First clinical expression of equine insect bite hypersensitivity is associated with co-sensitization to multiple Culicoides allergens

BACKGROUND: Insect bite hypersensitivity (IBH) is an IgE-mediated allergic dermatitis in horses incited by salivary allergens from Culicoides spp. IBH does not occur in Iceland, as the causative agents are absent, however a high prevalence is seen in horses exported to Culicoides-rich environments. AIMS: To study the natural course of sensitization to Culicoides allergens and identify the primary sensitizing allergen(s) in horses exported from Iceland utilizing a comprehensive panel of Culicoides recombinant (r-) allergens. METHOD: IgE microarray profiling to 27 Culicoides r-allergens was conducted on 110 serological samples from horses imported to Switzerland from Iceland that subsequently developed IBH or remained healthy. Furthermore, a longitudinal study of 31 IBH horses determined IgE profiles the summer preceding first clinical signs of IBH (T(IBH)-1), the summer of first clinical signs (T(IBH)) and the following summer (T(IBH)+1). In a group of Icelandic horses residing in Sweden, effects of origin (born in Iceland or Sweden) and duration of IBH (<4 years, 4–7 years, >7 years) on Culicoides-specific IgE was evaluated. Sero-positivity rates and IgE levels were compared. RESULTS: At T(IBH), horses were sensitized to a median of 11 r-allergens (range = 0–21), of which nine were major allergens. This was significantly higher than T(IBH)-1 (3, 0–16), as well as the healthy (1, 0–14) group. There was no significant increase between T(IBH) and T(IBH)+1(12, 0–23). IBH-affected horses exported from Iceland had a significantly higher degree of sensitization than those born in Europe, while duration of IBH did not significantly affect degree of sensitization. CONCLUSION: Significant sensitization is only detected in serum the year of first clinical signs of IBH. Horses become sensitized simultaneously to multiple Culicoides r-allergens, indicating that IgE-reactivity is due to co-sensitization rather than cross-reactivity between Culicoides allergens. Nine major first sensitizing r-allergens have been identified, which could be used for preventive allergen immunotherapy.