Signs of allergies in many horses appear with the arrival of summer and become progressively worse each year, according to Dr. Susan L. White and Dr. Lydia F. Miller, whose "AAEP Forum" column appears in the August edition of The Horse
. Often horses with this history are allergic to insect bites (insect hypersensitivity).
Several different clinical syndromes have been associated with insect hypersensitivity, such as Queensland or sweet itch, which is caused by Culicoides species (no-see-ums). However, any biting insect can be involved in insect hypersensitivity. In fact, many affected horses are allergic to the bites of more than one kind of insect.
The first signs can include redness and large, flat, circular swellings (wheals) or raised nodules with or without crusting. Intense itching (pruritus) often leads to skin damage, hair loss, secondary infections, and thickened, wrinkled skin.
The best therapy is to prevent insects from biting your horse, or at least reduce the number of bites. Effective management strategies include stabling during times of high insect activity; directing fans to the surface of the horse when stalled; and using long-acting insect repellents (either on the horse or impregnated in mesh blankets or other equipment).
Intradermal skin testing can help detect the insect or group of insects to which the horse is most allergic. If determined, a desensitizing vaccine can be custom-made for that horse. Approximately 50%-75% of horses respond favorably to desensitization; however, six to 12 months are needed before the horse receives the full benefit of this therapy.
Another strategy to control insect hypersensitivity is to decrease the horse's immune reaction through corticosteroids. Depending on the situation, these compounds can be applied directly to the affected area(s) or given systemically.