Dust mites cause common intradermal skin test reaction in dogs

For those nonseasonal pruritic patients where food elimination trials haven't been helpful and steroids seem to help or those patients with nonseasonal recurrent otitis or pyoderma, an allergy to house dust mites or food storage mites should be considered. Dust mites are the leading cause of allergy/asthma in humans. In dogs, it appears to exceed flea allergy as the most common intradermal skin test reaction. An estimated 30-80 percent of atopic dogs and cats skin test positive to dust mites (Photo 1). Of the two types of dust mites, Dermatophagoides farinae and Dermatophagoides pteronyssinus, cats tend to be allergic to both species whereas dogs in the United States tend to be D. farinae allergic (in the United Kingdom, D. pteronyssinus tends to be the more common allergen). What do we do with these dust mite patients to make them more comfortable? Did we as humans bring this about by domesticating our pets? The answer is probably, yes, and we need to address what we can do about it.

Photo 1: Chronic skin changes from facial pruritus due to dust mite allergy.

First, a bit of information about dust mites. Developing an understanding about them will allow us to educate our clients so they too can help their pets. Dust mites are found around the world and became a problem to humans as cave dwellers (Photo 2, p. 2S). Caves harbored birds and that is possibly where the first exposure took place. Dust mites are acarids and belong to the same order as Sarcoptes, Otodectes, Cheyletiella and food storage mites. They may cross react on skin testing with one another i.e. food storage mites can cross react with dust mites, dust mites can cross react with Sarcoptes. In other words, if a patient with scabies is skin or blood tested for allergy, there is a good chance he will test false positive to dust mite.

History lesson

Dust mites are present year round but particularly favor August to November when the houses are closed up because of cool evenings and increasing humidity. It is interesting that many of us see an increase in pruritus in our allergic patients during this time of year. The mites feed on epidermal debris, yeast, molds, food remnants and their own dead bodies. Thankfully because of increasing human allergy to these mites (and food storage mites), more is becoming known about dust mites in general. They have translucent "skin" so they favor darkness. They emerge especially at night and swarm around warm bodies, which provide an increased humidity and temperature. Dust mites prefer mattresses, upholstery, carpeting (particularly the carpet backing) and fabrics. You may never feel the same again about your bedding after reading this! Whereas people feel adequate treatment for dust mites includes frequent vacuuming and "having the furnace ducts cleaned", nothing could be less true! The allergen we or our pets react to with dust mites is present in their bodies and excrement so it is not only important to kill the dust mites but to denature the allergens. And since they live in fabrics, having the ducts cleaned will only reduce the house dust content of the home. When speaking of our pets, attention must be paid to where they sleep, cloth toys, and where they spend a great deal of their time in the home. In one study, the highest levels of dust mite measured in homes with pets included the pet's bed and finished basements. Knowing which areas of the home harbor the most dust mites helps when it comes to treating the environment for mites. The car with cloth upholstery is another area that should not be forgotten. Linoleum, wood floors and leather furniture do not appear to harbor many mites.

Uninvited guest

Photo 2: Electronic micrograph of a dust mite.

The diagnosis of dust mite allergy is achieved by first determining that the patient is atopic by obtaining a good history i.e. age of onset 6 months to 3 years old, predisposed breed, facial/foot involvement, recurrent otitis/pyoderma, no response to food elimination trial and ruling out ectoparasite infestations. In cats, the symptoms can vary from eosinophilic granuloma complex lesions to miliary dermatitis to excessive grooming to chin acne or a waxy otitis (Photo 3). With dust mite allergy, patients are usually nonseasonal with possibly a flareup during the end of summer/fall particularly here in the Midwest. There is usually a positive response to steroids but since the allergy is nonseasonal, year round steroids are a dangerous way to treat. Once it has been determined that the patient is atopic, the next step is skin or blood testing to confirm the dust mite allergy.

Diagnosis

Once the diagnosis of dust mite allergy has been made, options for treatment include environmental treatment for dust mites, immunotherapy or a combination of fatty acids and antihistamines where effective. Avoidance of the allergen is usually not practical, as our pets tend to sleep with us and be part of the household. Interestingly we had one patient that was an outside dog for five years that was brought to live inside and in the course of two years, became dust mite allergic. Since we now have a dust mite treatment protocol for the environment in the form of an odorless spray (the former treatment was a carpet powder with a fragrance that allergic humans had problems with) we recommend environmental treatment initially. It can be successful 70 percent of the time in a course of two to four weeks. Retreatment is usually necessary every four to six months. The treatment consists of two sprays, one kills the dust mite, the other denatures the allergen. Mattresses, comforters, carpets and upholstered furniture need to be treated as well as automobile interiors and cloth toys. Dog beds should be treated and perhaps replaced every six months. Aveho Biosciences manufactures a borate DustMite and flea control as well as Allerase Anti-Allergen spray. They also offer an allergen detection system (Mite-T-Fast) to determine whether unhealthy levels of dust mites exist. More information is available for both veterinarians and client education on the company's Web site (www.avehobiosciences.com). Where environmental treatment is not effective, other options include antihistamines/fatty acids, immunotherapy injections for the dust mite allergy, or oral cyclosporine 5mg/kg daily.

Treatment

If not much is known about dust mites, even less is known about food storage mites. Storage mites include members of Lepidoglyphus, Tyrophagus, Acarus and Glycyphagus species. Currently in the United States Tyrophagus putrescentiae is being tested for via skin or blood testing in the dog and cat. Food storage mites are present in dry foods, cereals, grains, straw and cheese. Like dust mites, they can be responsible for nonseasonal symptoms including pruritus, erythema and recurrent otitis. Storage mites may possibly be responsible for other problems such as upper respiratory problems or even discoid lupus. They are well known in humans to be a cause of asthma and allergic rhinitis. One study in humans finds storage mites to have overtaken dust mites as a leading source of allergy. It will be interesting to see if studies show the presence of dust mites in dry pet foods/snacks and if so, variability between brands and formulations. Treatment for food storage mite allergy includes changing to a canned or cooked diet without dry grains, cereals, or cheese for four to six weeks and environmental treatment. Immunotherapy is controversial and personally I have found my patients do better on the diet change. Since food storage mites and dust mites can cross react, if a patient is found to be positive to both, I generally have the owner perform the diet change first before starting environmental treatment for dust mite or immunotherapy.

Food storage mites

Photo 3: Eosinophilic granuloma complex lesions in a cat with dust mite allergy.

It appears that dust mites and food storage mites overpower us in their numbers and will be here long after we're gone. Unfortunately, it doesn't take but a small amount to elicit allergic reactions in us and our pets. Hopefully with more knowledge of these mites, their lifestyle, and ways to eradicate them (or cohabitate with them!) we can make our allergic patients more comfortable.