Food Allergy: Diagnosis By Food Trial

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 can, at the same time, look like nothing you’ve ever seen.

Take inhalant allergies, for comparison. When a pet is allergic to dusts, 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. In dogs and cats, the skin acts as the “shock organ” instead of the respiratory tract as it is in people. 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 causes isolated lesions in an area not usually associated with allergies.

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.  These patients have 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.

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, poultry, 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 never “priming” the immune system to respond with an allergic reaction to lamb.

Why was that the standard in bygone days and not today? Because some bozo messed it up for everybody! 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, the 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 twenty 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, kangaroo 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?


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! Likewise, he can’t chew on rawhides (which are made from the skin of cows, which is beef). Treats must be homemade, specially formulated from the elimination diet reshaped or cooked to make them into a hypoallergenic treat.

An option widely used today is Hill’s Prescription Diet z/d.   z/d is made with protein that has been hydrolyzed. Hydrolysing the proteins allows it to be used by the body for nutrition, yet not being recognized as protein by the immune system.  Then they are unable to contribute to food-related reactions.

Hills also makes hypoallergenic treats that your pet can eat during his food trial.

The Hills Pet Food Company has given us a terrific and effective alternative diet that also saves pet owners from having to cook during food trials.

Perhaps best of all, if the food trial successfully demonstrates that your pet has a food allergy, he can eat Prescription Diet z/d for a lifetime. The nutrition is that complete and balanced.

If your pet has skin problems that simply can’t be resolved by the means that control most common allergies, he may need to have a food elimination trial to investigate the possibility of food allergy. See your pet’s doctor for the specific needs of your pet.

See you tomorrow, Dr. Randolph.


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  1. I have doing some research on sebaceous cysts and warts in dogs. I have a 4 year old spayed standard poodle (I noted you have a poodle too!). She is my Delta Society and READ program partner. I understand that the poodle breed is prone to these things and I have had the veterinarian look at different things that crop up and we have aspirated them on occasion to make sure that they were just cysts. But it appears that she is getting more and more warts and cysts as she ages. I was beginning to wonder if there may be some connection between this problem and food allergy. I know that she has a sensitive stomach, and have already found that beef does not do well with her. As far as digestion goes she has done very well on Innova dry foods, and I also have her on Pro-Zyme probiotics and give her yogurt. So her stools and digestion have settled out very well on that diet. But she always has some active cysts going on. I leave them until they rupture by themselves (I can tell when one is giving her a problem and ready to rupture because she starts tending to it) We then very, very carefully express the contents and then I Betadine it and put antibiotic ointment on it and keep and eye out. It generally heals with no problem, but the hair grows back in very black (She was a coal black puppy but matured out to a salt and pepper….heavy on the pepper) So she has some black spots all over her. She seems in excellent health in all other ways and she is an absolutely wonderful dog. I bought her from a very good breeder who bred family-owned dogs and issued health certifications and a year warranty with purchase. I paid $850 for pet quality. She has been worth every penny. I am just concerned about her great propensity for these growths and scared that I might overlook something someday. If you have any suggestions to consider I would really appreciate it!

    • I assume you’ve read our post on dog warts. As they are caused by a virus, just like in people, spreading them from an infected site to a new site is very easy for the dog to do herself. Sebaceous cysts are more common in dogs with allergies, though your pet seems to have gastrointestinal problems usually not associated with food allergy but more commonly associated with inflammatory bowel disease (which someday may prove to have an allergy relationship). Thirty years ago there was a company that made an autogenous wart vaccine, but I haven’t heard of anyone doing that in decades. If your pet’s doctor has a dermatologist he works with regularly (most veterinarians do) he can ask. Speaking of dermatologists, they can freeze warts off, thus eliminating the virus and reducing spread (until another papilloma virus finds your dog). Expect the hair in the frozen location(s) to come back white, or some other color. Otherwise, it sounds like what you’re doing is working well.

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