Food Allergies and Intolerance in Pets

By: Catherine Lenox, DVM, Diplomate ACVN

TVMA Member
Houston, TX

Published March 2014

Food Allergies and Food Intolerances

Adverse reactions to food include allergies or intolerance, which can be common in cats and dogs. Food intolerance is a broad term encompassing all adverse reactions to a pet’s diet that are not mediated by the immune system. Food allergies are caused by an immune reaction. It is often difficult to differentiate between food allergies or food intolerances, therefore they might better be described as adverse food reactions.  These kind of allergies and intolerances have similar clinical signs—primarily skin-related (cutaneousOf, relating to, or affecting the skin.) and/or stomach or intestine-related (gastrointestinalOf or relating to the stomach and the intestines) signs.

Food allergies are in response to a dietary allergen, which is a protein that stimulates the immune system. Common allergens include chicken, beef and fish proteins. On the other hand, food intolerances are not always in response to proteins and can include reactions to other components of the diet such as an inability to digest certain nutrients (e.g., lactose intolerance Is the inability to digest lactose, a sugar found in milk and to a lesser extent dairy products.) or toxins in the diet.

Diagnosis by Food Trial

Dogs and cats can show the signs of allergies and intolerance in several ways. Gastrointestinal signs associated with allergies and intolerance can include vomiting, diarrhea and decreased appetite. The skin can also show signs of reactions, including itching, redness, chronic ear infections and self-induced trauma to the skin from chewing, scratching or licking.

Diagnosis of adverse food allergies and intolerance is based on a dietary elimination trial, or “food trial.” This involves removing the suspect ingredient(s) from the diet to see if the clinical signs resolve after elimination of the ingredient in question. Sometimes the dog or cat will be challenged at the end of the elimination period, which involves reintroducing the suspect ingredient(s) to see if the clinical signs return. This ensures that the suspect ingredient(s) was the true culprit. Many pet owners will choose to skip the reintroduction step if clinical signs resolve during the trial, but it is difficult to determine what solved the problem if other treatments were also used during the trial. Blood tests also can be run for food allergies, but they are not always accurate.

Veterinary therapeutic diets, also known as prescription diets, that are available from your veterinarian should be used for food trials. Standard over-the-counter diets should not be used for trials. Over-the-counter diets are typically not produced on dedicated production lines; therefore, they may be contaminated with other food products, making it difficult to totally eliminate suspect ingredients. Veterinary therapeutic diets that can be used for a trial include novel ingredient diets or hydrolyzed protein diets. Novel ingredient diets contain ingredients to which the pet has never been exposed. Novel proteins typically include duck, venison, rabbit or other protein sources that are not commonly found in pet food. Some novel ingredient diets also have carbohydrate sources that are unique as well, including potato, oat or green pea. Hydrolyzed protein diets contain a protein source that is broken down on a molecular level to a size that is too small to stimulate the immune system.

Food Trials for a Special Diet

If your pet has a pre-existing health problem that necessitates a special diet, such as kidney disease, urinary stones or pancreatitis, an owner should consider consulting with a veterinary nutritionist for advice on how to perform the food trial. A veterinary nutritionist can recommend a complete and balanced homemade diet that meets all dietary needs of a patient, or they may prescribe a different type of veterinary therapeutic diet. Veterinary nutritionists are board-certified by the American College of Veterinary Nutrition and can be found at

What if the Problem isn’t an Allergy or Intolerance?

A proper food trial should last two to four weeks when gastrointestinal signs are present and eight to 12 weeks when skin (cutaneous) signs are present. Patients with gastrointestinal signs tend to show improvement faster than patients with skin (cutaneous) signs. If the pet does not show improvement within these time periods, it is unlikely that adverse food allergies and intolerance are the cause of the problem. In those cases, additional diagnostics may be necessary to determine the underlying cause of the clinical signs.

Dr. Catherine Lenox owns a consulting business, Lenox Veterinary Nutrition Consulting, PLLC.  She does nutrition consults (mainly homemade diet formulation) and acupuncture for pets and owners in Houston.

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