dust mite allergy

House dust mites are eight-legged creatures that are part of the Arachnid family — relatives of spiders, chiggers and ticks. The mites that commonly cause allergies are of the Pynoglyphoid genera, especially the species Dermatophgoides of pteronyssinus, Dermatophygoides farinae and Blomia tropcaclis.

House dust mites in our homes thrive on human and other animal skin dropping, including feathers, and even flour! In fact, these mites also love a nice serving of mould as part of their diet as well.

Mites thrive on surfaces that provide a reservoir of skin cells and other edible material. This includes bedding, carpets, stuffed toys and soft furnishings such as couches and chairs.

What part of the dust mite causes allergies?

The house dust mite is not the main problem in itself. It is doesn't bite or transmit disease. What people become allergic to are the proteins in the mites and their faeces. And with as many as 18,875 mites counted in a gram of dust (the weight of a paper clip), producing up to 20 faecal pellets each day, the term 'the air is thick' takes on a whole new meaning.

As dust mites are totally blind, they have evolved a system to decide whether particles they have just come across are food or waste that they have come across are food or waste that they gave already extracted all nutrients from. After they have ingested a parcel of skin dander, they envelope it with a wrapping of their own digestive proteins. These enzymes extract all the nutrients they need before discarding the wrapped package as faecal pellets.

If the mites then come across these pellets later, they can recognise the chemical signature of mite droppings and avoid ingesting them again. Unfortunately, allergic people can react to the enzymic proteins in these faecal pellet wrappers.

Old mattresses easily carry up to a million live dust mites. Given enough food supply, each mite can process and package four faecal pellets per day. With four million allergenic particles disturbed when people move around in bed, it is no wonder that dust mite allergy sufferers experience a worsening of symptoms at night time.

What are the symptoms of dust mite allergy?

Dust mite allergies are often characterised by 'perennial' or 'year-round' hayfever or allergic rhinitis: frequent sneezing, a runny, stuffy, itchy nose and irritated eyes. Asthma and eczema can also be triggered by a dust mite allergy. Symptoms often worsen when the individual comes in close proximity to disturbed dust, for example cleaning out closets or shelves, lying on the carpet, or getting into bed at night.

In New Zealand, mite numbers appear to peak in March and April, and allergens persist at high levels through to July. The lowest allergen levels are in September and October, but cold weather doesn't necessarily mean the end of allergy. That's because the mite faecal particles remain in the home, mixed in with dead and disintegrating mite bodies, which also cause allergies. Symptoms may also lessen in the summer because more time is spent outdoors and heaters are turned off.

Diagnosing a dust mite allergy

Skin prick testing or Rast test, along with a detailed symptom history, is the best way to test for house dust mite allergy.

Combating the house dust mite

House dust mites cannot drink. They obtain water partly from their food but the bulk of water need is obtained by a mechanism of hydrophilic (water-loving) crystals in their armpits. These finely structured crystals can obtain enough water for the mite when the relative humidity in the air is above 60 per cent. The mite must tone down its activity if the relative humidity drops below this figure.

In Auckland, the relative humidity is above 80 per cent year around, which explains the very high levels of dust mite counts in this city environment. The hydrophilic crystals also protect the dust mite from dehydration when conditions are dry. Crystals can be damaged (eg by being frozen). Such damage increases the rate of desiccation that can hasten the death of the mite when exposed to sunlight. Persistent heat above 60 degrees C also kills the mite and its eggs.

The best treatment for dust mite allergy is avoidance of the dust mite and its faeces. The following measures can be used to control house dust mite populations:

• Reduce their food supply. Have a bath or shower before going to bed. Do not eat on the bed or soft furnishings where house dust mites abound. Do not keep animals in bedrooms.
• Use dust mite covers on your bedding. These are a very efficient way of keeping the dust mite inside mattresses and stopping the food supply from entering into mite colonies. Plastic sheets are good only when they are brand new.
• Remove wall-to wall carpeting. Fixed carpets, especially those with thick underlay, provide excellent protection for the dust mites. Even the strongest vacuum cleaners cannot produce enough draft in the depth of the carpet piles to dislodge them. Rugs are the best way to go.
• Vacuum cleaners are efficient in removing easily dislodged dust mite particles. Be sure to have a fine enough exhaust filter (such as a HEPA filter), otherwise particles will just be redistributed throughout the room.
• Every two to three months, soft toys should be frozen in the freezer (this breaks down the hydrophilic crystals in the dust mite's armpits) and then subjected to sunlight for six hours or put in the dryer for half an hour.
• Boiling or heating materials to above 60 degrees C is efficient in killing the mites, as it prolonged sunning (although this depends on the strength of the sunlight and length of time exposed).
• Install a ventilation system in your home or use a dehumidifier to keep household moisture levels low
• Insecticide sprays turn out to be efficient only very temporarily and all marketed products also affect other insects that are predators or competitors of the ubiquitous mite. As the mite eggs survive and hatch, there is no longer adequate competition to hold their numbers down and the population may even expand.

This article is adapted from Auckland Paediatrician, Dr Allen Liang's address at the Allergy New Zealand AGM in 1999.