The allergenic effect is actually due to certain proteins in the droppings of house dust mites (including the proteins Der p1 and Der p2). The droppings dry out, break down, and get mixed into house dust, which also contains the shed skin and faeces of the mites. When the dust is breathed in, allergic reactions can develop.
The immune system then generates IgE antibodies in reaction to the proteins, which are actually harmless. These antibodies bind to the body’s defence cells, known as mast cells, which then release anti-inflammatory substances such as histamine.
Histamine and other messenger substances stimulate the glands to release secretions and irritate the nerves, causing itching, sneezing and vasodilation that leads to redness and swelling of the mucous membranes.
Places where house dust mites live
House dust mites are found in every home and are not a sign of poor hygiene. They prefer warm, damp, dark places. Mites thrive at a humidity of 70 to 80 per cent and temperatures above 25 degrees. They are not present in dry climate areas, even in dusty conditions. They need a damp environment to survive, since they take moisture from the air rather than drinking. They feed mainly on the dead skin shed from humans and animals. Their genus name ‘Dermatophagoides’ comes from the Greek, meaning ‘skin eaters’. In homes, they prefer bedrooms and the interiors and surfaces of mattresses, blankets and pillows. These places provide them with optimal living conditions. Humans release moisture into the air while sleeping and shed a lot of dead skin. Mites can also be found in the kitchen, where cooking activity creates a warm and damp environment. Mite allergens are found in dust-filled textiles in the highest concentrations; these include upholstered furniture, carpets, curtains and plush toys as well as beds. Allergen levels are at their highest between May and October, the peak breeding season of house dust mites. Most of the mites die during the winter, but the allergen-containing dust is stirred up by heating systems. This often causes the symptoms experienced by affected patients year-round to worsen during the winter.
The allergy becomes particularly troublesome at night or in the morning with symptoms such as a runny or congested nose. Other symptoms include sneezing and sometimes itchy, watery eyes.
Constant exposure to allergens may lead to chronic inflammation of the nasal mucosa, which may in turn lead to nasal turbinate hypertrophy. This leaves patients unable to breath through their nose, and they then end up breathing through their mouth (which may lead to more frequent infections in the ear, nose and throat area).
A hypersensitive reaction can also occur in the bronchial mucosa after some time, causing bronchial asthma. Symptoms include coughing (at night), difficulty breathing, and wheezing.
In rare cases, the skin is affected (itching, redness and hives). A house dust mite allergy can also trigger an atopic dermatitis flare-up.
In some cases, people with a house dust mite allergy also experience an allergic reaction after eating crustaceans such as prawns, crabs or lobsters. Like mites, these animals belong to the phylum Arthropoda and contain proteins similar to those found in mites (Der p10). The sensitised immune system considers the crustacean proteins as ‘dangerous’ and triggers an allergic reaction. Such cross-reactions may also occur after eating molluscs such as mussels or snails. Allergic reactions to storage mites occur less often. This type of mite can be found in the damp walls of pantries or in the hay inside stalls or pet cages.
As a first step in diagnosing a suspected house dust mite allergy, the doctor asks the patient about his or her symptoms. The questions include where the symptoms occur, at what time of day and during which seasons, how long they last and whether there are any allergic diseases in the family. After a physical examination, skin and/or blood tests are conducted in order to confirm the presence of an allergy. These tests reveal an allergic disposition to certain house dust mite allergens but do not provide evidence of an actual allergy. This is why a provocation test may be necessary if there are any doubts. For this test, small amounts of the suspected allergen are applied to the patient’s nasal mucosa under medical supervision. The patient is then observed for swelling of the nose, sneezing or watery eyes. A pulmonary function test is conducted if the patient suffers from respiratory symptoms. For this test, the doctor measures the amount of air that the patient inhales and exhales, and the speed at which air is exhaled. Changes to the airways can be determined through the results.
Incidentally, pollen, animal hair and mould can also cause symptoms that are similar to those of a house dust mite allergy. For this reason, allergies to these substances should be ruled out.
The most important therapeutic measure is to reduce allergen exposure by reducing the number of house dust mites in the environment.
- The bedroom in particular should be kept cool and dry. The optimal air humidity is 45 – 55 per cent (this can be measured with a hygrometer), and the optimal room temperature is 18 degrees Celsius. To reduce humidity, it is recommended to air out rooms briefly several times a day and not to keep houseplants in the bedroom (this also reduces mould levels).
- The mattress should also be well ventilated (not placed directly on the floor; a mattress on a bedframe is better than a storage bed) and made up with special bedding that does not allow mite allergens to penetrate. The costs for this bedding, also known as encasings, are covered by statutory health insurance. Double beds with twin mattresses must have encasings on both mattresses.
- The bedding should be aired out every morning. (Cotton) bedding should be laundered every one to two weeks at 60 degrees Celsius. Pillows and blankets should be washed at the same temperature every three months. If the materials do not allow for this, encasings can be used.
- There is no consensus as to whether carpets should be removed. If there is floor heating under the (short-pile) carpet, using it reduces the moisture level in the floor and consequently reduces the number of mites. Carpets bind to dust particles that would otherwise swirl about on smooth floor surfaces. However, it is very difficult to remove mite particles with a standard vacuum cleaner. Smooth floor surfaces (wood, tile, linoleum) are therefore the better choice if they are regularly vacuum cleaned and damp mopped.
- Vacuum cleaning increases the concentration of airborne mites. Therefore people with a dust mite allergy should ask non-allergic individuals to perform this task, or wear a protective mask while doing so. It is also advisable to equip the vacuum cleaner with a HEPA filter, a particulate filter that traps extremely fine particles. But they are not of much use if the vacuum cleaner is not sealed and dust escapes out of the cracks.
- It is also recommended to wash the textiles in the room regularly (curtains should be washed at 60 degrees Celsius if possible) and to keep them to a minimum (no decorative pillows, no upholstered bedframes or furniture; choose furniture with smooth surfaces such as leather or wood). Plush toys should not be placed in the bed; as an alternative, they can be washed regularly at 60 degrees Celsius. If this is not possible, they can also be placed in the freezer for 12 hours. This kills the mites, but the toys must then be washed (at the appropriate temperature) in order to remove the allergens.
- Pets should not be allowed in the bedroom because the skin flakes they shed give mites more to feed on.
Non-medicinal measures include regular nasal rinses. Some medications can also provide temporary relief from allergy symptoms, but they do not treat the cause. The same substances are used in this case as in pollen allergy therapy, in particular the newer antihistamines and cortisone sprays.
One form of therapy that treats the cause of house dust mite allergy is (allergen) specific immunotherapy (SIT). This can be an option when measures to reduce the number of mites in the home prove unsuccessful or for cases in which symptoms are severe and have persisted for over two years.
The purpose of SIT is to allow the immune system to become used to the house dust mite allergens over a longer period of time. Patients are given an injection of an allergen solution once a week in increasing doses into the layer of fat under the skin (subcutaneous immunotherapy or SCIT). The highest dose is reached after one to three months, after which the injection is only administered once a month. This therapy takes three years to complete and therefore requires persistence and cooperation from the patient. There is no guarantee that the allergy will ‘disappear’ once the treatment is completed. But studies show that a full course of SCIT can reduce symptoms and the need for medication over the long term. The treatment also reduces the risk of developing asthma. The available studies on the effectiveness of SCIT for house dust mite allergy are not as extensive as those for pollen allergy.
Young people who do not react to many different allergens have the best chances of success.
Besides SCIT, another option is to administer the allergens as drops. The doses are given to the patient under the tongue (sublingual immunotherapy or SLIT) and then swallowed. This treatment can be carried out at home and also takes three years. The effectiveness in adults has been documented in individual studies; however, the available data is more contradictory than for other cases such as SLIT for grass pollen allergy.
Prof. Dr. med. Dr. h.c. T. Zuberbier
Last changes made: July 2016