Intradermal skin test in equine medicine.

Abstract

Atopic dermatitis (AD) is a common pruritic skin disease in horses. To manage AD it is important to identify the allergens and delete as many factors as possible. In human and small animal dermatology, intradermal testing (IDT) is a common and often used diagnostic tool to identify the offending allergens. In equine medicine the value of the use of intradermal tests in horses is doubtful and opinions are strongly divided, as the intradermal test are not standardized for horses yet. This study evaluates the results of intradermal testing on the left and right side of the neck in atopic horses. It then compares the IDT results with two different enzyme-linked immune sorbent assays (ELISA). The purpose of this study is to contribute to the validation of intradermal testing in atopic horses and is the first study assessing the concordance between intradermal tests on both sides of the neck in horses with AD. Ten private-owned horses with clinical signs such as pruritis and urticaria, are included in this study. All ten horses were intradermally injected with sixteen different allergens on both sides of the neck and were evaluated on four timepoints post injection. Only eight horses showed positive results on both sides of the neck. Bilateral positive results (n=26) were seen for 7/16 allergens, on the time points 30 minutes and 1 hour post injection. The farinae mite showed eight bilateral positive reactions, the house dust mite showed five bilateral positive reactions and the hay mite, English ryegrass and the tree pollen mixture showed three bilateral positive reactions. The herb pollen mixture and lichwort showed two bilateral positive reactions. Only four of these allergens showed good to excellent agreement (K= 0,61-1,00) between the left and right IDT 30 minutes post injection, one allergen also showed a statistic strong correlation (r>0,8; p<0,001). For every horse, four serum samples were obtained and analysed by two different laboratories (laboratory A and laboratory B). Laboratory A received two blood samples from each horse and analysed the serum samples using the monoclonal antibody cocktail-based (mac)ELISA. Laboratory B received two blood samples from each horse and analysed the serum samples using the single monoclonal (sm)ELISA. Laboratory A showed twelve duplicate positive results for nine different allergens, laboratory B showed eight duplicate positive results for five allergens. The majority of the allergens with duplicate positive results in laboratory A (8/12) showed a statistic strong correlation (r>0,8; p<0,001) and excellent kappa agreement (K=0,81-1,00); for laboratory B these results were only achieved in 4/8 allergens. Between laboratories, only the farinae mite and the house dust mites showed high IgE levels in both laboratories with a statistic strong correlation (r>0,8, p<0,001) and excellent kappa agreement. The IDT results on 30 minutes post injection showed high variability compared to the results of the macELISA and smELISA in respectively laboratory A and laboratory B. Only a moderate to good agreement was seen for two allergens in both laboratories (the farinae mite and the house dust mite). When taking in consideration all these results, only four allergens, the farinae mite (D. farinae), birch pollen, English ryegrass (L. perenne) and the storage mite (L. destructor) are reliable when an IDT is performed and evaluated 30 minutes post injection. In this study serological blood tests could not support the diagnosis. A high variability between the IgE results consists between two laboratories. There is need for better reliability of both IDTs and IgE serological tests.