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Allergy and asthma
 
House dust mite allergy
Reviewed by Dr Paul Klenerman, specialist  and Professor Brian Lipworth, professor of allergies and respiratory medicine

What is house dust mite allergy?

House mite allergy is a hypersensitive reaction to proteins in the excretion of dust mites. The protein attacks the respiratory passages causing hay fever and asthma. It will aggravate atopic dermatitis in people who have a tendency to this problem.

House dust mites are found in all homes. They are microscopic organisms that thrive in warm and humid houses with lots of food - human skin. The mites prefer to live in beds and, because we spend about a third of the day in bed, we inhale large quantities of dust mite allergens.

How do you become allergic to house dust mites?

The excretion of the mites contains a number of protein substances. When these are inhaled or touch the skin, the body produces antibodies. These antibodies cause the release of a chemical called histamine that leads to swelling and irritation of the upper respiratory passages - typical asthma and hay fever symptoms. The predisposition for allergy is often hereditary.

Unlike pollen, dust mites are present all year round causing constant allergy - 'perennial' allergic rhinitis. The excretion from the mites dries out and can be launched into the air when someone walks over a rug, sits down in a chair, or shakes the bed clothes, giving allergic people immediate symptoms.

What are the symptoms of house dust mite allergy?

What makes the symptoms worse?

  • Air pollution such as tobacco smoke or car fumes.

How does the doctor make the diagnosis?

It is often enough to tell the doctor when, where and how you get the symptoms. Skin tests and various blood tests can be used for confirmation.

Good advice

It is best to do everything possible to avoid hypersensitivity to house dust mites. People who have perennial rhinitis, inflamed mucous membrane of the nose, or are allergic to house dust mites should try to adapt their homes.

  • Have as little furniture as possible in which mites can live.

  • Clean walls, woodwork and floors with wet cloths. The floor can be polished.

  • Only use rugs that can be washed once a week.

  • Use bedding that can be washed often, cotton sheets, washable bottom sheets and synthetic blankets or duvets. Don't use woollen blankets or quilts.

  • Make sure your chairs are made of wood or plastic.

  • If you can, use plastic curtains and dust them daily.

  • Use wet cloths and a vacuum cleaner with a no bag vortex and allergen filter to clean the house thoroughly, preferably every day, but at least twice a week.

  • Avoid dust traps like teddy bears, cushions, dried flowers, bric-a-brac and toys.

  • Wash bedding etc at a temperature of at least 60°C to kill the house dust mites.

  • Leave bedding, duvets, pillows and mattress hanging outside for an hour every day or as often as practical.

  • Put duvets and pillows in plastic bags and put them in the freezer for 24 hours at least once a month.

  • You may want to sleep on a cheap mattress that you can exchange for a new one at least every six months.

  • Dust mites hate dry and cold air, so try to air the house every day and don't use an air humidifier, which will only make matters worse. If the lower edge of the window is moist when you wake up in the morning, there is too much humidity in the air.

  • Do not spray the house, it may worsen your symptoms.

  • Do not touch dusty objects like books and old clothes.

  • When you are likely to be exposed to substances that give you a reaction, eg when you are house cleaning, you should wear a mask.

  • Don't allow smoking in the house.

What complications are possible?

  • You are predisposed to other respiratory diseases.

  • You are also predisposed to otitis, inflammation of the ear.

  • You may have trouble sleeping and suffer from chronic fatigue.

  • Hospitalisation following a severe asthma attack.

Future prospects

If you are allergic to house dust mites, it is important that you don't expose yourself to the dust mite allergen because it increases your chances of developing asthma. The best remedy against house dust mites is described under the heading 'Good advice' above.

Your symptoms can be controlled by treatment, but you can't escape your hypersensitivity. If severe dust mite allergy is the only form of allergy you suffer from, your doctor may want to try hyposensitisation - a 'vaccination' against the allergen. This tolerance treatment involves regular allergen injections in increased doses over a period of five years, but is not routinely given and is not always successful.

What medicine is given?

    Medicines for allergies include:

    • Antihistamine tablets or syrup (eg loratadine, cetirizine). These lessen the allergic reaction by blocking the actions of histamine. They relieve hayfever symptoms.

    • Nasal sprays or drops containing sodium cromoglicate, corticosteroids (eg beclometasone) or antihistamines (eg levocabastine). These can be used to reduce nasal inflammation and control symptoms in the nose.

    • Eye drops containing sodium cromoglicate, nedocromil, or antihistamines (eg azelastine) reduce eye inflammation and can be used if eye symptoms are a particular problem.


    If the allergy causes asthmatic symptoms, some of the asthma medication below may be used

    • Relievers (bronchodilators): these are quick-acting medicines that relax the muscles of the airways. They are used when required to relieve shortness of breath.


    • Preventers: these act over a longer time and work by reducing the inflammation within the airways. They should be used regularly for maximum benefit. When the dosage and type of preventive medicine is correct, there will be little need for reliever medicines.


      Relievers

      There are three groups of bronchodilators.

        Beta-2 agonists

        Beta-2 agonists cause the airways to relax and widen. Examples of those which act for a short time (3 or 4 hours following a single dose) are salbutamol and terbutaline. These medicines are inhaled from a variety of delivery devices, the most familiar being the pressurised metered-dose-inhaler (MDI). When inhaled, these types of medicines work within minutes to open the airways, making breathing easier.

        Longer-acting beta-2 agonists include salmeterol and formoterol. Their action lasts over 12 hours, making them suitable for twice daily dosage to keep the airways open.

        Anticholinergics

        One of the ways in which the size of the airways is naturally controlled is through nerves that connect to the muscles surrounding the airways. The nerve impulses cause the muscles to contract, thus narrowing the airway. Anticholinergic medicines such as ipratropium block this effect, allowing the airways to open. The size of this effect is fairly small, so it is most noticeable if the airways have already been narrowed by other conditions, such as chronic bronchitis.

        Theophyllines and aminophylline

        Theophylline and aminophylline are given by mouth and are less commonly used in Britain because they are more likely to produce side effects than inhaled treatment. They are still in very wide use throughout the world.

        All three types of reliever can be combined if necessary.

      Preventers

      There are three main groups of anti-inflammatories.

        Corticosteroids

        Corticosteroids (or 'steroids') work to reduce the amount of inflammation within the airways, reducing their tendency to contract. They are usually given as inhaled treatment, eg beclometasone, although sometimes oral steroid tablets may be required for severe attacks. Although steroids are powerful medicines with many potential side effects their safety in asthma has been well established. It is also important to balance the problems that arise from poorly treated asthma against the improvement in health which occurs when the condition is well treated.

        Cromones

        There are two kinds of cromones: sodium cromoglicate and nedocromil. They also act to reduce inflammation of the airways. They tend to be best for mild asthma symptoms and are more effective in children than adults. The medicines are given by inhalation and are usually very well tolerated.

        Leukotriene receptor antagonists

        Leukotriene receptor antagonists are compounds released by inflammatory cells within the lungs and which have a powerful constricting effect upon the airways. By blocking this effect with these antagonist medicines the constriction is reversed. There are two such medicines currently available: montelukast and zafirlukast.

    Most cases of allergic asthma are best controlled with an inhaled corticosteroid, eg beclometasone, which is taken at regular intervals as a preventative measure. A beta-2 agonist, eg salbutamol, is used in conjunction with this to relieve symptoms when necessary. For patients who have associated perennial or seasonal allergic rhinitis (hay fever) the use of an antihistimine, eg cetirizine, is often useful.

Based on a text by Dr Flemming Andersen

Last updated 04.01.2005

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