Inspired by Nature • Powered by Science - Allergies in horses can develop at any age. Allergies are simply an imbalance of the immune system where your horse’s immune system perceives a threat from something harmless that he has come into regular contact with. Instead of eliminating microorganisms, the body suddenly starts attacking its own tissues, resulting in an overreaction to normal, everyday things such as grass, tree pollens, shavings, dust, molds, hay and straw, environmental pollutants, drugs, fly spray, or a new grooming product or shampoo. Many reactions cause a release of histamines, which cause swelling and other signs of inflammation. These symptoms are usually seen on the skin, but can also come in the form of swelling around the eyes and muzzle or even internally.