Equine atopic skin disease and response to allergen- specific immunotherapy: a retrospective study at the University of California-Davis (1991–2008) Casey T. Stepnik*, Catherine A. Outerbridge*, Stephen D. White* and Philip H. Kass† Departmentsof *Medicine and Epidemiology and †Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, CA 95616, USA Correspondence: Casey T. Stepnik, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA. E-mail: ctstepnik@ucdavis.edu Sources of Funding This study is self-funded. Conflict of Interest No conflicts of interest have been declared. Abstract This retrospective study reports on the clinical pre- sentation of equine atopic skin disease and evaluates response to treatment with allergen-specific immu- notherapy (ASIT) based on intradermal testing and ⁄ or serum testing. Computerized medical records from January 1991 to December 2008 yielded 54 horses included in the study. Presenting clinical signs (CS) included urticaria (n = 28), pruritus (n = 8) or both (n = 18). Forty-one of 54 horses received ASIT, and response to ASIT (n = 32) was evaluated via telephone survey. Eighty-four per cent (n = 27) of owners reported that ASIT reduced their horse’s CS; 59% (n = 19) were able to manage CS by ASIT alone. Three horses (9%) were managed with ASIT in combi- nation with doxepin and discontinued use of corti- costeroids. There was no statistical significance between type of test performed and reported success of ASIT (v 2 analysis, P = 0.53). Ninety-three per cent (n = 30) of owners reported use of antipruritic medi- cations prior to starting ASIT; 57% (n = 17) of these owners reported discontinuing those medications due to success of ASIT. Adverse effects were limited to swelling at the injection site, seen in 16% (n = 5). Seventy-five per cent (n = 24) of owners elected to discontinue ASIT after 6 months to 8 years (mean 2.2 years): 15 due to resolution of CS, six due to per- sistent CS, two because the horse was sold, and one due to cost. Ten owners reported no recurrence of CS after discontinuing ASIT; five had recurrence within a median of 2 years of discontinuing ASIT (range 1– 12 years). Allergen-specific immunotherapy is a safe and effective way to manage equine atopic skin dis- ease. Accepted 4 April 2011 Introduction Atopic skin disease in people and dogs is a heritable con- dition characterized by pruritus, with affected individuals genetically predisposed to develop IgE-mediated allergy to environmental allergens. 1 Atopic skin disease in horses is not as well characterized as in other species, despite sharing some similarities. While pruritus is considered the primary clinical sign of atopic skin disease in all species, 2–5 it is reported that equine patients may experience urticaria with or without concurrent pruritus as a manifestation of their atopic skin disease. 6,7 Clinical signs may occur seasonally or year round. 2,8 The diagnosis of equine atopic skin disease is primarily based on history, clinical presentation and exclu- sion of other pruritic and ⁄ or urticarial skin diseases. 3,6,9 Equine atopic skin disease is typically considered a lifelong disease that requires chronic management. 2,3 Management strategies include systemic medications, such as antihistamines, glucocorticoids, tricyclic antide- pressants or a combination of these, along with topical therapy and allergen avoidance. 6 The undesirable adverse effects and limitations on usage in competition sport of some of these systemic medications along with the impracticality of being able to avoid the offending aller- gens often make allergen-specific immunotherapy (ASIT) the preferred treatment choice. 2,3 Allergen-specific immunotherapy has been used successfully in the man- agement of atopic skin disease in horses. 2,3,7 Intradermal testing and serum tests have both been utilized for identi- fication of allergens to be used in immunotherapy formu- lated for horses. 5,6 Reports on the compatible history, clinical signs and treatment of these horses have been limited to descrip- tions in textbooks and limited isolated case reports. The purpose of this retrospective study was to report in detail a larger population of horses examined at the Veterinary Medical Teaching Hospital, University of California at Davis (VMTH-UCD), describe their clinical presentation of atopic skin disease and evaluate their response to treat- ment with ASIT. Materials and methods The computerized medical records of the VMTH-UCD from January 1991 to December 2008 were searched using the key words ‘atopic dermatitis’, ‘allergy test’, ‘intradermal skin testing’, ‘urticaria’ and ‘equine’. Horses were excluded if their history was not compatible with atopic skin disease and if there were no cutaneous lesions described in the medical record. Urticaria was included if other aetiol- ogies for the urticaria had been eliminated and other criteria for inclu- sion in the study were met. Within the scope of the retrospective ª 2011 The Authors. Veterinary Dermatology ª 2011 ESVD and ACVD, Veterinary Dermatology, 23, 29–e7. 29 DOI: 10.1111/j.1365-3164.2011.01001.x