Skin feeders: living with dust mites

By Katherine Fleming 8 June 2010
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These microscopic arachnids are ubiquitous and so numerous that, try as you might, they’re impossible to avoid.

TONIGHT, AS YOU SINK wearily into bed, you won’t be alone – even if you’re the only person in the room. Under your slumbering head, the pillow will teem with potentially thousands of unseen bedmates, feasting on your fallen skin flakes.

The good news is they don’t bite. The bad news is they’ll be procreating, defecating, dying and decomposing in your bedding, couches, carpets and clothes. Essentially, we all spend a good deal of time wallowing in dust-mite filth.

The humble dust mite – the species Dermatophagoides pteronyssinus is most common in Australia – is an arachnid, a relative of spiders and ticks. It’s less than half a millimetre long but what it lacks in size, it makes up for in abundance. The detritus in our old pillows is made up mostly of sloughed skin, but the microscopic dust mites, alive and dead, are also there in their thousands, along with mite dung.

CSIRO entomology researcher Dr Matthew Colloff, author of the book Dust Mites, puts the numbers of dead dust mites (only about 5 per cent are alive) in an average bed at between 100,000 and more than 1 million, depending on conditions. “If you’re above 500 mites per gram of dust, which contains skin scales, organic debris, mould, ash, crumbs and all sorts of things, you’re getting into pretty high mite numbers,” explains Matthew.  “The highest ever recorded [level] was about 12,000 per gram. Now, if you make a circle with your thumb and index finger, a gram of dust would fit in there, so imagine 12,000 mites in that kind of space.”

The fact that our skin scales are their main food source means generally, wherever we are, they are, says Dr Peter Dingle, associate professor in health and the environment at Murdoch University’s School of Environmental Science. “We shed literally millions of skin cells every day but they have to be dehydrated and de-fatted by mould,” he explains. “The mites wait for the mould to attack and then they feed on both.”

COMPLEX FATS AND PROTEINS also make skin scales tough to digest, says Matthew, but the mites have chambers in their gut that act as fermentation tanks. “They probably have skin scales sloshing around in an enzyme soup for prolonged periods,” he says. “It’s like skin-enzyme porridge.”

A seemingly less advantageous evolutionary quirk involves breeding. Dust mites mate for 24 hours at a time, probably because the male’s penis is only about as wide as the sperm. “Our best guess is mating is so prolonged because the sperm is coming out more or less single file,” Matthew says. “Why it would have evolved like that, no-one knows.”

In optimal conditions mites live for about six weeks, each producing about 20 faecal pellets a day and females lay about 30 eggs during their life span. During weeks, months and years, the debris formed by live and dead mites and their waste accumulates. It’s the faecal pellets – just 20 microns across and easily inhaled when bed-making or vacuuming make them airborne – and the mites’ decomposing bodies that cause problems for humans.

This material contains a series of proteins that are highly allergenic for susceptible people, causing everything from sneezing, itchy eyes and a blocked, runny nose, to a severe asthma attack. There are about 20 known allergens and most, but not all, occur in the mites’ faeces.

DR JANET RIMMER, a respiratory physician and director of the National Asthma Council Australia, says about 45 per cent of the Australian population suffer from allergies, and of those about 80 per cent are allergic to dust mites. While they’re most dangerous to the 20-30 per cent of us with asthma, they don’t always cause asthmatics to be allergic. And not all people allergic to dust mites have asthma. Historical studies of asthmatics being taken into hospitals or sanitariums have produced variable results.

“Those were obviously lower dust-mite environments and some people got better, but not everybody did,” Jane says. “Dust-mite allergens are playing a role but we don’t always know how much.” The mites’ role in allergies wasn’t discovered until the mid-1960s, and early investigations into reducing the effect on allergies and asthma by cutting exposure were hampered by social stigma – people then had no knowledge of mite allergens. “The news that their homes were infested with mites which were making  their children sick may have reduced their compliance with interventions,” Matthew says.

Since then, extensive research has failed to yield a cure. A 2008 review compiled for the international Cochrane medical database found none of the research measures to reduce exposure to house dust mites had any discernable benefit for asthmatics. “After pouring bucketloads of money into dust mites in the late 1980s and 1990s, I think the funding bodies got fed up with it because it hasn’t been solved, so they’ve gone on to things that are more sexy,” Matthew says. “But dust mites are still a huge public health problem and we haven’t cracked it. There’s a lot we still don’t know.”

REDUCING YOUR EXPOSURE:Dust mites are found everywhere – traces have even been identified on the Mir Space Station and in Antarctica. Many Australian cities offer warm, humid environments that make them well-suited as breeding grounds. Sydney and Melbourne are particular hotspots. It’s impossible to clear your world completely of dust mites. Anyone professing to be able to is talking rubbish, says Dr Matthew Colloff. But dramatically cutting exposure to mites (by about 95 per cent) can reduce symptoms, and the following steps can help:

1 – Expose mattresses to the sun; dust mites are very susceptible to dehydration.
2 – Wash bedding weekly with tea-tree or eucalyptus oil or in 55˚C plus water, which kills mites and washes away allergens. Ten minutes in a hot dryer will also kill mites, as will dry-cleaning, although it doesn’t get rid of allergens.
3 – Replace soft toys in bedrooms with wooden or plastic ones, or hot-wash them weekly. 
4 – Consider replacing carpet with hard flooring and keep bedroom windows open.
5 – Use a vacuum with a HEPA (high efficiency particulate air) filter. Note: allergen levels in the air will be higher for up to 20 minutes after vacuuming.