Allergen‐specific immunotherapy in horses with insect bite hypersensitivity: a double‐blind, randomized, placebo‐controlled study

BACKGROUND: Insect bite hypersensitivity (IBH) is a common cause of pruritus in horses, but there are few controlled studies on the efficacy of allergen‐specific immunotherapy (ASIT). Atopic dermatitis and IBH can present with overlapping clinical signs; multiple insect and environmental allergens could be indicated in these horses to achieve effective hyposensitization. Although the success of ASIT using Culicoides spp. whole‐body extracts is controversial, there are no controlled studies published that clearly show benefit from this form of therapy. HYPOTHESIS/OBJECTIVES: The objective was to evaluate the efficacy of ASIT in horses with IBH using commercially available extracts and tests. ANIMALS: Twenty horses with seasonal pruritus and positive intradermal reactions to a whole Culicoides extract. METHODS: An enzyme‐linked immunosorbent assay test (Allercept®) was used to detect concurrent allergen‐specific IgE for other insects and environmental allergens. The ASIT was formulated by adding the relevant serologically positive allergens to the Culicoides extract. After randomization, 10 horses received ASIT and the rest a placebo solution. Clinical response was assessed every 4 months during 1 year using a clinical scoring system based on the severity of four clinical signs at 10 different body regions. Horses were not stabled and, to minimize dropouts, an insect repellent was used weekly in both groups. RESULTS: Differences in clinical scores between groups were nonsignificant at any re‐evaluation, while both groups improved to a similar extent, probably due to the insecticide treatment. CONCLUSIONS AND CLINICAL IMPORTANCE: Using commercially available extracts and tests, we could not demonstrate a beneficial effect of 1 year multiple ASIT in nonstabled horses with IBH.