Insect Hypersensitivity
Basic Information 
Epidemiology
Genetics and Breed Predisposition
Clinical Presentation
Physical Exam Findings
• Skin lesions vary from mild with areas of broken-off hair only, papules, and crusts to complete alopecia and severe excoriation.
• The distribution of the lesions depends on where the involved insects are feeding and can affect any combination of locations, including the face, mane, ventrum, dorsal back, legs, and tail.
• “Classic” lesions of Culicoides hypersensitivity include “buzzed-off manes” and “rat tails.”
• Chronic lesions may include lichenification and hyperpigmentation and skin fold development (rugae) at the base of mane and tail.
• Some severely affected horses may also show behavioral changes (restlessness and irritability) and even weight loss, making the horse unsuitable for riding in some cases.
• Affected horses may also develop secondary bacterial infection of the skin (folliculitis). Signs of this complication range from erected hairs over small papules with or without crusts to deeper lesions with nodules and draining tracts in rare cases.
• Urticaria is an uncommon manifestation of insect hypersensitivity in horses.
Etiology and Pathophysiology
• Type I (immunoglobulin E–mediated) hypersensitivity reaction to saliva from various biting insects ± type IV (delayed or cell-mediated) hypersensitivity reaction.
• Insects found most commonly to be involved are Culicoides spp. (biting midges, “no-see-ums”) but also by mosquitoes, black flies, horse flies, horn flies, stable flies, sand flies and, less commonly ants, wasps, and bees.
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