FC-53 Serologic allergy testing in horses with insect hypersensitivity

The usefulness of the intradermal test (IDT) and the serological allergy test (SAT) for detecting antigen-specific IgE in allergic cats has not yet been established. In this study, we compared the results of IDT with those of SAT and evaluated the clinical usefulness of the two tests for detecting possible allergens in allergic cats. IDT and SAT using eight antigens were performed on 22 cats with intense pruritus after excluding ectoparasites and performing diet elimination tests. Approximately 50% of the cats reacted to at least one allergen by either IDT or SAT, and 36.4% of the cats reacted on both IDT and SAT. In contrast, seven healthy cats did not show any reactions on IDT or SAT. The most commonly detected allergen in both tests was house dust mites (IDT, 36.4%; SAT, 40.9%). Five cats reacted to one allergen and the others reacted to more than one allergen with IDT. Three cats reacted to one allergen with SAT. The following percentage agreement between the results of the two tests was calculated: house dust mites (86.4%), cat fleas (63.6%), grass mix (86.4%), common mugwort (81.8%), cat epithelia (90.9%), ragweed (86.4%), Japanese cedar (90.9%), and plantain (81.8%). The overall mean percentage agreement was 83.5%. In summary, the present study showed good agreement between IDT and SAT for cats, and SAT may be more sensitive than IDT, but less specific for detecting sensitized allergens.
Funding: Self-funded.     

FC‐51 Comparison of intradermal test and antigen‐specific IgE test in 22 cases of feline allergic dermatitis

The usefulness of the intradermal test (IDT) and the serological allergy test (SAT) for detecting antigen‐specific IgE in allergic cats has not yet been established. In this study, we compared the results of IDT with those of SAT and evaluated the clinical usefulness of the two tests for detecting possible allergens in allergic cats. IDT and SAT using eight antigens were performed on 22 cats with intense pruritus after excluding ectoparasites and performing diet elimination tests. Approximately 50% of the cats reacted to at least one allergen by either IDT or SAT, and 36.4% of the cats reacted on both IDT and SAT. In contrast, seven healthy cats did not show any reactions on IDT or SAT. The most commonly detected allergen in both tests was house dust mites (IDT, 36.4%; SAT, 40.9%). Five cats reacted to one allergen and the others reacted to more than one allergen with IDT. Three cats reacted to one allergen with SAT. The following percentage agreement between the results of the two tests was calculated: house dust mites (86.4%), cat fleas (63.6%), grass mix (86.4%), common mugwort (81.8%), cat epithelia (90.9%), ragweed (86.4%), Japanese cedar (90.9%), and plantain (81.8%). The overall mean percentage agreement was 83.5%. In summary, the present study showed good agreement between IDT and SAT for cats, and SAT may be more sensitive than IDT, but less specific for detecting sensitized allergens. Funding: Self‐funded.     

Serum allergen‐specific immunoglobulin E in atopic and healthy cats: comparison of a rapid screening immunoassay and complete‐panel analysis

Feline and canine atopic dermatitis are thought to have a similar immunopathogenesis. As with dogs, detection of allergen‐specific IgE in cat serum merely supports a diagnosis of feline atopy based on compatible history, clinical signs and elimination of other pruritic dermatoses. In this study, a rapid screening immunoassay (Allercept^® E‐Screen 2nd Generation; Heska AG, Fribourg, Switzerland; ES2G) was compared with a complete‐panel serum allergen‐specific IgE assay (Allercept^®; Heska AG; CP) in healthy cats with no history of skin disease and in atopic cats. The latter had no diagnosis of external parasitism, infection, food hypersensitivity or other skin disease explaining their pruritus, and expressed cutaneous reaction patterns typically associated with feline allergic skin disease (head, neck or pinnal pruritus, miliary dermatitis, self‐induced alopecia, eosinophilic granuloma complex). The proportion of cats positive on either the ES2G or the CP assays was not significantly different between the atopic and healthy cat groups. There was, however, strong agreement between the results of the ES2G and CP assay; overall, the two tests were in agreement for 43 of 49 (88%) serum samples. There was also strong agreement when individual allergen groups were evaluated (agreement noted: indoor, 41 of 49 samples; grasses/weeds, 37 of 49 samples; and trees, 41 of 49 samples). These results indicate that although neither test is diagnostic for feline atopic dermatitis, the screening assay is beneficial for predicting the results of a complete‐panel serum allergen‐specific IgE assay in cats.     

Comparison of intradermal and serum testing for allergen-specific IgE using a Fcepsilon RIalpha-based assay in atopic dogs in the UK

Atopic dermatitis in dogs is a common allergic skin disease that affects substantial numbers of dogs in the UK. The purpose of this study was to compare the results of an intradermal test (IDT) and an in vitro test in a large cohort of dogs. Dogs were intradermal tested with Greer allergens (Greer Labs Inc, Lenoir, NC, USA) using standard techniques. At the same time blood samples were drawn and submitted for evaluation by ELISA using the ALLERCEPT Definitive Allergen Panels for allergen-specific IgE, a commercial assay that uses a biotinylated recombinant extracellular domain of the high affinity Fc-epsilon receptor alpha chain protein (Fcepsilon RIalpha). The allergens used in the two tests included grass, tree and weed pollens, moulds, flea saliva/whole flea extract and house dust mite species. The optical density readings from the ELISA for each allergen were compared with the results of the IDT for 265 dogs. The prevalence of positive reactions in the ELISA was equal to or greater than the results of the IDT in the case of almost all of the allergens, but two notable exceptions were the house dust mites Dermatophagoides farinae and Dermatophagoides pteronyssinus. These two allergens were the most common positive reactions by IDT (prevalence D. farinae 78.9%, D. pteronyssinus 66.4%). The results of the two tests were significantly different (McNemar's test, P<0.05) for 16 of the 22 allergens. The sensitivities of the ELISA compared to the IDT (where there were more than 3 dogs with positive reactions in both tests) varied between 19.3 and 77.1% (D. pteronyssinus 19.3% and D. farinae 67.9%) and the specificities varied between 64.2 and 96.6% (D. pteronyssinus 96.6% and D. farinae 89.3%).     

A comparison of intradermal testing and detection of allergen-specific immunoglobulin E in serum by enzyme-linked immunosorbent assay in horses affected with skin hypersensitivity

Skin hypersensitivities (allergies) in horses are often diagnosed using clinical signs only. Intradermal testing or serological assays are diagnostic options to confirm the allergic nature of the disease and to identify the allergen(s). Our objective was to develop an allergen-specific enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody specific for horse IgE and to examine its potential for allergen detection in serum in comparison to intradermal testing. Intradermal testing with 61 allergen extracts was performed on 10 horses affected with skin hypersensitivity. Their sera were analyzed by ELISA for IgE antibodies to the same allergens. The kappa test of concordance was used for comparison of the results of both tests. Out of 61 allergen extracts, only two (Timothy and Quack) had kappa values greater than 0.60, suggesting a substantial agreement between skin testing and IgE ELISA. The statistical comparison of the remaining 59 allergens showed little or no concordance between the tests beyond chance. To identify parameters that may influence the sensitivity of the ELISA, the assay was modified to detect allergen-specific IgGb and IgG(T) in serum, and the protein content in all allergen extracts was determined by SDS-PAGE. The commercial allergen extracts revealed a high variation in detectable protein. High concentrations of allergen-specific IgG in horse serum were found to compete with IgE for binding to the plates. In conclusion, an ELISA using whole serum and crude allergen preparations provides limited diagnostic information in horses. The reliable diagnosis of allergens in equine skin hypersensitivity is essential to improve allergen-specific treatments, such as hyposensitization, or the development of allergy vaccines.     

Comparison of cellular and humoral immunoassays for the assessment of summer eczema in horses

The objective of this study was to compare and analyze three common diagnostic methods for summer eczema (SE) in horses, an allergic dermatitis caused by bites of Culicoides spp. Nine horses with a medical history of SE and nine control animals were intradermally challenged with whole body extracts (WBE) and the saliva of a native (C. nubeculosus) and exotic (C. sonorensis) Culicoides species. Blood and serum samples of the horses were examined for basophil reactivity by a histamine release test (HRT) and for Culicoides-specific serum immunoglobulin E (IgE) and G (IgG) by enzyme-linked immunosorbent assay (ELISA). The results of intradermal testing (IDT) at 30min (immediate reactivity) and 4h (late-phase reactivity) post challenge with most insect preparations revealed significant differences between horses with and without SE. Overall, the HRT showed the most accurate results with a sensitivity of 1.00 for all Culicoides preparations and specificities of 0.78 (WBE) and 1.00 (saliva). By contrast, delayed reactions of the IDT (24h), and levels of Culicoides-specific IgE and IgG in the native serum showed little or no distinction between allergic and non-allergic horses. However, the use of purified serum IgE and IgG indicated the possibility for elevated titers of insect-specific serum immunoglobulins in horses with SE. The IDT and HRT did not reveal obvious differences in onset and intensity of positive reactions for the native verses exotic Culicoides species, whereas the ELISA showed slightly higher numbers of positive reactions for serum IgG with the indigenous species. Saliva, as compared to WBE, was found to have improved sensitivity and/or specificity for the HRT and for the late-phase immune reactions as measured by the IDT. Overall, the results indicate that allergy tests utilizing effector cells (mast cells, basophils) are more accurate in diagnosing SE in horses than serological analysis by ELISA.     

Canine and feline atopic dermatitis: A review of the diagnostic options

Atopic dermatitis is an inherited pruritic skin disease in dogs and cats. This pruritic skin condition is due to the animal having an allergic reaction to environmental allergens. The environmental allergens that an individual dog or cat is allergic to are specific for that individual animal. Management options for affected dogs and cats include identification of the offending environmental allergens and subsequent avoidance of that allergen, or allergen-specific immunotherapy. Several diagnostic tests are available to veterinarians to try to identify these allergens. The pros and cons of each of these diagnostic tests will be addressed.     

Intradermal and serological testing for mites in healthy beagle dogs

Background – Intradermal testing (IDT) is widely used in veterinary medicine to select allergens for immunotherapy. The recommended concentration for mites is 250 protein nitrogen units (PNU)/mL. It is not known whether healthy dogs responding to this concentration have asymptomatic sensitization or irritation. Furthermore, interbatch and intersupplier variability of allergens has not been fully addressed. Hypothesis/Objectives – The incidence of positive IDTs in healthy beagles was recorded and the value of combining these results with serology to differentiate between asymptomatic sensitization and irritancy evaluated. Additionally, the interbatch and intersupplier variability of allergens was assessed. Animals – Seventeen healthy laboratory beagles with no history or clinical signs of canine atopic dermatitis were used. Methods – Intradermal tests were performed with four mite allergens from two suppliers (varying batches). An initial IDT at 250 PNU/mL was used to determine whether decreasing or increasing test concentrations were used in the subsequent titration IDTs. Additionally, two IgE ELISA tests from different manufacturers were performed. Results – Seven of 17 dogs showed IDT reactions at 250 PNU/mL. There were highly significant allergen interbatch and significant intersupplier correlations and agreement. The associations between the IDT reactions and the IgE serologies statistically identified two groups of dogs: one with positive serology and IDT reactions at 250 PNU/mL; and another with negative serology and IDT reactions. Conclusions and clinical importance – Our results suggest that dogs that have IDT reactions and positive serology are asymptomatically sensitized, while dogs that react at higher allergen concentrations, but have negative serology, do so as a result of irritant reactions.     

IgE ELISA using antisera derived from epsilon chain antigenic peptides detects allergen-specific IgE in allergic horses

Equine disease with an allergic etiology is common. Environmental antigens most often implicated as allergens in horses include molds, dusty hay, grass pollen, hay dust mites, and insect saliva. Although intradermal testing with allergen is a useful diagnostic tool for some species, skin testing frequently produces false positive results in horses. Allergen deprivation as a diagnostic tool is often impossible and at best it is ineffective at diagnosing the specific allergic reactivity. Synthesis of IgE after exposure to allergen is the instigator of the allergic process. While IgE exerts its effect after binding strongly to mast cell Fc receptors, the presence of free IgE in the serum can be used to quantify and determine the allergen specificity of the allergic disease. A lack of widely available reagents for detection of equine IgE has limited this approach in horses. We have used the nucleotide sequence of equine IgE to prepare a peptide-based immunogen to elicit equine epsilon chain-specific antisera. Selection of peptides was based on antigenic attributes of the deduced amino acid sequence of the equine epsilon chain. Six peptides were selected for conjugation to carrier molecules and rabbit immunization. Of these, one peptide elicited antisera that was successfully used in enzyme linked immunosorbant assay (ELISA) to screen horse serum from 64 allergic horses for allergen-specific IgE. Twenty-four of the 64 horses showed positive reactivity to one or more of the following allergens: grass, grain mill dust, mosquito, and horsefly. This study demonstrates the usefulness of peptide-based immunogens for development of antisera to rare or difficult to purify antigens such as IgE. Resultant antisera has great usefulness in diagnostic assays for equine allergy and as a research tool.     

Measurement of serum immunoglobulin E (IgE) specific for house dust mite antigens in normal cats and cats with allergic skin disease

The purpose of this study was to determine whether cats with allergic skin disease have significant concentrations of serum Immunoglobulin E (IgE) specific for antigens derived from the house dust mites (HDM) Dermatophagoides farinae (DF) and Dermatophagoides pteronyssinus (DP). Enzyme-linked immunosorbent assays (ELISA) were developed for this purpose. Binding of serum allergen-specific IgE was detected via the use of biotinylated Fc-epsilon receptor alpha chain protein (FcvarepsilonRIalpha). Following optimisation of the assay, serum samples from 59 cats with allergic skin disease and 54 clinically normal cats were screened. Results were expressed as ELISA units per ml (EU/ml) compared to a standard curve. Serological findings were correlated with the clinical presentation of affected cats. Cats with symptoms of feline allergic skin disease were grouped as follows: self-induced alopecia without lesions (group 1), papulocrusting dermatitis (group 2), eosinophilic granuloma complex (group 3), papular/ulcerative dermatitis of head and neck/facial dermatitis (group 4), and a combination of symptoms (group 5). Control normal cats comprised the final group (group 6). The Kruskal-Wallis test was used for statistical analysis. There was no significant difference between groups for DF- and DP-specific IgE concentrations with a p-value of 0.875 and 0.705, respectively. Although the FcvarepsilonRIalpha-based ELISA was able to detect house dust mite-specific feline IgE, the presence of this allergen-specific IgE correlates poorly with the presence of clinical manifestations of allergic skin disease. The results of this study question the clinical relevance of house dust mite-specific IgE in feline allergic skin disease.     

The prevalence of immediate and delayed type hypersensitivity reactions to Microsporum canis antigens in cats

Spontaneous recovery from Microsporum canis infections in cats is thought to be dependent on the development of a competent immune response. The purpose of this study was to determine the prevalence of positive delayed type hypersensitivity reactions in cats with and without dermatophytosis. Four groups of cats were intradermally skin tested with M canis extract and test sites were evaluated both subjectively and objectively at 0, 24 and 48 h after injection. Delayed intradermal testing (IDT) reactions were absent in cats not exposed to dermatophytosis (n=20); infected-recovered cats (n=38 culture negative lesion negative and n=43 lesion negative but culture positive) had significantly larger IDT reactions than unexposed cats and cats that were still actively infected (n=18). Based on the results of this study, IDT with M canis extract can be used to assess the cellular immune response of cats with dermatophytosis.     

IgE and IgG antibodies in skin allergy of the horse

In horses, allergies have been characterized by clinical signs and/or intradermal (i.d.) allergen testing. Our aim was to find the first direct evidence that immunoglobulin E (IgE) mediates equine allergy. In addition, we tested the hypothesis that immediate skin reactions in horses can also be mediated by IgG. Anti-IgE affinity columns were used to purify IgE from serum of one healthy horse and three horses affected with summer eczema, an allergic dermatitis which is believed to be induced by Culicoides midges. A modified Prausnitz-Küstner experiment was performed in four clinical healthy horses by i.d. injection of the purified serum IgE antibodies. The following day, Culicoides allergen was injected at the same sites. Skin reactions were not observed in response to allergen alone, and in two horses after stimulation at any previous IgE injection site. However, the other two horses showed an immediate skin reaction at the previous injection sites of IgE obtained from allergic horses. In addition, purified monoclonal antibodies to various equine immunoglobulin isotypes were injected i.d. into six healthy horses. Immediate skin reactions were observed in response to anti-IgE (6/6 horses) and anti-IgG(T) injections (5/6 horses). The specificities of both antibodies for IgE and IgG(T), respectively, were confirmed by enzyme linked immunosorbent assays. The results provide the first direct evidence that IgE mediates classical Type-I allergy in horses and plays a major role in the pathogenesis of summer eczema. The data also suggest that IgG(T) can bind to skin mast cells and might contribute to clinical allergy.     

The impact of oral versus inhaled glucocorticoids on allergen specific IgE testing in experimentally asthmatic cats

Glucocorticoids (GCs) are palliative for allergic asthma, but allergen-specific immunotherapy (ASIT), which relies on identification of allergens, represents a potentially curative treatment. The purpose of this study was to determine if oral or inhaled GCs would interfere with identification of sensitizing allergens. The hypothesis was that oral but not inhaled GCs would interfere with accurate allergen-specific IgE identification determined by skin and serum testing in experimentally asthmatic cats. Asthma was induced in 18 cats using Bermuda grass allergen (BGA). Cats (n=6/group) were randomized to receive oral GCs (10mg prednisolone q 24 h), inhaled GCs (600 μg budesonide q 24 h) or placebo (q 24 h PO) for one month. Intradermal skin testing (IDST) and serum BGA-specific IgE were measured prior to, during and after treatment. A paired t test was used to compare groups pre- and post-treatment (P<0.05 significant). IDST reactivity was eliminated in 4/6, 3/6, and 1/6 cats receiving oral GCs, inhaled GCs, and placebo respectively. Two weeks after stopping treatment, IDST was again positive in all cats. Serum IgE reactivity to BGA was not significantly diminished by any treatment. In conclusion, a two-week withdrawal from GCs is adequate for IDST, but may not be necessary for serum IgE testing.     

Allergen-specific IgG antibodies in cats with allergic skin disease

An enzyme-linked immunosorbent assay was developed for quantifying feline serum allergen-specific IgG directed against selected house dust, pollen and flea allergens. The assay was used to compare allergen-specific IgG concentrations in sera from healthy cats, cats with non-dermatologic illness, confirmed allergic cats and undiagnosed pruritic cats. Our results demonstrate that cats with confirmed allergic skin disease have significantly more IgG directed against house dust, flea and ryegrass allergens than other cat groups examined. These results support the theory that cats with allergic skin disease have an underlying T_H2 lymphocyte response that directs production of both allergen-specific IgG and IgE.     

Response to intradermal skin testing with four cat allergen preparations in healthy and allergic dogs

Intradermal skin testing with 4 cat allergens was performed on 40 dogs. The allergens used were a commercial preparation, cat allergen 1, cat pelt extract, and cat serum. Twenty dogs had inhalant allergies (canine atopy) and 20 dogs were healthy. The dogs in these 2 groups were further allotted to groups of dogs with or without exposure to cats. Cat pelt extract was the only allergen that caused a statistically significant (P less than 0.025) difference in the number of positive reactions in healthy vs allergic dogs, with healthy dogs having the greater number of positive reactions. Seven (35%) of the 20 dogs with no known exposure to cats had positive reactions to at least one of the allergens. In dogs that had been exposed to cats, positive reactions developed in twice the number of dogs without clinical signs of canine atopy, compared with those with clinical signs of canine atopy. These data showed that dogs do become sensitized to cat allergen, but that this sensitization may not indicate known exposure to cats or clinical disease.     

Allergic pulmonary and ocular tissue responses in the absence of serum IgE antibodies (IgE) in an allergic dog model

Allergen-specific serum IgE may be insensitive as a marker for IgE-mediated reactions at the mucosal level. Five of six atopic beagle dogs developed high ovalbumin (OVA)-specific serum IgE levels after sensitization. This study aimed to show that these dogs still express allergen-specific IgE at the pulmonary and ocular mucosal levels and in the skin even when corresponding serum IgE was below the detection limit. When serum IgE levels were negative, all dogs exhibited allergic reactions at the tissue level. Specifically, they displayed positive ocular reactions after an ocular OVA challenge. After airway challenge with aerosolized OVA, five out of six animals reacted with decreased compliance and increased resistance of the lungs. Furthermore, an eosinophilia in the bronchoalveolar lavage fluid (BALF) was observed. Four weeks after the last exposure to OVA, IgE-positive BALF cells were seen in all animals. Six weeks on, all dogs still displayed positive skin reactions to OVA. This indicates that not only skin testing but also detection of ocular and pulmonary allergic tissue reactions including cell-bound IgE in BALF can serve as more sensitive and lasting surrogate markers of hypersensitivity in the allergic dog model than detection of allergen-specific serum IgE levels.     

Results of intradermal tests in horses without atopy and horses with chronic obstructive pulmonary disease

OBJECTIVE: To compare results of intradermal tests (IDT), conducted using environmental allergens, in horses without atopy and horses with chronic obstructive pulmonary disease (COPD). ANIMALS: 38 horses (22 horses without atopy and 16 horses with COPD). PROCEDURE: All horses were examined (physical examination, hematologic examination, serum biochemical analyses, examination of bronchoalveolar lavage fluid). An IDT was conducted, using a full panel of 73 allergens consisting of grasses, weeds, trees, molds, and insects. Results of the IDT were evaluated 30 minutes and 4, 6, and 24 hours after injection of allergens. Horses without atopy were euthanatized, and gross and histologic changes of lung parenchyma were assessed. RESULTS: Horses without atopy had a greater number of positive immediate and late-phase reactions than did horses with COPD. Horses with COPD did not have a significantly greater number of positive reactions than horses without atopy at any time period for any allergen group (grasses, weeds, trees, molds, and insects). CONCLUSIONS AND CLINICAL RELEVANCE: Positive results of IDT document allergen-specific hypersensitivity but do not necessarily distinguish clinically relevant reactions from subclinical reactivity in horses with COPD. Interpreting the clinical relevance of results of IDT requires a thorough knowledge of the medical history, physical examination findings, and environment of each animal.     

Induced and spontaneous IgE antibodies to Dermatophagoides farinae in dogs and cats: evidence of functional heterogeneity of IgE

Enzyme-linked immunosorbent assays (ELISAs) were developed to measure IgE antibodies specific for Dermatophagoides farinae in dogs and cats. Although higher levels were detected in atopic dogs and cats than in normal animals without skin disease, the differences were not statistically significant. On the other hand, levels in dogs and cats that were reared under laboratory conditions, and thus presumably not exposed to house dust mites, were either very low or undetectable. IgE antibodies were induced in 10 laboratory-reared cats using low-dose antigenic stimulation in aluminium hydroxide. All cats developed detectable IgE, but not all developed positive skin tests. However, serum from those cats with positive skin tests were able to give positive Prausnitz–Küstner (PK) tests. The canine data, together with previous work on basophil histamine release, suggests that the distinction between atopic and normal dogs may result from a heterogeneity of either IgE or of the high-affinity mast cell receptor. The feline data can only be explained by the existence of a heterogeneity of IgE.     

Flow cytometric analysis of lymphocyte proliferative responses to food allergens in dogs with food allergy

Two different allergy tests, antigen-specific immunoglobulin E quantification (IgE test) and flow cytometric analysis of antigen-specific proliferation of peripheral lymphocytes (lymphocyte proliferation test), were performed to examine differences in allergic reactions to food allergens in dogs with food allergy (FA). Thirteen dogs were diagnosed as FA based on clinical findings and elimination diet trials. Seven dogs clinically diagnosed with canine atopic dermatitis (CAD) were used as a disease control group, and 5 healthy dogs were used as a negative control group. In the FA group, 19 and 33 allergen reactions were identified using the serum IgE test and the lymphocyte proliferation test, respectively. Likewise, in the CAD group, 12 and 6 allergen reactions and in the healthy dogs 3 and 0 allergen reactions were identified by each test, respectively. A significant difference was found between FA and healthy dogs in terms of positive allergen detection by the lymphocyte proliferation test, suggesting that the test can be useful to differentiate FA from healthy dogs but not from CAD. Both tests were repeated in 6 of the dogs with FA after a 1.5- to 5-month elimination diet trial. The IgE concentrations in 9 of 11 of the positive reactions decreased by 20-80%, whereas all the positive reactions in the lymphocyte proliferation test decreased to nearly zero (P<0.05), suggesting that lymphocytes against food allergens may be involved in the pathogenesis of canine FA.     

Results of intradermal tests in horses without atopy and horses with atopic dermatitis or recurrent urticaria.

OBJECTIVE: To compare results of intradermal tests (IDT) for environmental allergens at 30 minutes and 4, 6, and 24 hours after injection in horses without atopy and horses with atopic dermatitis (AD) or recurrent urticaria (RU). ANIMALS: 22 horses without atopy, 10 horses with RU, and 7 horses with AD. PROCEDURE: In all horses, medical history was obtained, and results of physical examination, hematologic examination, serum biochemical analyses, examination of bronchoalveolar lavage fluid, and IDT with 73 allergens were examined. RESULTS: Horses with AD or RU had a significantly greater mean number of positive reactions for IDT, compared with horses without atopy. Horses with AD had a significantly greater number of positive reactions than horses without atopy in every allergen group at all time periods, except for molds at 4 and 24 hours. Horses with RU had a significantly greater number of positive reactions than horses without atopy in every allergen group, except for molds at 30 minutes and 4 and 6 hours, trees at 4 and 6 hours, and grasses at 4 hours. CONCLUSIONS AND CLINICAL RELEVANCE: A significantly greater number of positive reactions for IDT in horses with AD or RU, compared with horses without atopy, provides evidence of type-I IgE-mediated hypersensitivity for these diseases. Evaluation of results of IDT performed in horses with AD or RU is useful in determining specific allergens for the formulation of immunotherapy along with providing identification of allergens that could be useful when creating avoidance strategies.