Summary Reasons for performing study: Insect bite hypersensitivity (IBH) is an IgE‐mediated allergic dermatitis caused by bites of Culicoides and Simulium species, and improved means of diagnosis are required. Objectives: The cellular antigen simulation test (CAST®) with C. nubeculosus and S. vittatum extracts was assessed in a population of IBH‐affected and healthy horses. Variations in test results over a one year period and possible cross‐reactivity between different insect extracts was studied. Methods: A total of 314 mature horses were studied using the CAST. Influence of severity of clinical signs, gender and age were evaluated, and 32 horses were tested repeatedly over one year. The kappa reliability test was used to assess agreement of the test results with different insect extracts. Results: Horses with IBH had significantly higher sLT release than controls with C. nubeculosus and S. vittatum. The highest diagnostic sensitivity and specificity levels were attained when using adult C. nubeculosus extracts with the CAST (78% and 97%, respectively), suggesting that most horses with IBH are sensitised against Culicoides allergens. A proportion of IBH‐affected horses was found to be sensitised to allergens of Simulium spp. in addition to those of C. nubeculosus. The CAST with C. nubeculosus had positive and negative predictive values ≥80% for a true prevalence of IBH of 12–52%. In the follow‐up study, the proportion of IBH‐affected horses with a positive test result ranged from 90% in November to 68% in March. Severity of clinical signs or age did not influence test results significantly. However, IBH‐affected males achieved significantly more positive test results than IBH‐affected females. Conclusions: The CAST with adult C. nubeculosus has high specificity and good sensitivity for diagnosis of IBH. Horses with IBH are mainly sensitised to Culicoides allergens, and some horses are additionally also sensitised to allergens in Simulium spp. Potential relevance: The CAST is likely to be a useful test for diagnosis of IBH, even allowing the identification of IBH‐affected but asymptomatic horses. This test may also help in further characterisation of allergens involved in this condition.

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BackgroundInsect bite hypersensitivity (IBH) is an IgE-mediated dermatitis caused by bites of Culicoides spp., which occurs frequently in horses imported from Iceland to continental Europe. IBH does not occur in Iceland because Culicoides species that bite horses are not present. However, Simulium vittatum (S. vittatum) are found in Iceland. In Europe, blood basophils from IBH-affected horses release significantly more sulfidoleukotrienes (sLT) than those from healthy controls after in vitro stimulation with Culicoides nubeculosus (C. nubeculosus) and S. vittatum. Aims of the study were: (I) using the sLT release assay, to test if horses living in Iceland were sensitized to S. vittatum and (II) to determine in a longitudinal study in horses imported from Iceland to Switzerland whether the sLT release assay would allow to predict which horses would develop IBH.ResultsHorses in Iceland, even when living in high S. vittatum areas, were usually not sensitized to S. vittatum or C. nubeculosus. Incidence of IBH in the 145 horses from the longitudinal study was 51% and mean time until IBH developed was 2.5 ± 1 year. Before import and after the first summer following import, there were no significant differences in sLT release between the endpoint healthy (H) and IBH groups. After the 2nd summer, when the number of clinically affected horses increased in the endpoint IBH group, a significantly higher sLT release after stimulation with C. nubeculosus but not with S. vittatum was observed. After the 3rd and 4th summer, the endpoint IBH group had a significantly higher sLT release with C. nubeculosus and S. vittatum than the endpoint H group. Some of the horses that remained healthy became transiently positive in the sLT release assay upon stimulation of their peripheral blood leucocytes with C. nubeculosus.ConclusionsHorses in Iceland are not sensitized to S. vittatum. In horses that develop IBH, sensitization to S. vittatum is secondary to sensitization to C. nubeculosus and probably a result of an immunological cross-reactivity. A sLT release assay cannot be used to predict which horses will develop IBH. A transient positive reaction in the sLT release assay observed in horses that remained healthy suggests that immunoregulatory mechanisms may control an initial sensitization of the healthy horses.