Itchy Pet Month: Food Allergy Dermatitis

Food Allergy Dermatitis: How To Perform A Proper Food Trial

By Tomeshia Hubbard, DVM 


Food allergy is an immune mediated non-toxic adverse reaction to foods. The most common type of food allergy is mediated by immunoglobulin E (IgE) antibodies and reflects an immediate type (Type I hypersensitivity) reaction which results in an acute onset of symptoms following ingestion or inhalation of food substances. IgE antibodies against specific food allergens are produced by the immune system and bind to high affinity receptors on the surface of mast cells and basophils. Upon re-exposure to the food substances, the IgE antibodies cross link and trigger the release of inflammatory mediators which include histamine, tryptase and tumor necrosis factor leading to the acute phase of the immediate hypersensitivity reaction. Additional newly synthesized mediators such as leukotrienes, prostaglandins and cytokines also lead to recruitment of inflammatory cells leading to IgE late phase responses. 

Adverse food reactions include allergic reactions termed food allergy and non-allergic reactions termed food intolerance. These terms are frequently used interchangeably because the exact immunologic process of the adverse food reaction is not fully known. Adverse food reactions are most commonly the result of a Type I, III or IV hypersensitivity reaction. 

Food allergy is approximately 10% as common as atopic dermatitis in dogs and probably as common as atopic dermatitis in cats. The age of onset can range from 2 months of age to 14 years of age. Most reports do not indicate a breed predilection; however, Labrador Retrievers, West Highland White Terriers and Cocker Spaniels may be over-represented. The history normally involves non-seasonal pruritus with little variability in pruritus from season to season. 

Clinical Signs 

The clinical signs of adverse food reaction in dogs include non-seasonal pruritus, otitis, dermatitis, recurring pyoderma, seborrhea, urticaria and gastrointestinal signs. The gastrointestinal signs can include flatulence, vomiting, diarrhea and having more than two bowel movements per day. Food allergies common affect the ears, rear and paws of dogs. The clinical signs in cats include pruritus of the head and neck, self induced alopecia and eosinophilic granuloma complex lesions. 


The only way to accurately confirm a diagnosis of adverse food reaction (food allergy) is to perform an elimination food trial. Provocative diet challenge is used to definitively diagnose food allergies. Most food allergic patients are not very responsive to corticosteroids, so this can also point towards a diagnosis of food allergy. Intradermal allergy testing, serologic allergy testing and patch testing are not adequate to confirm a diagnosis of food allergy due to the poor reliability of test results. 

Elimination Dietary Trial 

Choose the best elimination diet for the patient 

  • It should consist of a protein the pet has never been exposed previously (rabbit, kangaroo) 
  • It should not cross react with previously fed diets (venison may cross react with beef) 
  • It should be palatable to the pet 
  • The owner should be willing to feed and pay for it 

Novel vs. Hydrolyzed Protein 

  • Hydrolyzed protein diets are an additional option for an elimination food trial. There is some concern as it relates to the effectiveness of the hydrolyzed protein for allergic patients, in that 10%-40% of patients continue to show clinical signs on the hydrolyzed protein diets. Some current commercial options include hydrolyzed soy, chicken, feathers and salmon. Better results are achieved with hydrolyzed protein diets which contain a protein source the pet has not had a lot of exposure to. 

Therapeutic vs. OTC Diets 

  • Numerous over the counter diets are available, however many of them contain ingredients that are not listed on the bag. These OTC diet may contain poultry, beef, pork and dairy products that you may be unaware due to contamination from other diet that are prepared in the same factory. Therapeutic diets are more reliable because of the excellent quality control provided by the companies who sell these diets. Therapeutic diets are also more expensive due to this process and a medical prescription is required to purchase these diets. 

Starting the elimination trial 

  • Feed only the chosen diet (novel or hydrolyzed) for a period of 8 weeks to cleanse the system. The owner should not be allowed to feed the pet any treats, flavored medications or table food. All flavored flea and heartworm preventatives should be discontinued and replaced with topical products or non-flavored options. Be sure that all secondary infections (pyoderma and Malassezia) are completely resolved and adequate flea control is being used. Clients are allowed to feed their pets the canned version of the novel or hydrolyzed diet if available, which can help owners administer medications during the elimination trial. 

Starting diet challenges 

  • Following the 8 weeks of cleansing the system, the pet should be challenged with individual protein, carbohydrate and dairy sources. I normally have owners challenge with chicken, rice, beef, fish, pork, lamb, soy and dairy (cheese, milk). Owners will challenge with poultry first, as it is in most over the counter pet foods. Each challenge will be done individually for a period of 7 days or until the pet shows clinical signs of an adverse food reaction. If a reaction occurs, the challenge food is then discontinued and the novel/hydrolyzed diet is fed until the reaction subsides completely. Once the reaction has subsided, the next challenge is started. If there is no reaction during the 7 day period of the challenge, the pet is not allergic to that challenge food and it is considered safe for the pet to eat in the future. There should be a 3 days washout period (feed only the novel/hydrolyzed diet) between each challenge routinely. 


If the elimination trial appears to have been adequately performed and the pet does not improve, you should consider atopic dermatitis as a diagnosis. Remember adverse food reaction/food allergy is a life-long condition that cannot be cured, but can be managed long term. 

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