Horses are susceptible to food allergies, just like humans. Likewise, it is usually impossible to identify the precise cause. Certain grains or types of hay have been reported to cause hives in horses. The types of allergies have been associated with high protein concentrates, although this is not always the case. Horses could also be allergic to feeds that are present in other areas of the barn and not actually being fed to the allergic horse.
Horses can also be allergic to medications, either topical or internal.
Allergic reactions have also been observed after administration of tranquilizers such as acepromazine. Hives have even been reported to occur after administration of certain vaccines such as equine influenza or tetanus antitoxin. Insect bites can cause problems. Hypersensitivity associated with midges or gnat bites is a common problem, although allergic reactions to mosquito bites have also been reported. The most commonly affected areas include the back, ears, mane, and tail.