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In the presented study, the in-vitro allergy test “functional in-vitro test” (FIT) was performed in horses suffering from summer eczema and test results were compared to clinical manifestation of the disease in the year before using a clinical scoring system. We aimed to identify involved insect allergens, evaluate the severity of hypersensitivity and if the FIT is reliable in insect-free seasons. In addition, we were interested in the influence of a parasitic burden on the FIT 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). Classification of severity in the FIT was from 0 (=no reaction) to 4 (= severe reaction). We found the FIT to be a reliable test even in the insect-free season for summer eczema. Only 2 horses with clinical signs tested negative for all 8 insect allergens. 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 increasing sensitivity and specificity of the test. No correlation was found between the clinical scores and the results of the FIT. In fecal samples 69% of horses showed a parasitic burden with 85% positive for strongyles, but there was no correlation between parasitic burden and clinical scores as well as the severity of the parasitic burden and the severity of hypersensitivity in the FIT. Nevertheless, a trend was obvious towards lower FIT results in horses with a severe parasitic burden (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.