Allergy-prone individuals produce IgE (immunoglobulin E) class antibodies to a wide range of proteins found in the environment, especially inside buildings and homes. Today, in developed countries, most people may spend up to 95% of their time indoors, spending very little time outdoors; it is clear that this change in lifestyle has led to changes in the frequency of many chronic diseases, with a particular incidence of respiratory allergic diseases. In addition, most homes in industrialized countries, thanks to improved insulation systems, maintain uniformly warm temperatures and generally contain more furniture than 50 years ago. When we spend long periods of time in these enclosed environments, we breathe air that is rich in potentially allergenic substances and in much higher concentrations than outside, which facilitates the development of dust allergy.
How does dust mite allergy manifest itself?
House dust is the main source of indoor allergens. It is a complex ecosystem composed of a mixture of inorganic and organic materials, including human skin flakes, fibers, fungal spores, bacteria, viruses, pollens, insects, animal dermal by-products, food debris, houseplants and dust mites. The inorganic material in the dust does not cause allergic sensitization, while the organic component present may act as an irritant or allergen. Any protein present in the dust may behave as an allergen if it is able to become airborne, be inhaled and reach the airways inducing an IgE-mediated allergic response. It is important to know that susceptibility to respiratory allergic diseases depends on genetic predisposition and exposure under appropriate conditions, i.e. in sufficient quantity and time, to certain substances that behave like allergens.
Sensitization to indoor allergens is more important for the development of asthma than sensitization to outdoor allergens. Although it is possible to be allergic to one or more of the organic substances present in dust, dust mites are the main source of house dust allergens, producing potent allergens capable of sensitizing and inducing respiratory symptoms in sensitized individuals.
How long has dust mite allergy been known?
In 1921, R. Kern first recognized the role of house dust as an allergen when he found that many of his patients with rhinitis or asthma had positive allergy tests when he used dust samples from their own homes. Shortly thereafter, in 1928,
Decker proposed that dust mites play a major role in house dust, although he found no growth in the samples he tested. By the late 1950s, a large number of allergens had been identified in house dust, including dermal derivatives of animals, insects, and fungi. It was known even then that dust samples from damp homes caused very strong skin reactions, but there was no explanation for this. Finally, in 1967, researchers R. Voorhorst and F. Spieksma discovered that the allergenic activity of dust samples from homes in the Netherlands was mainly due to the presence of a mite, Dermatophagoides pteronyssinus (D. pteronyssinus) (see picture). From that moment on, techniques for the cultivation of mites were developed and extracts of these mites were used for the diagnosis of house dust allergy.
Are there differences in the composition of the mites from one region to another?
In Voorhorst and Spieksma's early work, these researchers found that dust samples from Dutch homes near canals contained more than 500 mites per gram of dust, while dust samples taken from the Valbella sanatorium in the Swiss town of Davos (the sanatorium where Thomas Mann set his famous book The Magic Mountain) contained negligible amounts of mites. This difference in concentration was the reason, they said, that children with dust mite allergic asthma improved dramatically when they were transferred to this Swiss sanatorium, by drastically reducing the level of dust mite exposure. Shortly thereafter, the association between asthma and positive allergic skin tests to dust mites was confirmed in other countries, including Australia, Japan and Brazil, and dust mites are now recognized as the major source of allergens in house dust.
Storage mites, or minor house dust mites, constitute a wide range of families, genera, and species found primarily in stored foods, grains, flours, farms, barns, and hay. Exposure to these mites, and their allergens, can also occur in homes, especially those with moisture problems. Many species of these minor mites have been identified in house dust, so the term house dust mite is used for all species of mites that can be found in the home environment and for which IgE-mediated sensitization processes have been described.
What are house dust mites?
Mites (acari or acarina, from the Greek akarés, 'tiny, which cannot be cut') are a subclass of arachnids.
not cut') are a subclass of arachnids, of which there are nearly 50,000
of which nearly 50,000 species are described out of a total of 100,000 to 500,000 species that have not yet been classified. Most mites are not visible to the human eye and grow to a few millimeters in length; for example, house dust mites measure between 0.2 and 0.5 mm.
0.5 mm. They are one of the oldest groups of land animals, with fossils known to date back 400 million years, and are distributed throughout the world, adapted to live in all known environments on the planet. Mites have been identified at altitudes from 5,000 m above sea level to 4,000 m deep, and are particularly abundant in coastal areas. The temperature range for their reproduction is very wide (5º-30º C), although the optimal temperature is 25º C. Some species have adapted to life in springs, tolerating very well the intense heat of hot springs in volcanic regions, while others have adapted to polar waters, tolerating extreme temperatures.
Mites are particularly abundant in areas with a lot of vegetation, among decaying wastes and in association with mosses and lichens; in fact, in wooded areas, they may constitute 70-90% of the total soil population, and play a vital role in both decomposition processes and soil integration.
decomposition processes and the integration of organic matter into the soil. In addition, mites are commonly found in homes, including mattresses, pillows and carpets. This high degree of habitat diversity corresponds to a very high degree of variability in shape, size, structure and behavior.
Which mites are affected by allergy?
It is estimated that only 25 of the more than 40,000 described mite species are associated with allergic diseases in humans. The mites most frequently involved in allergic processes belong to the order Astigmata, in which only three superfamilies are responsible for allergic problems:
Pyroglyphoidae: to this family belong Dermatophagoides (D.) pteronyssinus, D. farinae and Euroglyphus maynei, which are undoubtedly the inducers of the vast majority of allergic sensitizations in Europe and the United States.
Acaroidae: Acarus siro and Tyrophagus putrescentiae are the responsible species within this family.
Glycyphagodae: Blomia tropicalis and Lepidoglyphus destructor are increasingly recognized as being responsible for allergic pathologies.
These mites are harmless to humans and it is their fecal residues that are highly allergenic. Their growth cycle (from egg to adult) is 25 days at 25º C, and most of them live between two and three months, during which time they lay one or two clutches of eggs that usually contain between 20 and 40 units.
What is the habitat of mites?
Dust mites that cause allergic diseases are preferentially found in two different habitats: homes and warehouses.
The main environmental factors that influence the presence of dust mites are: temperature and relative humidity. The optimal temperature is between 25º and 35º C.
The optimal relative humidity for D. pteronyssinus is above 75% and for D. farinae (see photo) it is between 50 and 60%. Proliferation under these conditions is faster than at lower temperature and humidity. The presence of fungi also facilitates the development of mites, probably because of the prior digestion of lipids on which they feed.
The species that invade homes are called house dust mites, and belong mainly to the family Pyroglyphidae. They coexist with humans because they feed on dander shed by humans (about 1 gram per day) or on dander from pets. Their main habitat is the interior of houses, as they are found in large numbers in mattresses, sofas and furniture covered with fabric. In Spain, D. pteronyssinus and D. farinae are preferentially isolated.
The species found in warehouses are called warehouse or storage mites. In Spain, the following species stand out for their presence: Acaro siro, Tyrophagus putrescentiae (see image on the left on the next page) and Lepidoglyphus destructor. They feed mainly on cereals and other food particles present in house dust, and their main habitat is cereals and foodstuffs usually stored. However, under certain favorable circumstances, they can proliferate in large numbers indoors. Tyrophagus putrescentiae is most often found in areas where protein-rich foods such as ham or cheese are stored. Lepidoglyphus destructor (see photo at right) is easily isolated in grain stores and is also a very common genus in northern Spain, especially in Galicia. The most common deposit mite in tropical areas and the Canary Islands is Blomia tropicalis.
In what areas of the house are mites abundant?
Inside the home, dust mites have isolated themselves in sheets, pillows, carpets, curtains, bedding, stuffed toys and mattresses. Sofas and especially mattresses are an excellent microhabitat for dust mites because, given the depth of their padding, they retain a lot of moisture, which provides them with the three factors necessary for optimal development: heat (from the sleeper's perspiration) and food (human skin flakes). Humidity is the main limiting factor for their development; optimal levels of relative humidity are 75% at 15ºC. These values are easily reached in mattresses when they are occupied, on average eight hours a day. The heat and perspiration of their occupants produce an increase in their temperature that reaches 25º-30º C.
25º-30º C, and their relative humidity increases by 5-8%.
Thus, during these 8 hours per day, the mites of the mattress find favorable conditions for their development. This time can be longer if the bed is made immediately, without prior ventilation, so that a certain degree of temperature and humidity can be maintained for almost 16 hours.
In homes in temperate climates, the number of mites varies seasonally, with low numbers in early summer and a gradual increase as fall approaches, followed by a decrease in the fall-winter. During the summer months, as the heating is not used and the house is more ventilated, the relative humidity of the air increases. On the other hand, in winter, the doors and windows are less open and, with the use of heaters, a warm but very dry climate is created inside, which is not at all favorable to their growth. In the bedrooms, on the other hand, the daily use of mattresses allows them to survive in better conditions than on the carpets.
Despite the decrease in the number of live dust mites during the winter months, the allergenic particles they produce remain in the environment and decrease more gradually. Thus, although the number of dust mites fluctuates throughout the year, the symptoms caused by their allergens are generally perennial.
What causes dust mite allergy?
Fecal particles produced by dust mites are the main source of allergens. Each mite can produce about 20 fecal particles per day that are capable of causing allergic symptoms even after the mite has died. In recent years, extraordinary progress has been made in the study of the characteristics of mite allergens and more than twenty different molecular groups have been identified, many of which are extracellular proteins with enzymatic activity.
Recent studies have shown that the critical levels of mites for asthma production are between 100 and 500 mites per gram of dust or, in other words, a level of Der p1 (major allergen of D. pteronyssinus) equal to or greater than 2 microns per gram of house dust.
Dust mite allergens can only be detected in the air during activities that produce turbulence, such as vacuuming or shaking bedding. Each mite can lay 20 to 50 eggs and produce a new generation about every 21 days. Thus, dust mites, dead or alive, can be found by the hundreds in every gram of house dust, especially in mattresses, pillows and bedding. This fact may explain why most people with dust mite allergies do not associate dust exposure with an exacerbation of their respiratory symptoms. In fact, dust mites act more as a chronic, cumulative source of allergens that cause bronchial inflammation-hyperreactivity than as triggers for acute rhinitis or asthma attacks.
How does dust mite allergy manifest itself?
The symptoms of dust mite allergy are, in most cases, respiratory: rhinitis and asthma. In some regions of our country, sensitization to dust mites affects more than 30% of the population and 90% of asthmatics.
In the case of rhinitis, the patient suffers from episodes, preferably in the morning when getting out of bed, of sneezing (more than 10 consecutive sneezes), nasal itching, abundant hydrorhea (nasal discharge) and intense nasal congestion. These symptoms usually go away
when you leave the house, only to reappear at night when you go to bed, or when you handle large amounts of water
or when handling large amounts of dust. About 30% of these patients may also have asthma symptoms in the form of coughing spells, chest tightness, wheezing and intolerance to exercise or laughter. Symptoms are usually perennial, meaning they occur throughout the year, although they may have exacerbations in the spring and fall. Ocular symptoms are rarer than in pollen allergy. It has also been suggested that mites may play an important role in the pathogenesis and exacerbations of atopic dermatitis. Finally, mites are also capable of contaminating certain foods made with enriched flours used in dough or cooking and causing anaphylaxis (the most severe manifestation of the allergy).
How is dust mite allergy diagnosed?
As in any other allergy procedure, medical questioning is fundamental and often sufficiently orienting. The physical examination must include observation of the nasal mucosa by anterior rhinoscopy, respiratory auscultation and observation of the skin. To establish a diagnosis of certainty, the allergist must perform allergy tests by prick test with a selection of allergens that vary from one region to another. These tests are simple, quick and very reliable when performed by experienced personnel. In some cases, it will also be necessary to perform specific IgE assays in the blood and, more rarely, controlled exposure studies to the allergen, whether nasal, ocular or bronchial.
Can dust mite allergy be prevented?
Dust mites are natural inhabitants of our home environment and their presence does not mean a lack of cleanliness in the home. It is not clear whether environmental control can prevent dust mite allergy, and existing evidence suggests that primary prevention is not possible. Studies show that the application of environmental control measures may have a beneficial effect on the onset and severity of asthma, but does not appear to be very helpful in preventing or delaying the onset of dust mite sensitization, especially in areas with high exposure to dust mite allergens (e.g., the Canary Islands or Galicia).
Highly effective measures: Remove carpets and all objects that accumulate dust from the bedroom; use dust mite covers on the mattress and pillow (the mattress and pillow should be vacuumed for 10 minutes once a month and exposed to the sun for 30 minutes 2 or 3 times a year); wash sheets and blankets at least once a week with hot water (over 50º C) ; Regular cleaning of sofas, carpets and other areas of the house; periodic checking of the house and repair of detected humidity problems; reduction of the relative humidity in the whole house, or at least in the bedroom, to less than 50% (regular use of a dehumidifier can significantly reduce the dust mite population).
Intermediate measures: use of air purifier to retain dust mite allergens, use of acaricides and air conditioning.
What treatments are useful in dust mite allergy?
In addition to the dust mite avoidance measures mentioned above, a number of medications are now available to alleviate and control the troublesome symptoms caused by this type of allergy. In addition, for correctly diagnosed patients, and depending on the progression of their allergic symptoms, the allergist may also prescribe treatment with dust mite vaccines to comprehensively treat their underlying condition.