of asthma has been found in high altitude areas of Europe, where levels of dust mites in furniture and mattresses are generally lower. The introduction of changes in house construction and cleaning habits over the past 50 years has favored the proliferation of dust mites.
1.- The use of carpeting and the introduction of vacuuming instead of shaking the carpets out of the windows has probably increased the level of dust mite allergens indoors, as most dust mites are aerosolized during vacuuming. In addition, vacuuming does not effectively remove dust mites from bedding, upholstery, rugs, or carpets.
The increased use of central heating in modern buildings, rather than local heat sources, provides optimal temperature and humidity conditions for the development of dust mites in all parts of the house.
The introduction of detergents that can be used with cold water to clean bedding has also favored the development of mites, which can only be killed by hot water.
Modern homes, with their centralized ventilation and humidification systems to save energy, especially after the oil crisis of the 1970s, have also increased the infestation of dust mites.
The invasion of television in recent decades has led to a dramatic increase in the amount of time spent indoors and thus an increase in exposure to dust mite antigens and other indoor allergens.
Dust mite allergens can only be detected in the air during activities that produce turbulence, such as vacuuming. The particles in which these dust mite allergens are found are only about 10 µm in diameter, and it is estimated that the average person can only inhale about 200 of these particles per day (Figure). These findings may explain why most dust mite allergic patients do not correlate dust exposure with their asthma exacerbations. Indeed, dust mites are much more important as a chronic and cumulative source of allergens that cause bronchial inflammation-hyperreactivity than as triggers of acute asthma attacks.
Various environmental control studies have demonstrated the ineffectiveness of isolation avoidance measures (e.g., putting on dust mite blankets only). In contrast, studies that used CONJUNCT cleaning and dust mite allergen avoidance measures (e.g., dust mite covers on mattresses and pillows, removal of carpets from the bedroom, the rest of the house, and the workplace, washing sheets at 60ºC, and using a powerful vacuum cleaner) have been shown to be effective in minimizing exposure to their allergens and reducing asthma symptoms and medication needs.
It has been shown that after only one month of residence in alpine [high altitude] clinics, where they are exposed to very low levels of dust mites, asthma patients not only show improvement in their asthma symptoms and medication needs, but also in their non-specific bronchial hyperresponsiveness and breath nitric oxide levels.
Patients with IgE-mediated asthma who have been shown to be sensitive to dust mites by skin and/or in vitro testing are candidates for these preventive environmental control measures. It has been proposed that the degree of intensity of these environmental measures should be made according to the severity of the asthma and the purchasing power of the patient.
The bedroom is the most important room to perform these environmental control measures, although other areas of the home and even the workplace, such as living rooms that may contain upholstered furniture or carpets, are also important.
Key measures to reduce exposure to dust mites are, in order of importance:
1.- Remove dust mite reservoirs:
- Carpets or rugs in the home, especially in the bedroom and living room, the use of small washable rugs is an acceptable alternative to carpeting.
- Replace the fabric upholstered chair with a leather one.
- Books should be kept in closed cabinets or boxes.
- Eliminate stuffed animals
Put special dust mite covers (xtagstartz 10 µm pores) on mattresses and pillows (they allow perspiration but not allergens to pass through them). Also put a cover on the box spring. Remove old and/or wool or feather mattresses or pillows (figure).