We often don't think of our pets having a food allergy. Most of the time if our dog or cat has a skin problem we think of a contact allergy and not food. Flea dermatitis is another allergic reaction but this time to fleas. Any mammal and even birds can have flea dermatitis. Food on the other hand can cause all types of reactions from hives, hot spots to difficulty breathing.
Typically, food allergies are not caused by something new that your pet has not eaten before but rather something that they have eaten with no problems until recently before the reaction to the food. Allergies are caused by the body suddenly rejecting something that is has been exposed to for a long period of time. It is almost like the body saying , "enough already, stop eating this."
Finding the cause of a food allergy can cause a pet owner to have constant headaches over the matter. Just finding diets that work for your pet can be time consuming if you just deal with processed pet food. What you may want to try is buying regular meat and vegetables at the grocery store and making "stews" for your pet. That way you can find out if it is the actual food that is the cause or an additive that the pet food manufacturer has put in the food.
Here is a short list of what many pets and horses are allergic to:
Notice that most of them are what pet food companies use to add protein or as a filler in foods. I have a mare (horse) that is allergic to soybeans and will break out in hives all over her body. She is also highly allergic to Purina horse feeds and will also have hives and difficulty breathing.