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 dont 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?
- House dust mite reduction measures as outlined below
Desensitising or immunotherapy.
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
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.