The house dust mite is the commonest allergen encountered in New Zealand and most other developed countries. It is the major cause of allergy in persons 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 house dust mite. 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 increase both in humidity and indoor allergens like house dust 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 house dust 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 house dust mites like?
The house dust 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 house dust, but in most parts of the world Dermatophagoides (skin eating) pteronyssius and D.farinae predominate.
House dust 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 mite lives – 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 coprohagic (eat their faeces) Despite coprohagia, large numbers of faecal particles can be found in bedding and carpets. Each mite produces about 20 waste particles a day. Because the waste 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 throughout July. Theoretically the dust mite population should decline in winter months when it 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 house dust mites don’t bite or transmit diseases — they are only harmful to people who become allergic to them.
How is allergy to house dust mite 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 bump (like an insect bite) will appear at the site.
How can house dust mite allergy be treated?
This treatment literally reduces your sensitivity to the house dust mite. Desensitising injections (extracts) contain a small quantity of house dust mite. The body defends itself by producing protective antibodies. If enough are produced in the body, the next time you come in contact with house dust mites they will protect you. Not unlike immunisation.
The initial course consists of increasing doses of dust mite given once a week for about 10 weeks. After the top dose is reached, you will need maintenance injections at 4 to 6 weekly intervals for 3 years. In this way long lasting immunity is achieved.