Discuss facts, fallacies of dust mite allergies

House dust mites are the most common allergen in people, dogs and cats. How did this get to be so common, and why is dust mite allergy such a problem? Here are some answers to your questions.

What is a dust mite?

First of all, house dust and dust mites are not the same thing. House dust is composed of human skin scales, bacteria, yeast, molds and other assorted components depending on your location. In horse and buggy days, women's long skirts brought lots of horse dander and horse dung into the home, so that was a main component of house dust at that time. Since the emergence of cars, horse dander or dung is no longer a prominent component of current house dust. Now nylon fibers and furniture foam are significant components. A recently published book, Dust Mites by Matthew Colloff who studied dust mites for 20 years, gives us a better understanding of these arachnids and, consequently, what can be done to help those with dust mite allergies.

Common house dust mite: A model of Dermatophagoides pteronyssinus. (Photo: Geoff Brightling/Getty Images)

Dust mites are in the same order as spiders and scorpions and have been found in fossils more than 28 million years old. The main species we deal with include Dermatophagoides farinae and Dermatophagoides pteronyssinus. The origin of the word "dermatophagoides" means "looks like something feeding on the skin." Dust mites actually do feed on exfoliated skin cells containing keratin, bacteria, fungal spores and viruses. Fungal spores provide minerals, vitamins and other nutrients for dust mites, as a diet composed of only keratin would be incomplete. In mattresses that contain dried semen, dust mites actually have an increased growth rate. Interestingly, dust mites prefer skin cells from atopic individuals and geriatrics because they have lower levels of skin lipids than those without atopy.

What environments do they thrive in?

Temperature and humidity are most important for dust mite growth and reproduction. A food source of atopic skin cells will not cause a growth in dust mites without the right temperature and humidity. Without the proper humidity to maintain body water content, dust mites are unable to feed. Textiles such as carpets, fabrics, cloth toys and mattresses are the best areas for dust mites to flourish. Exfoliated skin scales shed on a hard surface become too dry and unattractive to dust mites. Materials that are insulative such as textiles, animal hides or vegetation such as straw are good habitats. The fungal hyphae contained in hay, straw and forage provide an additional food source for the mites.

Photo 1: Eosinophilic granuloma complex lesions on the ventral abdomen of a cat allergic to house dust mites. (Photos: Courtesy of Dr. Alice Jeromin)

Since each home is different as far as textiles, temperature and humidity content, dust mite numbers vary from house to house. The more people in the home, the higher the humidity, which contributes to an increased mite population. A dust sample from carpet usually yields more than five species of mite dwellers, ranging from dust mites to storage mites such as Tyrophagus putrescentiae. Tyrophagus putrescentiae like cereals, grains and cheese as well as products with an oil content such as peanuts.

Some wonder how long it takes for dust mites to occupy a newly built home — mite populations are usually established within one year of occupation. Cultures of dust mites released onto a sofa migrate through clothing to other parts of the home and the family car over a l0-day period. The highest concentrations are found in carpeted basements, human bedding and dog beds. Older mattresses and carpets have higher mite populations.

Photo 2: Chronic facial skin lesions in a dog with dust mite allergies.

How can they be controlled?

Various products claim to help eliminate dust mites. In fact, many acaracides have been field tested in clinical trials, including pyrethroids, organochlorides, benzyl benzoate, thiabendazole, triclosan, silver and tributyltin oxide. Most have been unsuccessful. For example, permethrin-treated carpets showed no success in reducing dust mite numbers. With benzyl benzoate, the exact acaracide mechanism of action is unknown but is thought to be a gut poison to dust mites. It works via contact and lethal ingestion by the mites. Laboratory trials show benzyl benzoate to be highly effective, but field trials have been disappointing. The chemical was unable to penetrate fabrics at the manufacturer's suggested dose and application time yet showed some success when applied at a higher concentration and left down longer (four hours vs. 12 hours).

Other treatments such as dehumidifiers reduced the humidity of room air, but the upper layers of the carpet and mattress still had the required humidity needed by dust mites to feed and reproduce. Steam cleaning has been found to be helpful, but ultraviolet light treatments have not been studied as of yet.

So what are some simple proven methods of controlling dust mites in the home? Washing bedding, cloth toys and fabrics at least once weekly in hot water (> 50 C) or cold water adding tea tree oil or benzyl benzoate is helpful. Vacuuming picks up dead mites because the live dust mites actually attach and hold onto fabrics. As an adjunct to other methods, vacuuming using a HEPA filter can be helpful, but without a HEPA filter, it can actually make matters worse. Mattress and pillow covers serve as barriers and along with the above suggestions, can be helpful. Airing bedding out in the hot sun or cold weather for a good l2 hours has also shown to reduce dust mite numbers. Replacing dog beds at least every six months and washing them along with cloth toys as per the above instructions at least weekly is a good idea.

Ultimately, treatment of dust mite allergy with immunotherapy, cyclosporine or antihistamines provides the most relief for our allergic patients. Discussing the facts and fallacies of environmental treatment with clients can save them time and money by avoiding unsuccessful methods.

Dr. Jeromin is a pharmacist and veterinary dermatologist in private practice in Cleveland, Ohio. She is a 1989 graduate of The Ohio State University College of Veterinary Medicine and an adjunct professor at Case Western Reserve University's College of Medicine in Cleveland.

References

For more information: Colloff M. Dust mites. Csiro Publishing, Australia, 2009.