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The aim of this study was to compare results of four different allergy tests from healthy and RAO-affected horses and to evaluate the clinical relevance of positive tested allergens via specific inhalation provocation test. Nine RAO-affected horses and 6 healthy horses underwent a complete lung examination including clinical parameters, arterial blood gas analysis, bronchoscopy with cytology of tracheobronchial secretion (TBS) and bronchoalveolar lavage (BAL). For allergy testing venous blood samples were collected for the functional in vitro test (FIT) and serum samples for the Fc-Epsilon receptor test and the Allergie-40 Profil IgE (Pferd) ELISA (IBL-Hamburg), and were submitted to specialized laboratories. An intradermal skin test (IDT) with 38 different allergens was performed by the author in all 15 horses. Subsequently a histamine inhalation provocation test (HIPT) was performed in all horses and the intrapleural pressure was indirectly measured via an esophageal probe. For each horse two individual allergens were chosen for allergen inhalation provocation test (AIPT) based on ICT results and were inhaled in increasing concentrations (one allergen per day).
None of the four allergy tests revealed a significant difference between healthy and RAO-affected horses. There was also no correlation between the results of the allergy tests. The histamine concentration which caused signs of dyspnoea was significantly lower in the group of healthy horses. The intrapleural pressure results showed a large individual variability and were also increased in some healthy horses. A significantly positive reaction within the AIPT was only detectable within two RAO-affected horses 6 hours after allergen inhalation with Aspergillus fumigatus and Cladosporium herbarum. The measured intrapleural pressure of these two horses increased close to 300% in comparison to baseline (PBS inhalation). During inhalation no changes of pressure were detectable in these two horses. Therefore, a pure type I immediate hypersensitivity reaction is unlikely to be involved in causing the symptoms of RAO in this group of horses. Because of the delayed reaction after allergen provocation other immune mechanisms (e.g. type IV hypersensitivity) could be reasonable.
Because all allergy tests used in this study can only detect type I immune reactions, these tests are probably not suitable for an etiological diagnosis of RAO according to these results.
It is plausible that in this study low intensity immune reactions against the allergen challenge remained undetected because of great individual variations of baseline parameters. Further studies with a larger sample size and more sensitive evaluation parameters are needed in order to eludicate the complex and important immunopathogenesis of RAO.