Everything from molds and spores in the air and grain to insect bites can trigger an allergic reaction. They are areas of swelling that begin as small lumps, generally on the side of the neck, and progress across the shoulders and throat. Hives will often indent or pit when a finger is pressed into the swelling. Horses may appear depressed, have a slight fever, and the areas may or may not itch. Hives can appear due to certain types of food, plants, drugs or insect bites.
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.