Food allergy can be one of the greatest everyday diagnostic challenges in the practice of veterinary medicine.

Food allergy in dogs and in cats can look like so many things. And, at the same time, it can look like nothing you’ve ever seen.

Take inhalant allergies, for comparison. When a pet is allergic to dust, pollens, molds and fabrics — the same things that cause “hay fever” in people — the pattern of skin lesions is usually diagnostic. Pets, you see, don’t respond with coughing, sneezing and runny eyes when they’re allergic to these allergens. Instead, the skin acts as the “shock organ” instead of the respiratory tract as it is in people. Still, the diagnosis is usually not hard once you learn to recognize the patterns.

Food allergy, on the other hand, can affect any part of the body. It can cause skin lesions that are bilaterally symmetrical (the same on both sides of the body), or it can cause isolated lesions in an area not usually associated with allergies.

Complicated diagnosis

Sometimes food allergy doesn’t even affect the skin. Though it’s unusual, sometimes the “lesions” are more like problems people have with food allergies, causing stomach and intestinal upset that results in vomiting, diarrhea and weight loss.

To further complicate the diagnosis of food allergy, there are no easy tests. There is no “Let’s take a blood sample” diagnosis here. Instead, we have to rely on food trials. Food trials involve the feeding of foods that the pet, and therefore his immune system, have never “seen” or been exposed to, before.

Protein sources

That said, what kinds of food might a pet be allergic to? Purina? Gaines? Sunshine Brand? No, it’s not the brand of food but the ingredients in the food. Usually, specifically, the protein source in the food.

Common sources of protein in pet foods are beef, chicken, egg, dairy and soy. If your dog has food allergies that act up on a Purina food, he’s likely to have the same problem if you change brands and the new brand has the same protein source, such as chicken.

What’s required, then, is to use a food that includes a protein source that your pet has never eaten. In bygone days, lamb used to be the standard for food allergy testing. Since lamb meat is too expensive to be used in everyday commercial foods, your pet’s doctor could be pretty sure that pets had never had exposure to lamb, thus “priming” the immune system to respond with an allergic reaction to lamb.

Changing standards

Why was that the standard in bygone days and not today? I think it probably went something like this: Word got around that veterinary dermatologists put dogs with “bad skin” on lamb for a protein source, combined with rice for a carbohydrate source during food trials for food allergies. However, some bozo didn’t understand the concept of food allergies and food trials — he just knew that these specialists were improving dogs’ skin with lamb and rice. So, he made a commercial food with those ingredients and proclaimed it “The Food For Dogs With Sensitive Skin! Your Dog’s Haircoat Will Gleam Within Weeks!”

Well, if a food elimination trial is to work, one must feed a protein source that a pet has never before consumed. Now that there are 20 or more brands of food with lamb and rice as their main ingredients, we can no longer be sure that our patients have never eaten any lamb, so we can no longer use lamb as our purified diet protein source.

Rabbit, venison, fish, and some really exotic meats have taken the place of lamb in food elimination trials for that reason. One meal of rabbit and your diagnosis is made, right? Wrong.

Just as with people, it takes a long time for a food elimination trial to be fully worked out. Three months is the minimum time veterinary dermatologists recommend, and it is not unusual for trials to last nine months or more.

During that time, it is crucial that your pet eat nothing but the special food, water and his heartworm preventive. Even heartworm preventives have to be chosen carefully. If your pet’s doctor suspects that your dog has a beef allergy, he can’t take a chewable heartworm preventive that is in a beef-flavored chewy.

— Jim Randolph is a veterinarian at Animal General Hospital in Long Beach, Miss.