Hypersensitivity of horses in British Columbia to extracts of native and exotic species of Culicoides (Diptera: Ceratopogonidae).

Six horses from British Columbia severely affected by Culicoides hypersensitivity, a seasonal dermatitis caused by the bites of Culicoides spp., were inoculated intradermally with extracts of six species or forms of Culicoides from British Columbia, United States, and Israel. Two native and four exotic species were thought to cause the disease in their own geographical area. The horses developed large welts within 20 min after injection of any of the six extracts, indicating an immediate (type I) reaction. The skin reactions caused by each extract peaked at or after 24 h, indicating an additional delayed (type IV) reaction. These reactions confirm that the reactions to the extracts were allergic in nature. The horses were physically irritated by each extract and developed transverse ridges in the injection area similar to those seen in natural cases of the disease, again indicating an allergic reaction. Previous studies showed that control horses injected with Culicoides extract showed only a small, short-term reaction, with no delayed component, no irritation, and no transverse ridge development, further indicating that the observed reactions were allergic in nature. The affected horses reacted to all extracts of Culicoides, despite the fact that they had not been previously exposed to most of the species, indicating that the allergen(s) was present in all the extracts tested.