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Adverse Food Reactions and Food Elimination Trials | Dr. Catherine Lenox | VetGirl Veterinary CE Blog

Today's VetGirl guest blog is by Dr. Catherine Lenox, DVM, CVA, DACVN. Today, she'll discuss all you need to know about performing food elimination trials in your veterinary patients... and for how long!

What clinical signs indicate the need for a food elimination trial?
There are two reasons to do a food trial: gastrointestinal disease and dermatologic disease, also known as cutaneous adverse food reactions. Signs of gastrointestinal disease that may be diet responsive include vomiting, diarrhea, weight loss, and hyporexia. Clinical signs of cutaneous adverse food reactions include nonseasonal pruritus, otitis externa, miliary dermatitis, and eosinophic plaques or granulomas. The food trial is an important diagnostic tool for patients with gastrointestinal disease and dermatologic disease because it helps determine if these signs are related to the diet, or if they exist regardless of the diet fed to the patient.

How do food trials for gastrointestinal disease and cutaneous adverse food reactions differ?
The same protocol is used for both types of food trials, but the length of time differs. For gastrointestinal disease, the trial can last 2-4 weeks. Signs of improvement should be seen within that time if the clinical signs are diet-responsive. For cutaneous adverse food reactions, the time to achieve clinical improvement is longer; therefore, food trials for dermatologic diseases should last about 12 weeks.

What should I do before starting a food trial?
In addition to your typical history and physical examination, a thorough dietary history should be obtained in order to know what the pet is currently eating and what the pet has eaten in the past. The dietary history includes not only the primary food the pet consumes, but also the treats fed, table scraps, dietary supplements, flavored medications, and anything else the pet may be eating. The owners should be questioned about all past diets in order to select a diet for a food trial.

What is the protocol for a food elimination trial?
In order to do a food trial, all known dietary antigens should be eliminated in order to eliminate foods to which that the immune system may react, or to which the patient may be intolerant. This means all of the different ingredients the pet has consumed should be removed from the diet and new proteins and carbohydrate sources should be used. All flavored medications, treats, table scraps, and anything else the pet consumes should be eliminated from the diet as well.

Does serum allergy testing for food allergies rule out the need for a food trial?
Adverse food reactions are not necessarily mediated by an allergy, so serum allergy testing for food allergies is not the gold standard for diagnosing adverse food reactions. Gastrointestinal disease or cutaneous adverse food reactions can be mediated by the immune system (a true “allergy”) or may be due to a nonimmunologic reaction. Serum allergy testing does not look at immune reactions that occur at the level of the gastrointestinal tract nor does it catch food intolerances that are not mediated by IgE.

How do I select a diet for a food elimination trial?
A veterinary therapeutic diet or a complete and balanced diet formulated by a veterinary nutritionist should be used for a food trial. Over the counter commercial diets available from pet stores and grocery stores typically have cross contamination from other ingredients processed in a plant, as demonstrated by a recent study. Veterinary therapeutic diet manufacturers utilize dedicated production lines and take other precautions to avoid cross-contamination. The diet selection should be either a hydrolyzed protein diet or a novel ingredient diets.

What is the difference between a hydrolyzed protein and a novel ingredient diet?
Hydrolyzed protein diets contain a protein source that is broken down on a molecular level to pieces that should be too small to stimulate the immune system. In hydrolyzed diets, the carbohydrate source is intact (that is, not hydrolyzed). Novel ingredient diets include protein and carbohydrate sources to which the patient has never been exposed. Hydrolyzed diets are ideal for patients with unknown dietary histories or patients that have consumed most common novel protein sources.

If the patient has other health problems, what steps do I take to make sure the food trial does not affect other diseases?
Consulting with a veterinary nutritionist (ACVN diplomate) is a good option in these cases that have multiple problems. There are a couple moderate calorie options for overweight patients who need a hydrolyzed protein or novel ingredient diet, but most pet food companies do not have options for patients with, for example, renal disease and adverse food reactions. A complete and balanced homemade diet can be a good option in these cases that have multiple problems that all necessitate dietary modification. Veterinary nutritionists can be found at

What should I do at the end of the food elimination trial?
If a patient’s clinical signs improve during the food trial, the patient should be challenged with the dietary antigen at the end of the food trial to establish a true adverse food reaction. However, this is not always done – many owners are thrilled to have clinical improvement and do not want to try the old food. If the food trial does not work, move on to other diagnostics such as endoscopy or skin testing to determine the underlying cause of the clinical signs.

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