In-vitro allergy tests for diagnostic of summer eczema in the horse - Correlation of allergic hypersensitivity in functional in vitro test (FIT) and clinical manifestation under the influence of parasitic burdens

In the presented study, the in-vitro allergy test " functional in-vitro test" (FIT) and the " cellular antigen stimulation test or " basophile degranulation test"" (CAST/BDT) were performed in horses suffering from summer eczema and test results were compared to clinical manifestation using a clinical scoring system. We aimed to identify involved insect allergens, evaluate the severity of hypersensitivity and if the tests were reliable in insect-free seasons. In addition, we were interested in the influence of a parasitic burden on the tests results. 51 horses known to be affected by summer eczema were tested during winter using the FIT on 8 insects (biting mites, biting housefly, blackfly, mayfly, moth, gnat, horsefly, housefly) and 4 insects with the CAST/BDT (biting mites, gnats, black fly, horse fly). Classification of severity in the FIT was from 0 (= no reaction) to 4 (= severe reaction). With the CAST a positive reaction above 250pg/ml sulfidoleucotriene were judged as positive reaction. Owners were also asked with a questionary about the clinical signs in the year before and the antiparasitic management. All horses were examined clinically with an eczema-score and feces samples were taken. We found both tests reliable even in the insect-free season for summer eczema. Only 2 horses with clinical signs were tested negative for all 8 insect allergens in the FIT. 3 blood samples could not be analyzed by the laboratory. With 81.3% most horses reacted to moth, followed by 79.2% reacting to biting mites and 62.5% to gnats. Almost 60% of horses showed allergic hypersensitivity in the FIT to houseflies, which are irrelevant for summer eczema. 7 horses reacted to all insect allergens tested. The results allow the suspicion that insect allergens tested in the FIT maybe cross-react. No correlation was found between the clinical scores and the results of the FIT. In the CAST 2 blood samples could not be analyzed by the laboratory. 7 horses with clinical signs were tested negative for all 4 insect allergens in the CAST. With 71.4% most horses reacted to biting mites, followed by 61.2% reacting to gnats, 43% react to blackfly and 39% to horse fly. In fecal samples 59% of horses showed a parasitic burden with 85% positive for strongyles, but there was no correlation between parasitic burden and clinical scores. But the severity of the parasitic burden and the severity of hypersensitivity in the FIT correlates significantly. Horses with a severe parasitic burden showed significantly lower FIT results ( P= 0.0324) compared to those with a mild or no parasitic infection. Therefore, false-positive results of the allergy test due to a high parasitic burden do not seem likely.