Family doctor

What is an allergy to housedust mites?

The housedust mite is the most common allergen encountered in New Zealand and most other developed countries. It is the major cause of allergy in people with year-round complaints of stuffy nose, sneezing, and watery eyes - what some people describe as a "permanent cold". In addition to these allergic reactions, dust mite can trigger asthma and eczema.

Of those who develop asthma in the first 10 years of life, over 75% will show a positive skin prick test to housedust mite allergy. When challenged by inhaling dust mite, a similar percentage will show an immediate asthma response.

Nowadays most people spend more than 90% of their lives indoors. Over the past 30 years, the home environment has changed enormously with the introduction of fitted carpets, soft furnishings, and central heating. Indoor ventilation has decreased, with considerable increases in both humidity and indoor allergens like housedust mites. High-dose exposure to dust mites in infancy has been implicated as a cause of the increasing prevalence of asthma and other allergies in children who are genetically predisposed to develop allergies (atopics).

In some climatic areas the prevalence of childhood asthma goes hand in hand with abundance of housedust mite and resultant sensitisation. In Papua New Guinea highlands, lifestyle changes and associated use of dwellings which harbour dust mites has seen a ten-fold increase in the prevalence of asthma.

What are housedust mites like?

The housedust mite is a microscopic eight-legged creature belonging to the subclass acari (comprising thousands of mites and ticks). Other well known mites include storage mites found in flour, and sarcoptes mites causing scabies. Many different species of dust mites have been reported in housedust, but in most parts of the world Dermatophagoides (skin eating) pteronyssius and D.farinae predominate.

Housedust mites live on human skin scales and other debris. They have a very precarious water balance. They absorb water through their leg joints from the environment. They have no mechanism of searching for or drinking liquids and are thus entirely dependent on ambient humidity. They survive and multiply best when the relative humidity is at 75-80%. Although it is common practice to measure the ambient (atmospheric) humidity, it is the humidity within carpets, sofa, mattress or clothing - where the mites live - that is relevant. As humidity falls, mites will withdraw from the surface but even in very dry conditions, it may take months for mites to die or allergen levels to fall.

Dust mites are approximately 0.30mm in length, too small to be seen with the naked eye. Mites excrete partially digested food and digestive enzymes as faecal particles. These faecal pellets are the major form in which the mite allergens accumulate in house dust. Most mites are coprophagic (eat their faeces). Despite coprophagia, large numbers of faecal particles can be found in bedding and carpets. Each mite produces about 20 waste particles a day. Because the faecal particles are so tiny and light they float easily into the air when the bedding is shaken or the carpet is walked upon or vacuumed. When these floating pellets are inhaled by a sensitive person, allergic symptoms result.

Each female mite can increase the population by 25 to 30 every three weeks. In New Zealand, mite numbers peak in March and April and allergens persist at high levels through to July. Theoretically the dust mite population should decline in the winter months in areas where the climate is cool and dry, but some sensitive people report an increase in symptoms in winter. This is because the mite faecal particles remain in the home and because the body parts are also allergenic. Forced air heating systems stir up the problem; as the dust becomes dry, more become airborne. During the winter months the warm woollen clothing and blankets are also a big source of dust mites. It is important to remember that housedust mites don't bite or transmit diseases - they are only harmful to people who become allergic to them.

How is allergy to dust mites diagnosed?

A detailed history taken by your doctor, looking for clues like: all year round (perennial) symptoms vs. seasonal symptoms due to pollens, symptoms worse at night or early mornings, symptoms aggravated by making the bed or vacuuming, shifting house, "spring cleaning", or anything that would disturb settled dust.

The skin prick test is the simplest, quickest and cheapest way of confirming the diagnosis. It is done by applying a drop of dust mite extract to the skin and pricking the skin and waiting for 15 minutes - if you are allergic an itchy, red lump (like an insect bite) will appear at the site.

Principles of housedust mite reduction:

Kill Mites

Raised ambient temperature, ultra-violet light and a relative humidity of less than 55% are all lethal to house dust mites.

These can be achieved as follows:

• Wash bedding, pillows, duvets and mattress covers in HOT water of greater than 55 degrees Centigrade.
• Expose the mattress to the sun's UV light for a few hours each month.
• Turn an electric blanket on to HIGH for a few hours each month. Many mites will be killed near the mattress surface.
• Where available, use an air conditioner or dehumidifier to maintain bedroom humidity below 50%.
• Chemical agents which kill mites are available in New Zealand, but these have not been found to be very effective.

Remove Mites & Mite Protein

Even if mites are dead, their bodies and excreta can still cause allergic reactions. Removal can be achieved as follows:

• Intensive vacuuming of mattress, carpets and furniture is needed once a week. The more vigorous and effective the cleaner the better. It is vitally important that housedust mite and other allergenic particles once sucked up into the vacuum cleaner are prevented from leaving the cleaner through its exhaust opening. The most effective way of controlling this efflux is undoubtedly the HEPA (High Efficiency Particulate Air) filter. HEPA filters retain 99.9% of particles in the range of 0.3 to 0.5 micrometers and will retain housedust mite, cat and mould allergens. Vacuum cleaners which incorporate both a double dust bag and HEPA filters are the best choice for allergy sufferers.
• Weekly hot water washing (55 degrees Centigrade) will kill mites and remove bodies and excreta. Curtains should be laundered at regular intervals.
• House dust mites and their excreta accumulate on dusty surfaces, in drawers and on clothes. Using a damp cloth, dust all exposed surfaces in the bedroom daily. Clean out the drawers regularly.

Mite - Impenetrable Barriers for Bedding

Mites are mobile, crawling through beds and carpets. Plasticised fabrics provide an impenetrable barrier to mites and their excreta.

Washable mite-protective covers for the mattress, bed base, pillows and duvet are available. The modern versions are water permeable and so do not cause sweating.

Bedding & Furnishings

Any furnishings with soft, fluffy surfaces provide growing sites for house dust mites. Similarly they will grow in dust allowed to accumulate about cluttered surfaces and books.

• All bedding should be washable. Synthetic materials, sometimes described as "allergy free" are best. Pillows, blankets and duvets in particular should be washable.
• Soft mattress covers or underlays such as sheepskins should not be used. Kapok or down duvet fillings should be avoided.
• Curtains should be of the type which can be laundered.
• Avoid soft covered furniture in the bedroom.
• Keep all surfaces uncluttered and store books outside the bedroom.
• Carpets are a problem as they can act as a continuous reservoir of mites. While regular vacuuming is helpful it is not as successful as having shiny surfaces such as wood or vinyl.
• Sheepskin rugs provide an ideal haven for house dust mites and so should be avoided.

N.B. MITE PROTEIN REDUCTION MEASURES ARE NOT A REPLACEMENT FOR APPROPRIATE MEDICAL MANAGEMENT OF ALLERGIC DISEASE BUT DO PROVIDE COMPLEMENTARY AND SOMETIMES CRITICAL ADJUNCT TO ONGOING TREATMENT WITHOUT ADDING TO THE DRUG BURDEN.