Allergy or hypersensitivity to feed ingredients is very rare in horses and is manifested by generalized skin weals or hives (urticaria) with or without itchiness (pruritis). Owners sometimes attribute signs such as fizzy behavior or loose droppings to feed allergy, but these are more likely the result of other factors such as starch overload, than true feed allergy.
The only way to confirm a diagnosis of feed allergy is to remove the horse to a different environment with completely different bedding and diet, then after a period of a couple of weeks to let symptoms subside, to re-introduce suspect feeds from the original diet one at a time to see if symptoms recur.
The so called ‘’protein bumps’’ or ‘’sweet feed bumps” are collections of small weals often on the flanks and belly and sometimes oozing a small amount of clear serous fluid which may form crusts. They may be related to dietary protein content, but this is not proven, and they generally resolve by themselves in a day or two without treatment. In more severe or persistent cases a veterinarian may prescribe short acting corticosteroid or antihistamine treatment to resolve the bumps.