Intradermal challenge of Icelandic horses with extracts of four species of the genus Culicoides.

Twenty-three Icelandic horses were challenged with extracts of four species of biting midges: Culicoides pulicaris, C chiopterus, C obsoletus and C impunctatus. Fourteen of the tested horses were affected with summer eczema. The horses were challenged intradermally with 0.1 ml of whole-body extracts of midges at a concentration of 0.01 or 0.005 per cent weight/volume. The skin reactions were measured after 30 minutes, 60 or 180 minutes and four, 24 and 48 hours after injection. Antigen titration showed that the reaction was dependent on the antigen concentration. Eight of nine unaffected horses failed to respond to any of the four antigens; the remaining animal responding to two of the four antigens. Ten of the 14 affected horses responded to at least three of the four antigens, while two of the animals in this group failed to respond to any. The mean responses to C chiopterus, C obsoletus and C impunctatus, read after 30 minutes, 60 minutes and four hours were significantly higher in the affected horses than in the unaffected horses. A significant difference was also found in the mean response to C chiopterus and C impunctatus, read after 24 hours.