The Intradermal Skin Test in the Horse: Value as a Diagnostic Modality in Equine Allergies

Recent studies have provided conflicting results in regards to equine intradermal skin testing and its use in defining causative antigens in IgE mediated diseases such as equine recurrent airway obstruction (RAO). This study was divided into two experiments. In the first experiment of this study, the hypothesis tested was normal horses would have minimal variability in the wheals formed by intradermal injection of positive control stimulants. This was evaluated by examining the repeatability of skin test wheals created by 5 concentrations of histamine, compound 48/80, and phaseolus vulgaris (PHA) within a normal horse and between 12 normal horses at 0.5 hours, 4 hours, and 24 hours post injection. Minimal variability was detected within individual horses and between 12 horses for histamine and compound 48/80 at 0.5 hours and for PHA at 4 hours. This information suggests that the intradermal injection of positive control substances is a repeatable test in normal horses.

In the second experiment of this study, the hypothesis tested was normal horses react differently to intradermal injection of positive control stimulants (histamine, compound 48/80, PHA) and/or an environmental antigen (Aspergillus) in comparison to horses affected with RAO. This was evaluated by identifying differences in wheal responses between normal horses and RAO affected horses. Concentration response curves were created in normal and RAO affected horses to the aforementioned stimulants at 0.5 hours, 4 hours, and 24 hours post injection. No statistically significant differences were noted in concentration response between normal and ROA affected horses when compound 48/80 and PHA were evaluated. RAO affected horses demonstrated a greater slope at the 0.5 hour time when compared to normal horses suggesting that RAO affected horses are hypersensitive to intradermal injection of histamine. Injection of Aspergillus mix at 4000 protein nitrogen units/ml caused an intradermal wheal reaction at the 24-hour time in 4/5 RAO horses. This reaction was not noted in normal horses. This information suggests that there may be a positive relationship between causative antigens (i.e. Aspergillus) that may induce clinical RAO and positive intradermal skin test results.

An additional aspect that was evaluated in both experiments involved histologic examination of skin biopsies taken from wheals created by intadermal injection of histamine, compound 48/80, PHA, and Aspergillus at various times post injection. In the first experiment, intradermal injection of histamine caused severe dermal edema and margination of neutrophils and eosinophils at 0.5 hours. Compound 48/80 demonstrated mild to modest dermal edema at 0.5 hours while PHA demonstrated severe dermal edema, hemorrhage, and lymphactic ectasia at 4 and 24 hours. PHA also demonstrated a neutophilic inflammation at 4 hours that progressed to a mixed lymphohistiocytic and neutrophilic inflammation at 24 hours. In the second experiment, no edema and modest to moderate neutrophilic inflammation was noted in normal horses after intradermal injection of Aspergillus at 24 hours. In contrast, RAO affected horses demonstrated mild to modest edema and a mild to moderate mixed inflammatory response (lympho-histocytic, neutrophilic, eosinophilic) after intradermal injection of Aspergillus at 24 hours suggesting a delayed type response.