Dust mites and disease - quick review

Without sight, they navigate using their exquisite sense of smell and by paying attention to signalling vibrations from other mites.

Five stages of life are; egg, 6-legged larva, two 8-legged nymph stages and finally a breeding adult. They grow best at 25C (77F) at 65-75% relative humidity (RH) and live for about three months. Adults breed for 26 to 34 days with healthy females producing up to 60 eggs.

To stop mite larvae and mite droppings (allergens) from passing through fabric, cloth pore size should be woven to 10 microns or less.

Adult dust mites are about a third of a millimetre long, up to 75% water in weight, blind, do not drink (as we know it), and produce up to 20 droppings a day. Like rabbits, mites consider their droppings as a source of food.

Mites have no 'stomach' but a chambered gut that produces digestive enzymes designed to break down scraps of food. Both leftover food and enzymes can be found in the mite's droppings.

House dust mites eat a special fungus that can survive the mite's toxic gut. The fungi then emerge from mite's dropping to grow again. The fungi may be helpful to the mite by breaking down food.

Adult mites travel about using suckers and hooks on each of their 8 legs and take in oxygen through their waxy shell like cover.

Scientists took 4 mites and let them breed in the best conditions for four months, when they finally looked they had 1217 mites.

Where they live

House dust mites evolved about 23 million years ago as nest dwellers, living in colonies scavenging on organic matter. Rats and birds nests were early homes for a mite colony.

As they evolved they developed a 'taste' for mouldy discarded skin scales. Scientists think the scales help make a 'perfume' to attract other mites, a necessary aid for a blind animal.

Modern man has become an indoor species and in doing so he has invited mites to come and live in his warm, humid, still and cluttered home.

One hundred years ago man's battle was with storage mites that spoiled grain stores, now it's the turn of the house dust mite that can threaten human health.

If the environment is right, various species of dust mites (called domestic mites) can live indoors in homes, barns and sheds living on items such as grains, hay, clothing, carpets or mattresses.

In an old mattress a doctor found both dead and alive mites, mould, fungi, pollens, bacteria, and discarded rotting skin scales.

By covering a mattress with micro-porous material, mites are too big to enter the mattress, but if left alone they can colonise on the top of the covers in old dust. Micro-porous covers should be damp dusted frequently to prevent mite colonisation.

A clinical study took a colony of house dust mites, coloured them red and let them go in a home. Later one was found in the family car - most travelling mites don't survive to establish a colony!

When on the look out for dust mite nest sites, don't forget the blanket in your pet's basket. Dogs and cats can also suffer allergy to mites.

Control mite infestation by reducing indoor humidity to less than 51% relative humidity (RH). Below 51% RH is considered hostile to mites but comfortable for man. Below 40% RH can be 'too dry' for human comfort. Excess humidity can be controlled with dehumidifiers.

Why mites cause allergy

Allergens (triggers) from dust mites can come from their body parts, saliva, or their droppings.

Allergic diseases from exposure to dust mites can include asthma, rhinitis (hayfever), conjunctivitis, sinusitis, atopic dermatitis (eczema) or any combination of these.

Three different species of house dust mites and three species of storage mites cause most of the allergy problems worldwide.

Storage mites and house dust mites are different animals with similar but different allergens. If you're allergic to mites, make sure you know which one to avoid.

Mite droppings are wrapped in a film that dissolves on contact with moisture. Once dissolved they release their contents of active enzymes, scraps of food, and living fungi.

Of the 22 known allergens from dust mites, seven are active digestive enzymes and several are classified as unknown.

One of the major allergens is an enzyme similar to a product used in the food industry to tenderise meat. Active mite enzymes enter the body by melting the 'glue' that binds delicate cells together. For most people a 'repair' is quick, but for those allergic to mites a reaction may occur.

Disturbed by movement or cleaning, dust mite droppings and allergens can be pushed into the air and drift on air currents for a short period of time. Begin house dust mite avoidance measures with your bed first or wherever you rest for long periods of time.

Allergen levels found on floors in homes with or without carpeting have been shown to be equal, implying that the allergens can be from various nest sites in or around the home.

There are six simple ways to kill dust mites; boil them, spray them (ie. benzyl benzoate), freeze them, exclude them, dry them out or microwave infested items on high for five minutes. Drying is best because house dust mites are up to 75% water in weight. Killing mites is not the same as allergen avoidance. A dead mite will leave behind allergens which must be washed away or avoided.