Simply changing to a different brand or even different meat source listed on the label is often diagnostically ineffective because many ingredients overlap in the different diets. There are two types of diet trials used to effectively diagnose food allergy in dogs and cats, elimination diet trials and hydrolyzed diet trials. Elimination diet trials are my preferred approach to most cases. In this type of trial, the previous ingredient list is reviewed and a protein and a starch that have not been fed previously are then fed exclusively for a period of 8 to 10 weeks, while watching for improvement.
Ideally, the protein and starch are cooked at home, with no other ingredients, dyes, preservatives or processing to interfere. However, most clients prefer to use prepared pet foods and we carry prescription diets with unusual ingredients, such as venison or kangaroo. It is often difficult, these days, to find diets using starches that the pet has never been exposed to, making it more difficult to completely eliminate all previous ingredients. Luckily, the proteins found in starches are a much less common cause of allergies in pets.
Another way to approach testing and treatment of food allergic patients is to use hydrolyzed diets. The protein portions of these diets are broken into smaller pieces, theoretically making it more difficult for the allergic process to occur. Think of the protein as a puzzle piece, and that puzzle piece perfectly fits a puzzle piece that the body makes. When enough pieces fit together, the allergy process begins. Hydrolysis is like tearing up the food puzzle piece into many smaller pieces so it is harder for the body’s puzzle pieces to find it, thus interrupting the allergic process. Theoretically brilliant, but in my experience, these diets are not quite as effective as complete elimination of the offending protein. Therefore, I don’t use them during the test phase, but sometimes they are satisfactory for maintenance feeding, once the allergy is diagnosed.
Regardless of the diet chosen, it is gradually introduced to prevent gastrointestinal distress, and then must be fed exclusively until directed otherwise, usually for 8 to 10 weeks. Nothing else should pass the pet’s lips. Think peanut allergy, even protein particles in the air can be enough to set off a reaction. So, all other foods, treats, and flavored medications, such as heartworm preventative must be temporarily eliminated. When possible, we even eliminate medications in capsules, because they are made from gelatin, which often comes from undefined protein sources.
Once the pet’s problem is resolved, or if it does not respond after 8 to 10 weeks, the old food and treats are re-introduced, watching for changes in itch or skin condition to complete the diagnostic process. If food allergy is diagnosed, the pet owner may choose to continue the prescription diet, look for similar commercial versions for maintenance feeding, or do provocation testing. In that case, suspected ingredients are introduced one at a time for about 2 weeks, until the offending protein is revealed, and can therefore be avoided.