Effects of dexamethasone and hydroxyzine treatment on intradermal testing and allergen-specific IgE serum testing results in horses

To determine whether dexamethasone or hydroxyzine affect intradermal testing (IDT) and allergen-specific IgE serum testing (ASIST) results in horses, these tests were performed serially in five horses without signs of atopic dermatitis before and after treatment with the drugs. IDT consisted of saline, histamine (1:100,000 w/v) and eight commercial extracts; results were evaluated as subjective scores by comparison to saline (0) and histamine (4) and as objective measurement of wheal diameter (mm). After baseline testing, dexamethasone (20 mg) was administered intramuscularly daily for 7 days. Testing was repeated 3-4 h, 7 days, and 14 days after the final dose of dexamethasone. Hydroxyzine (500 mg) was subsequently administered orally twice daily for 7 days. Testing was performed 3-4 h, 3 days, and 7 days after the final dose of hydroxyzine. No differences were found between pre- and post-treatment subjective IDT scores for either drug. However, wheal diameter for histamine and house dust, dust mite mix, and black ant extracts decreased (P < 0.05) at all times post-injection for IDT performed 3-4 h after the final dose of both medications. Wheal diameter returned to pre-treatment levels 14 days after discontinuation of dexamethasone and 7 days after discontinuation of hydroxyzine. No significant changes in ASIST results were found. In conclusion, treatment of horses with dexamethasone or hydroxyzine for 7 days had no effect on ASIST results but did decrease IDT wheal diameters. Based on findings of this study, withdrawal times of 14 and 7 days for dexamethasone and hydroxyzine, respectively, prior to IDT can be recommended.