Atopic dermatitis (AD) is a genetically predisposed pruritic inflammatory allergic skin disease that is common among humans. The incidence of AD has increased rapidly worldwide during the past 30 years [19]. As previously described [15], this condition is also common in dogs and recognized as a multifactorial disease involving IgE-mediated allergic reactions to various environmental allergens, alterations of skin barrier function, microbial colonization, and abnormal cutaneous immunity. In general, canine AD is diagnosed based on medical history, physical examination, exclusion of other diseases that cause similar clinical signs, and allergy test results [11]. Among these, the intradermal skin test (IDST) is considered the gold standard. However, it seems that IDST for dogs and cats is not practical for general practitioners. Allergen-specific IgE serology is now widely used in clinical practice [4]. In general, allergen-specific IgE serology appears to produce false positive results (low specificity) and cannot be used to confirm a diagnosis, but offers useful information about avoidance allergens and the inclusion of allergens for allergen-specific immunotherapy [4,15]. Causative allergens vary according to geographical region, climate, environmental pollution/hygiene, and residential environments [7,19]. Allergen-specific IgE serology is also required for large-scale investigations of canine AD. The purpose of the current study was to measure the sensitization rates of environmental allergens in dogs with AD using an allergen-specific IgE serology test and identify the causative allergens.

Forty-six out of 101 dogs (45.5%) were males, seven (15.2%) of which were castrated. Fifty-five of the 101 dogs (54.5%) were females, four (7.3%) of which were spayed. The ratio of male : female animals was 1 : 1.2. There were 16 different breeds included in the study: mixed (n = 13), cocker spaniel (n = 15), Yorkshire terrier (n = 13), Shih Tzu (n = 13), Maltese (n = 12), Pekingese (n = 4), miniature pinscher (n = 4), beagle (n = 4), Dachshund (n = 3), miniature Schnauzer (n = 3), pug, Cavalier King Charles spaniel, Jindo dog, Chihuahua, and Bichon Firse (one each). Animal age at presentation ranged from 5 months to 13 years (median, 3 years) and most dogs were less than 5 years old (77.9%). The dogs developed clinical signs and skin lesions between 6 months and 2 years, and duration of the clinical signs prior to diagnosis ranged from 2 weeks to 8 years.

Identify causative allergens associated with AD has been a very important issue in veterinary dermatology because this condition is the most common cause of pruritus in dogs [25]. Many studies have been conducted to assess the value of allergen-specific IgE serology in cases of canine AD and canine food allergic dermatitis [4,5,12,23]. Interpretation of the test results should be performed cautiously and only used once a clinical diagnosis of AD has been made because positive test results are not specific for canine AD [11]. Traditionally, house dust mites and house dust have been considered the most important allergens in humans and dogs [7,19]. In canines, human dander, feathers, kapok, molds, weeds, grasses, and trees are also important allergens [25].

In the present study, 101 dogs with AD were tested with 92 allergens (70 environmental allergens and 22 food allergens). Among the environmental allergens, the sensitization rates for house dust and house dust mites were high; these are considered important allergens in veterinary and human medicine. The high sensitization rate (61.4%) for house dust mites was similar to ones previously reported in studies performed in the same geographic location [7,26,28]. However, the sensitization rate for house dust in the present study (55.2%) was higher than those observed in previous investigations. The sensitization rate for house dust was just 5.3% among 35 dogs in 2002 [28], and 17.5% among 22 dogs in 2005 [26].

In addition, reactions to airborne pollen (grasses, weeds, and trees) and molds were relatively common in this study although pollen is not usually considered an important allergen. Grasses produced negative results in three previous studies [7,26,28]. Sensitization to weeds was not observed and the rate for trees was just 5.3% among 22 dogs in 2005 [26]; few or no allergens in these groups caused positive reactions in 2002 [28]. The most recent study results obtained in 2010 [7] showed that Rhizopus had a higher positive rate (49.1%) compared to the one observed in the current study (15.8%), which was the lowest sensitization rate among molds. According to other previous reports, sensitivity of the allergen-specific IgE serology test to molds was lower than the IDST, and serology test results are less reliable for detecting molds hypersensitivities [5,14].

Flea and insect groups produced relatively weak positive reactions in this study. Flea populations tend to grow in warm temperatures and high humidity, and these insects are very common allergens of canines in the world [25]. However, flea allergies were rare in the present study because most of the evaluated dogs lived primarily indoors. The results of our study also suggested that the sensitized rate was low. On the other hand, mosquitos are very common in Korea, and it was recently observed that these insects have an increasingly close relationship with AD in humans and dogs [7]. However, the sensitization rate for mosquitos was only 9.4% in our investigation.

Differences among the causative allergens when comparing studies might be due to several reasons. Direct comparison between IDST and allergen?specific IgE serology test results are impossible because the two techniques detect IgE from different sources such as skin and serum [5,25]. Discrepancies between IDST and serology data may be explained by variations in testing methodologies, allergen extracts, and sensitivity [3,5,10,22]. In previous studies of causative allergens of AD dogs in Korea, an IDST was performed. Thus, complete correlations between results from those publications with ones from the current study cannot be identified.

As previously described [19], geographic and climate conditions are closely associated with environmental allergens. In particular, pollens and molds are affected by temperature and humidity [1]. Not surprisingly, these factors represent recently emerging allergens that cause allergenic disease in humans [17]. Positive responses to environmental allergens identified in the present study can be related to increased average temperatures and CO₂ concentrations in Korea due to global warming and rapid urbanization. Previous studies conducted in our country were performed during different years (2002, 2005, and 2010), and the general sensitization rates of allergens might be increased overall 10 years. In addition, previous investigations included a relatively small number of dogs and were conducted in different regions of Korea [7,26,28].

Secondary infection associated with Malassezia or staphylococci is quite frequent, and recent studies have focused on the relationship of the associated antigens with allergies in dogs [15,16]. Antibodies to microorganisms in AD dogs had never been reported in Korea. In the present investigation, the sensitization rates for Staphyloccus and Malassezia were 19.8% and 21.9%, respectively. Results from previous studies revealed that AD dogs produce higher levels of IgE to these microorgainisms. [2,16]. These data indicate an immunopathogenic role of the microorganisms; however, the exact pathogenesis and clinical significance are uncertain. In humans, Staphylococcus aureus plays an important role in AD development and staphylococcal superantigens have been implicated in AD pathogenesis [9]. Thus, secondary infection by Malassezia or Staphyloccus should be monitored and promptly identified in AD dogs.

Animals with food allergic dermatitis were excluded from our using an elimination diet. However, AD dogs are believed to be sensitive to various allergens, and cross-reactivity of airborne and food allergens has been detected in humans [8]. Among food allergens, chicken, turkey, brown rice, brewer's yeast, and soybean are considered common. These allergens are frequently used ingredients for commercial dog food that most dogs in this study were mainly fed. The meaningful of food-specific IgE test is still controversial [13], but which also can be used for limited-antigen dietary trials [6]. Based on the results from the present study, chicken, turkey, brown rice, brewer's yeast, and soybeans can be included in avoidance diets. Rabbit, venison, duck, and tuna can also be used for dietary trials of canine AD.

The clinical history, characteristic signs, and several diagnostic criteria [18,21,27] were used for diagnosing canine AD in the current investigation. The sensitization rates for house dust and house dust mites were high, while allergies against airborne pollen and molds were relatively common in our study. The sensitization rates for Malassezia or staphylococci were approximately 20%. Reducing the allergen load and exposure as well as promptly identifying and treating secondary infections may be helpful for controlling the symptoms of AD dogs. There are several reports indicating that the SAT has high specificity and relatively low sensitivity [4,15]. Thus, some false positive reactions might have been obtained in our study. However, both the IDST and SAT are mostly used for allergen-specific immunotherapy to treat AD [11], and have been found to have equal efficacy for identifying allergens as a part of immunotherapy regimens for AD dogs [20,24].

In summary, we measured the sensitization rates for selected allergens in AD dogs using allergen-specific IgE serology. This SAT is easy to perform, less stressful to the patients, has a low cost compared to the IDST, and produces fewer false negative results. When accounting for the patient's history, clinical signs, and exposure to allergens and eliminating other causes of pruritus, allergen-specific IgE serology might be valuable for controlling canine AD.