An adverse reaction to food is a repeatable, adverse response to a component of the diet.  Dermatological and/or gastrointestinal disturbances typically occur.  There are two types of adverse food reactions:

1.  Food allergy: Is an immune-mediated reaction to an antigen in the diet.  The most common type of allergens in dogs and cats are those arising from protein sources commonly found in their diets.  The most common food allergens come from beef, dairy, and chicken protein in dogs and beef, fish, and dairy protein in cats.  Clinical signs often occur following repeated exposure.
2.  Food intolerance: Is a non-immunologic reaction to a component of the diet that is repeatable and occurs on first exposure and/or subsequent exposures.  Intolerances can be predictable (lactose) or unpredictable (idiosyncratic).
   
   

How do we distinguish between atopy and food allergy?

Unfortunately, the clinical signs of allergic skin disease from food allergy and atopy largely overlap. The most common dermatological sign of food allergy is a non-seasonal pruritis, but these patients can also present solely with otitis externa in dogs or miliary dermatitis in cats.  Although allergic skin disease is reported to be the most common manifestation of food allergy, the true prevalence of gastrointestinal signs from food allergy are unknown.  This is likely because numerous non-allergic gastrointestinal diseases can also respond to an exclusion diet.  In addition, the gastrointestinal signs associated with food allergy are non-specific and include vomiting, diarrhea, flatulence, abdominal pain, weight loss, and failure to thrive. 

In general practice, it has been reported that atopy and flea allergy dermatitis are diagnosed more frequently than food allergy in dogs and cats presenting with only signs of allergic skin disease.  In these cases, atopy and flea allergy should be investigated first.  However, if a patient presents with both gastrointestinal and dermatological signs, food allergy is more common, and an exclusion diet should be strongly considered.

Does the time between contact with the offending antigen and clinical signs assist in differentiating a food allergy from other types of gastrointestinal disease?

Although it would be extremely helpful in the diagnosis if the clinical signs from a food allergy occurred immediately after consuming the offending substance, unfortunately, this temporal relationship does not always occur.  Food allergy can elicit a type I (immediate via IgE), type III (immune-complex), and type IV (delayed hypersensitivity) immune reaction. It is important that owners understand this when discussing food allergy, as they may initially disregard this differential diagnosis given the possible lapse in time between ingestion and clinical signs.  I find it imperative that an owner understands this, as it will improve their compliance with your diet recommendations!

How do we confirm a diagnosis of food allergy?

If you have a patient whom you suspect has a food allergy, the only way to support this diagnosis is with a resolution of the clinical signs when the offending antigen/allergen is removed from the diet and subsequent return of these signs when challenged with the original diet.  Although this is supportive of a food allergy, especially in a patient with dermatological and gastrointestinal signs, in a patient with just gastrointestinal signs, this improvement can be seen with food allergy, food intolerance, and food-responsive enteropathy, or it can be purely a coincidence.

Serologic testing for the diagnosis of food allergy has been investigated but is reported to be unreliable, with significant variation between laboratories.  In addition, results of serologic testing have not been shown to correlate with a patient’s response to an exclusion diet.  Intradermal and patch testing have also been shown to be unreliable.

Depending on the patient’s history and physical examination, frequency and severity of the clinical signs, as well as the financial limitations of the owners, the below tests may be warranted before considering a diet trial.  Please note this is not an exhaustive list, as many additional tests would depend on the patient’s clinical signs (Ex. Basal cortisol, TLI, cobalamin, etc). 

Tests to consider: 

• Fecal analysis
• Hematology and serum biochemistry
• Abdominal imaging (ideally ultrasound)
  
  

What are the available options for an exclusion/ elimination diet?

Categories of exclusion diets include home-prepared novel protein (ideally nutritionist-formulated), commercial novel protein, hydrolyzed protein, and elemental diets.  Although all diets can be equally effective, hydrolyzed diets offer an advantage in cases where the pet has an unclear diet history or has been exposed to numerous dietary proteins. While hydrolyzed diets do offer a lower risk of eliciting an allergic reaction, some patients may still react; and therefore, I often attempt to select a hydrolyzed diet that utilizes a novel protein.  The downside to these diets is palatability, so in some patients, the use of a hydrolyzed diet is not an option. In these cases, I recommend a commercial novel protein diet.

Prior to embarking on a diet trial, the owner’s expectations need to be managed so that they are compliant and do not become frustrated.  The owner must understand that it can take several weeks before an improvement in the gastrointestinal signs (1-4 weeks) and dermatological signs (4-8 weeks) are noted and that while on the elimination diet, the patient should not be fed any other diet, table scraps, flavored medications, supplements, or treats.  Lastly, all our patients are different, and I have had numerous patients that have required trials on two to three diets before finding a diet that results in the resolution of their clinical signs.  Long-term management of these patients requires avoidance of the identified ingredient or long-term use of a hydrolyzed or nutritionally complete novel protein diet.

References:

Hall EJ, Day MJ. Diseases of the small intestine.  In: Ettinger SJ, Feldman EC, et al. Textbook of veterinary internal medicine: diseases of the dog and cat. 8th edition. St. Louis (MO): Elsevier Saunders; 2017. P 1545-1547.

Purina Institute. Therapeutic nutrition: Food allergy and food intolerance. https://www.purinainstitute.com/centresquare/therapeutic-nutrition/food-allergy-versus-food-intolerance. July 2023.

Purina Institute. Therapeutic nutrition: Diet elimination trials. https://www.purinainstitute.com/centresquare/therapeutic-nutrition/diet-elimination-trials. July 2023.

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About the Author: Jessica Adamany,  BSc, DVM, DECVIM-CA, MRCVS

Dr. Jessica Adamany is originally a Maryland resident and graduated from the Virginia–Maryland Regional College of Veterinary Medicine in 2011. After completing a rotating internship at the Western College of Veterinary Medicine in Saskatoon, Canada she moved overseas where she undertook her internal medicine residency. She currently works at Pride Veterinary Centre, a large multi–disciplinary referral practice in England. Here she is involved in the supervision of 4 internal medicine residents and is the co–director of the rotating internship program. She is passionate about teaching and case discussion and enjoys the positive connections built through teamwork.