Insect Hypersensitivity/Allergies

Insect hypersensitivity is the most common cause of pruritus in horses. A hypersensitivity reaction to bites by various Culicoides spp. is the best documented cause of the syndrome, but allergic reactions to bites by black flies, mosquitoes and other flies are also recognized. Equine insect hypersensitivity is a problem with a worldwide distribution as is reflected by the many different names for the syndrome. Nearly a 1000 Culicoides spp. are found worldwide and common names for these insects include gnats, no-see-ums, biting midges, sand flies, and punkies. They are tiny winged insects 1-2 mm in length that breed in standing water (ponds, lakes, irrigation canals, marshes, swamps, and watering troughs). Their numbers are greatest under warm and humid conditions.
Clinical signs of insect hypersensitivity can be observed in any breed or age of horse but several breeds appear to have a hereditary predisposition: Icelandic, German Shire, Arabian, Connemara, Swiss Warmblood, Quarter Horse, and ponies. Signs may start between 2 and 4 years of age and often become more severe as horses age. Typically, pruritus starts in the spring, becomes worse in the summer, and regresses in the fall in temperate regions or is year round in tropical regions. Pruritus frequently becomes more severe and of longer duration each subsequent year, especially when concurrent environmental allergies (atopic dermatitis) are present. With hypersensitivity to Culicoides spp. pruritus is most commonly observed along the dorsum including small papules and secondary excoriation along the mane, rump and tail base. With progression, lesions may extend to the neck and shoulder regions, ears, and face. Less commonly, a more ventral distribution of skin lesions develops, dependant on the feeding sites of the offending insects. With chronicity and continued self-trauma, “buzzed-off manes” and “rat tails” may develop along with lichenification and in severe cases, skin folds or rugae in the base of the mane and tail, behavior changes (e.g., restlessness and irritability making the horse unsuitable for riding) and even weight loss. […]