Local investigation into the role of Culicoides species diversity (Diptera: Ceratopogonidae) in recurrent horse dermatitis cases in southwest France

Background: Insect bite hypersensitivity in horses (“sweet itch”) is a common pruritic, chronic, seasonal, and recurrent dermatitis affecting approximately 10% of horses in France and is a major concern for the horse industry and private owners. This dermatitis results from an allergic reaction to the saliva of specific biting flies (Diptera: Nematocera), primarily from the Culicoides genus. Given the frequent occurrence of this health problem and the limited investigation in France, we conducted a field survey in the vicinity of a riding stable in southwestern France with a reported chronic case of recurrent horse dermatitis to (i) characterize the Culicoides species associated with horse populations and (ii) estimate the relative abundance of the different species identified based on the trapping site location. Methods: For this purpose, three Onderstepoort Veterinary Institute (OVI) traps were set up for one night once a week, from mid-June to the end of July 2022, the known adult peak activity period. Traps were placed either indoors or outdoors at horse facilities. Results: Culicoides obsoletus/scoticus were more abundant (58.3%), followed by C. circumscriptus (12.1%), C. nubeculosus (11.5%), C. punctatus (5.6%), C. festivipennis (3.8%), C. pulicaris (2.3%), C. riethi (2.3%), C. parroti (2.2%), and the remaining species, C. lupicaris, C. dewulfi, C. brunnicans, C. flavipulicaris, and C. picturatus, collectively representing only 1.4%. Importantly, C. obsoletus/scoticus and C. circumscriptus were found indoors in notable proportions (54% and 11.4% of captures, respectively). Conclusions: The findings highlight the continuous exposure of horses to Culicoides bites during the warm season, including at night and indoors, from C. obsoletus/scoticus, but also including low-impact species like C. punctatus and C. pulicaris. This underlines the need for ongoing research and surveillance.