Wayne S. Rosenkrantz,     Tustin, California

Importance of House Dust Mites in Atopic Dermatitis

House dust mite (HDM) hypersensitivities are extremely common throughout the world. In a review of 469 cases of atopic dermatitis at the Animal Dermatology Clinics in southern California, dust mite allergens caused the most reactions on allergy testing (Dermatophagoides farinae, 55.2%; Dermatophagoides pteronyssinus, 39.7%). In other reports, positive skin test results or serum immunoglobulin E specific for HDM allergens have been as high as 6% to 90% in canine atopic dermatitis cases. House dust allergy is common even in clean homes and is a major cause of year-round allergy symptoms in humans as well as pets. House dust is a mixture of many mites and their body part allergens. The most common HDMs belong to members of the family Pyroglyphidae and include D. farinae, D. pteronyssinus, and Euroglyphus maynei. Other mites of importance include some mites that have been classified as storage, mold, or grain mites belonging to the family Acaridae (Acarus siro and Tyrophagus putrescentiae) and mites belonging to the family Glycyphagidae (Glycyphagus domesticus, Blomia tropicalis, and Lepidoglyphus destructor). Such mites have been found in canine dry dog foods, more commonly once they have been opened and stored for periods longer than 5 weeks (Brazis et al, 2008). However, some of these can be found in high levels outside food sources and also are considered environmental mites that could contribute to mite hypersensitivity reactions.

The concentration of dust mites in the home environment varies, depending on the part of the country or world in which one lives. Environmental factors such as moisture or humidity are very important in supporting and promoting dust mites in home environments. The most important antigen fractions come from D. farinae and D. pteronyssinus (Der f 1 and f 2, Der p 1 and p 2, and Der f 15, a high-molecular-weight antigen). It is possible that some mites may be more resistant to dryer or less humid environments. D. farinae is considered more resistant to dryer climates and even may be found in desert climates such as Las Vegas. In more humid or subtropical environments B. tropicalis may be found and in some cases may be the most prevalent. The HDM content varies from home to home as well as within areas of the home, depending on the type of furniture, building materials, and presence of pets. The highest allergen concentrations are found in pillows, mattresses, carpeting, and upholstered furniture. Levels also have been detected in automobiles and on the hair coats and bedding of dogs (Randall et al, 2005). There may be as many as 19,000 HDMs in 1 g of dust, but more typically 100 to 500 mites live in each gram of dust. Each mite produces about 10 to 20 waste fecal particles per day and lives for about 30 days. Egg-laying females can add 25 to 30 new mites to the population during their brief lifetime.

Isolated Allergens and Methods of Detection

Allergens are divided according to their chemical properties and allergen cross-reactivity; they have cross-reacting and species-specific epitopes. Antigens are named by mite species and group antigen designations (e.g., Der p 1, Der f). Group 1 antigens Der p 1 and Der f 1 are the most allergenic for humans; they are associated with or found in mite feces and probably represent intestinal enzymes cysteine protease. Group 2 Der p 2 and Der f 2 are more stable than group 1 antigens in the environment; they contain molecules associated with male reproductive tracts and have more species cross-reactivity. The other classified allergens include group 3, fecal trypsin; group 4, amylase; group 6, chymotrypsin; group 8, glutathione S-transferase; group 9, another serine protease (an important human allergen); group 10, tropomyosin; group 11, paramyosin; group 13, fatty acid–binding protein; group 15, chitinase; group 16, gelsolin; group 17, calcium-binding protein; and group 18, chitinase. There are other as yet unclassified or identified allergenic epitopes considered less important or minor allergens.

The current information regarding which antigens dogs react to indicates little reactivity to group 1 and 2 antigens. There are higher-molecular-weight allergens recognized by atopic dog sera (Nuttall et al, 2006). One of the high-molecular-weight antigens has been characterized as Der f 15, a digestive chitinase (McCall et al, 2001). Other high-molecular-weight antigens that may be important in the dog include another chitinase (Der f 18) and tropomyosin (Der f 10). These allergens may be very difficult to denature and destroy, which makes them more difficult to eliminate from the environment.

Currently, identifying HDM allergens in the environment or on the pet rarely is performed, and the association between mite levels and clinical disease has not been studied in the dog as it has in humans. Therefore when HDM sensitivity is detected, HDM control generally is recommended when feasible and especially when HDM is the only or primary allergen to which the dog reacts. When the owner or clinician wants to identify mites or mite allergen in the environment, the following techniques are available: mite counts per gram of dust, specific antigen assay (enzyme-linked immunosorbent assay [ELISA] or amplified ELISA), and measurement of mite fecal matter, with measurement of fecal guanine levels. Dust and mites usually are collected by vacuum techniques. Fecal material can be sampled with a semiquantitative measurement of guanine (fecal product) using a commercially available test kit (Acarex); less than 0.6 mg of guanine per gram of dust can be detected, and the result provides an estimate of dust mite fecal antigens. ELISA antigen tests using monoclonal antibody reagents for groups 1 and 2 Der p and Der f antigens are available. An in-home dust mite allergen test also is available to detect allergen levels in carpets, bedding, and upholstered furniture. The Aclotest Dust Mite Detection Kit uses a dipstick with polyclonal antibodies to detect HDM allergens and takes 30 to 60 minutes to perform. This test kit is sensitive to 0.5 µg per gram of dust and detects D. pteronyssinus and D. farinae allergens, including Der p 1 and Der f 1. More rapid in-home tests have been evaluated with lateral-flow technology. The author previously used a swipe test that allowed direct testing of carpets, upholstery, and bedding, but it is no longer available commercially (Polzius et al, 2002). Another lateral-flow device by Indoor Biotechnologies detects Dermatophagoides spp. group 2 allergens with a gold-labeled monoclonal antibody. This test uses a dust-sampling device that collects extracted dust from a defined area. The test is quick (10 minutes) and easy to use, and results have a strong correlation with results of ELISA testing (Tsay et al, 2002).

Although species specificity exists, there may be some cross-reactivity between HDM allergens and related allergens such as those from storage mites or even Sarcoptes scabiei or Otodectes. Results of any of these environmental tests need to be reviewed carefully and always interpreted in conjunction with clinical history and physical findings.