Culicoides obsoletus (Diptera: Ceratopogonidae) as a causal agent of Culicoides hypersensitivity (sweet itch) in British Columbia.

Six horses severely affected by a seasonal dermatitis similar in both histopathology and epidemiology to Culicoides hypersensitivity (CH) and six unaffected or normal horses were inoculated intradermally with an extract of Culicoides obsoletus (Meigen), the most common Culicoides in southwestern British Columbia. Affected horses developed large welts within 20 min after injection, representing an immediate (type I) reaction; welts were largest 24 h or more after challenge, indicating in addition a delayed (type IV) reaction. This reaction was discernible for greater than 3 wk in some of the affected horses. Normal horses developed small welts which peaked 2-4 h after challenge. Affected horses were irritated by the injections and developed characteristically ridged skin at the injection sites, similar to that seen in natural lesions, whereas the normal horses showed neither discomfort nor clinical signs. C. obsoletus is one of the most numerous Culicoides species biting horses, feeds on parts of the body where lesions are found, and is, we believe, responsible for dermatitis in the Pacific Northwest. The skin test was repeated 2 yr later in three of the affected horses. In two, the reactions were similar to those in the first test, but in the third horse, the reaction was greatly reduced after the second test. This paralleled a decrease in the severity of its natural clinical signs. Thus, a skin test may be useful in the diagnosis of Culicoides hypersensitivity.