Intradermal testing of horses: A more realistic view and its application

Intradermal tests (IDT) as diagnostic tool for hypersensitivity in horses are known to provide variable, sometimes contradictory results. Reasons for such obstacles and confusing results have been investigated in repeated intradermal testing of 24 Icelandic and 6 worm blooded horses. Due to physiological differences in the local outfit of the skin with different kinds of cells and of cells in variable priming and / or sensitization stages as well as with different kinds and quantities of mediators the dermal response to a given substance can be very different depending on the site of application. Therefore misjudgement of a skin reaction at one site (micro area) can easily occur if evaluated by comparison with a "reference reaction" at another site. This risk is inherent in the presently used "reference assessment" of skin reactions towards a test substance (e.g. an antigen), depending on the skin reactivity to two reference substances, histamine as a positive reference plus a solvent as a negative reference. However, much more compatible with skin physiology is the evaluation of a skin reaction exclusively at the injection site. A judgement if the skin reaction is positive or negative can be given by means of the development of the local reaction with time (30 min up to 24 hours) in comparison to the initial pustule caused exclusively by the injected volume (of 100 mu L) without any skin reaction. The results obtained prompted us to suggest a more physiological performance plus a modified evaluation procedure ("kinetic assessment") of IDT. By these means IDT may well be a useful tool to support etiological diagnostics of type I allergic diseases: It provides information about functional sensitization of an individuals mast cells in vivo. Therefore it is essential to perform and assess skin reactions close to skin physiology.