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