Addressing Food Allergies and Intolerances in Pets
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Addressing Food Allergies and Intolerances in Pets

by Catherine Lenox, DVM, DACVIM (Nutrition)
Mar 21, 2024

What is the difference between a food allergy and a food intolerance? 

Food allergy and food intolerance are causes of gastrointestinal and skin problems. They often have similar appearances, but for each, there is a different mechanism causing the signs. Food allergies are caused by the immune system reacting to a food or food component, while signs of food intolerance are not mediated by the immune system. Food intolerance can be caused by an inability to digest or absorb something in the food or another reaction unrelated to the immune system.

An example of food intolerance is lactose intolerance in people – people can have severe signs after eating dairy products with lactose, but it’s not an allergic reaction, it’s caused by an inability to digest the lactose.

Food allergies and food intolerances are often combined into one category, adverse food reactions, or AFR.

What are signs that my dog or cat might have AFR?

Dogs and cats often both present with skin signs – itching, scratching, and causing trauma to the skin. There can also be secondary bacterial or yeast infections and/or hair loss. However, many other skin conditions can manifest this way, so your veterinarian will often rule out flea allergies or other causes of itching before trying to diagnose AFR.

A big indication that a dog or cat has AFR is when gastrointestinal signs occur with skin signs. These gastrointestinal signs can include diarrhea, vomiting, gas, loud gastrointestinal gurgling, or increased frequency of defecation and they can be the only signs of an AFR, or they can occur with skin signs such as itching or scratching.

Dogs may also get frequent ear infections, and cats can have upper respiratory signs associated with food allergies. Food allergies and intolerances are typically nonseasonal, meaning the signs occur year-round instead of during one part of the year.

What are common foods that cause AFR?

The most common causes of AFR in dogs are beef, chicken, and dairy. Fish is a common food that causes AFR in cats, in addition to chicken and beef (Source: Critically appraised topic on adverse food reactions of companion animals (3): prevalence of cutaneous adverse food reactions in dogs and cats - PubMed (

How can a Board Certified Veterinary Nutritionist or a primary care veterinarian diagnose AFR?

Although there are many different options for diagnosing AFR, the gold standard is a food elimination trial followed by a challenge under the supervision of a veterinarian. During a food trial, all potential problem foods should be removed from the diet and a specific diet that helps eliminate allergens and problem ingredients should be fed for the duration of the trial. This means many treats, human foods, table scraps, supplements, flavored medications, and toothpaste must also be avoided during the trial because of common allergens or problem food components. Your veterinarian will get a thorough diet history to help determine potential sources of allergens in your pet’s diet.

If skin signs are involved, potential allergens must be avoided for about 8 weeks, and if gastrointestinal signs are the only concern, the food trial can be about 2-3 weeks. If the patient does not have AFR, there will be no improvement at the end of the trial. If improvement occurs, the challenge portion should be done to confirm the diagnosis. A challenge involves feeding the previous diet or single ingredient and monitoring for a return of signs. The signs can often return within a few hours, but the challenge is considered unsuccessful if no reaction occurs within one week. For a true diagnosis, the food trial has to be done properly, with no additional foods or treats fed for the duration of the trial. This can be a concern if there is no response to the trial but AFR are strongly suspected.

With skin signs, medication is often used to help reduce itching at the start of the food trial. If the pet improves, the medication should be stopped before the end of the food trial to see if the medication or food was the cause for improvement.

What types of diets can be used to diagnose and manage AFR?

The only diets that can be used to truly diagnose AFR eliminate all potential dietary allergens to allow for an effective food elimination trial. This means that commercially available (pet store) brands are usually ineffective for diagnosis because they are not produced to be free of potential allergens. Diets that should be used include veterinary therapeutic (recommended by your veterinarian) hydrolyzed protein diets, veterinary therapeutic novel protein diets, or properly formulated home-cooked novel protein diets. Hydrolyzed proteins are broken down on a molecular level to improve digestion and absorption and can reduce the chance of immune reactions. Novel proteins are proteins the pet has never eaten before, which reduces the change of a reaction to the protein source. Hydrolyzed protein diets are often preferred because they are easy to digest and absorb, and because using a novel protein diet requires your veterinarian to know about every protein source your pet has ever eaten during his or her life.

If a pet has other medical conditions or if the pet owner prefers to cook for their pet, a home-cooked novel protein diet can be properly formulated by a board-certified veterinary nutritionist for the trial. For all types of diets, the diet used for the trial can be fed long-term if the diagnosis of food allergy or intolerance is confirmed, provided it is complete and balanced and meets all the pet’s nutritional requirements.

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