A Diagnostic Approach to Food Allergy

Food allergy patients are very challenging to diagnose but are very rewarding to treat. Once the offending allergens that cause a food allergy in a dog or cat are known, avoidance of these allergens results in control of the pruritus in most of these patients. The difficult step is identifying the offending allergens. Most clients do not fully understand how to perform a food trial correctly.

Adverse food reactions (AFRs) are defined as an abnormal reaction to an otherwise harmless ingested component in the diet. These reactions can be based on an immunologic basis (food allergy, also referred to as a dietary/food hypersensitivity), a nonimmunologic reaction (food intolerance), or a toxic reaction (intoxications). This article will be limited to discussing food allergy (FA) as it relates to dermatology patients. FA is defined as an abnormal adverse immune response caused by exposure to a specific ingredient in food, usually a protein. These responses can be IgE antibody-dependent, cell-mediated, or both. Whereas GI disease can be due to a food intolerance or a true food allergy, cutaneous disease is limited to a true food allergy.

Pruritus is the most common presenting complaint for canine and feline patients with food allergies. There is tremendous variation in the age, breed, distribution of lesions and degree of pruritus in these patients. FA should be considered in the differential diagnosis for any patient with non-seasonal pruritus.

Although there are saliva tests, intradermal tests and numerous serologic tests available to diagnose food allergies, NONE of these tests have ever been shown to be accurate. Not all tests have been studied, so it is not accurate to state that these tests have been shown to be inaccurate. Current recommendations by most veterinary dermatologists is to avoid using these tests to diagnose a food allergy due to their inaccuracy. Therefore, the only currently recommended test to diagnose a food allergic patient is via dietary avoidance trials. The assumptions for addressing a food allergic patient are as follows:

1) Assume the patient has a food allergy (this includes ruling out other potential causes of pruritus)

2) Assume the patient’s food allergy is due to one or more ingredients, usually assumed to be proteins, that he/she is ingesting on at least a periodic basis (daily to every few days)

In order to diagnose a food allergy, we need to:

3) Assume we can feed a truly hypoallergenic diet that avoids all allergens for the patient

4) Make sure the clients are 100% compliant in following directions for the diet trial

5) Make sure the patient is 100% compliant in eating ONLY the hypoallergenic diet

There are two main categories of food elimination diets: novel protein diets and protein hydrolysate diets. Novel protein diets work on the premise that the patient has not ever been exposed to the new protein and therefore has not been sensitized to it. These diets can be commercial or home-cooked diets. Protein hydrolysate diets work on the premise that the protein molecules have been hydrolyzed / reduced to a size that will not elicit an adverse reaction. Neither type of diet works in every food allergic patient. Novel proteins may not be novel. That is, there may have been unknown prior exposure to the

novel protein. Numerous studies suggest that cross-contamination occurs in dog and cat foods just as it does in human foods. Think of a child with a severe peanut allergy. Even though a candy bar may not state that it contains peanuts in the ingredient list, it should be avoided if it was produced in a facility that has peanuts in it. Likewise, a pet food that may not list ingredients such as lamb, rabbit or kangaroo in the ingredient list but may contain them if they are present in the same facility. Lamb and fish have been considered “novel” in the past, however they are now very common ingredients and are now considered inadequate as novel proteins. Most pet food manufacturers do not test their products for foreign proteins, so claims such as “grain free” or “limited ingredients” may not be entirely accurate. Many foods labeled grain free still contain grain and some grains contain protein. Additionally, some patients with a specific protein allergy may still react to a hydrolyzed protein diet that contains that protein (i.e., a dog with a chicken protein allergy may still react to a chicken protein hydrolyzed diet). This suggests the size of all chicken protein hydrolysate molecules is not small enough to prevent an allergic reaction. Home-cooked novel protein diets may eliminate this problem, but compliance and convenience may be issues.

My current recommendations for addressing a potentially food-allergic patient start with recommending the client try to forget everything they have heard, or think they know, about food allergies. Most of what they have heard or read on the internet is inaccurate. There are two important steps in performing a diet trial:

1) The hypoallergenic diet phase: Avoid feeding all offending allergens for a sufficient length of time to see at least a 50% improvement in pruritus;

2) A challenge phase: Challenge with the allergens that were avoided in step 1) to see if the pruritus returns.

In discussing the options for a strict food elimination diet trial, I recommend using a novel home-cooked diet or a prescription novel protein source (such as Royal Canin Rabbit / Potato) or a prescription protein hydrolysate diet (such as Hills Z/D Ultra, Royal Canin H/P, or Purina H/A diet for 3 months, or Royal Canin Ultamino diet for 6-8 weeks). Beef, chicken, dairy and wheat proteins are the most common food allergens. Contrary to popular opinion, more food allergies are due to animal proteins than plant proteins. It is important to avoid commercial, non-prescription (such as Natural Balance, Wellness, Blue Buffalo, etc) as some common diets have been shown to contain other proteins (cross-contamination) in addition to the primary hypoallergenic protein, thus compromising their efficacy. In addition to feeding an appropriate hypoallergenic diet, the other primary problem with many food elimination diet trials is client / patient compliance, in that they are not 100% strict. Any small morsel of food that the patient ingests and is allergic to will ruin the diet for several days to a week. The hypoallergenic diet will provide a complete, balanced diet. It will not make the patient any better if he/she has a food allergy! It is important to stress that any improvement in a reduction in pruritus occurs due to the patient avoiding exposure / ingestion of offending food allergens. This distinction is important to communicate to the client, so they understand the importance of a “STRICT” diet. Once there is at least a 50% reduction in the level of pruritus, which may take several weeks to several months, a diet challenge can be initiated. This part of the diet trial will confirm the improvement in pruritus is due to the hypoallergenic diet and not to another factor (such as a decrease in environmental allergens). At this point it is important to ask the question, “Does this patient have a food allergy?” In order to answer this question, feed all food ingredients that were avoided during the hypoallergenic diet phase of the trial. This includes feeding all

previously fed dog or cat foods, treats and human foods. This list typically includes a variety of ingredients. Most importantly, it should include proteins such as beef, chicken, dairy, chicken eggs, pork, fish, corn, wheat, soy, lamb and consider adding unique proteins such as buffalo, venison, white and sweet potato, peas and any other ingredient that previously fed to the patient. If a food allergy is present, the pruritus will increase significantly within a few hours to a week. It is usually dramatic. A gradual increase in pruritus over several weeks does not confirm a food allergy and some or all of the diet trial should be repeated. If a significant increase in pruritus occurs, it confirms the patient has a food allergy. In order to identify which allergens are causing the pruritus, an individual allergen trial needs to be performed. This entails adding a single ingredient to the hypoallergenic diet for up to one week. For example, add cooked hamburger as the single ingredient challenge for beef. Use cooked chicken meat as the single ingredient challenge for chicken. If pruritus increases with the single ingredient challenge, the patient is allergic to that ingredient. If no increase in pruritus occurs, the patient is not allergic to that ingredient. All ingredients that were added to the diet during the challenge phase should be given as individual ingredients. This is laborious and time consuming but will allow complete identification of all offending allergens. Most food allergic patients react to 2-5 different allergens, not just one. Once all the offending allergens have been identified, a new diet that does not contain any of the offending allergens can be used as a maintenance diet.

Submitted by: Dr. Kevin Shanley


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