Determination of irritant threshold concentrations of multiple tree, grass, weed and mould allergens for intradermal testing of horses residing in the southern USA

Background Appropriate allergen threshold concentrations (TCs) for intradermal testing (IDT) have not been established in horses for many pollen and mould allergens. Objectives To determine the TCs in non‐allergic horses and describe the frequency of late phase reactions for 26 allergens, including trees, grasses, weeds and moulds in horses residing in the southern Unites States. Animals Twenty four clinically normal horses in the southern United States. Methods Threshold concentrations for different allergens were determined using IDT subjective measurements at 30 minutes. Delayed reactions were evaluated at 4 and 24 h. Results Threshold concentrations (all PNU/mL) were established for eight tree allergens (black willow 1,000, box elder 1,000, live oak 1,000, pecan 2,000, white ash 4,000, red oak 4,000, red mulberry 2,000 and green ash 2,000); two grass allergens (Johnson grass 250 PNU/mL and Kentucky blue grass 500 PNU/mL); two weeds (carelessweed 1,000 PNU/mL, great ragweed 500 PNU/mL) and one mould (Curvularia 8,000 PNU/mL). The TC was not determined due to excessive reactivity at the lowest concentration tested (1,000 PNU/mL) for bahia and perennial rye grass. Eleven other allergens did not meet the criteria to establish a TC when evaluated at 30 min due to lack of positive reactions. Multiple allergens caused positive reactions in ≥10% of horses at 4 h. Reactions at 24 h were rare with the exception of one horse. Conclusions and clinical importance This study identified intradermal TC for multiple pollen and mould allergens in horses. These values may prove useful for optimizing allergen concentrations for IDT of allergic horses. Résumé Contexte Les concentrations seuils (TCs) d'allergène adaptées aux tests intradermiques (IDT) n'ont pas été déterminées chez le cheval pour de nombreux allergènes de pollens et moisissures. Objectifs Déterminer les TCs chez les chevaux non‐allergiques et décrire la fréquence des réactions retardées pour 26 allergènes dont les arbres, les herbes, les graminées et les moisissures chez des chevaux vivant au sud des Etats Unis. Sujets Vingt quatre chevaux cliniquement sains du sud des Etats Unis. Méthodes Les concentrations seuils des différents allergènes ont été déterminées par mesure subjective des IDT à 30 minutes. Les réactions retardées ont été évaluées à 4 et 24h. Résultats Les concentrations seuils (PNU/mL) ont été déterminées pour huit des allergènes d'arbre (saule noir 1,000, chêne vert 1,000, érable 1,000, pacan