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Cutaneous adverse food reactions are a lifelong disorder, but when diagnosed correctly they can be easily managed through diet.
The concept of elimination is relatively simple, but can be very challenging due to the cost of prescription foods and the length of time it takes to trial the diet. Food allergies, known as cutaneous adverse food reactions (CAFR), are immunologic reactions to food allergens. CAFR is thought to affect approximately 20 percent of dogs and 13 percent of cats with allergic skin disease. Proteins such as beef, chicken, dairy, soy, fish, and eggs are common allergens thought to trigger food allergies in dogs and cats.
Common symptoms
The most common symptoms associated with cutaneous adverse food reactions include: nonseasonal pruritus, usually on the face and neck, perianal pruritus, anal sacculitis, otitis externa, and recurrent secondary infections. In some cases, concurrent noncutaneous symptoms of a food allergy may include: gastrointestinal signs such as vomiting, diarrhea, bloating, conjunctivitis, respiratory signs, urticaria, seizures, and behavioral problems.
Diagnosing or ruling out food allergies is a very important step in the allergenic workup for any itchy pet. However, there is a lot of misinformation about the accuracy of food allergy testing.
Elimination diet trials currently remain the gold standard for diagnosing cutaneous adverse food reactions in dogs and cats, but are they the best test? While the short answer is yes, there are certain factors to consider.
Which factors should be considered?
Elimination diet trials with prescription foods or homemade diets with subsequent protein loading are considered the most accurate tests for diagnosing cutaneous adverse food reactions. They are performed by feeding a single new or single hydrolyzed protein source and monitoring for improvement in cutaneous signs associated with CAFR. If improvement is seen during or at the end of the diet trial, subsequent protein loading is recommended to confirm the diagnosis of a cutaneous adverse food reaction.
Although the concept of elimination is relatively simple, it can be very challenging due to the pricing of prescription foods and the long duration of the diet trial. Therefore, it is important to understand and explain the rationale behind an elimination diet trial before making recommendations to pet owners.
Trial duration
A 2015 study found that the ideal duration of a diet trial is eight weeks. By eight weeks, 90 percent of food-allergic dogs and cats show improvement in clinical signs associated with CAFR.
Unfortunately, one of the main challenges in elimination diet trials is the persistence of caregiver follow-up. The same review also found that 80 percent of dogs will show improvement in clinical signs by five weeks and 80 percent of cats will show remission within six weeks of a diet trial. Shorter periods, while not ideal, may improve compliance and may help diagnose cutaneous adverse food reactions in the majority of cases.
Diet trial options
Homemade diets are still considered an ideal option for elimination diets because they do not contain any additives that rarely trigger food allergies in dogs and cats. However, they are often impractical for caregivers to prepare and are less likely to be nutritionally balanced. Therefore, most veterinarians focus on commercially available prescription novel protein or hydrolyzed protein diets for elimination diet trials.
Another challenge in diet trials is the cost of prescription diets, especially in larger dogs or multi-pet households, and it is hoped that over-the-counter alternatives will be less costly. However, it is important to understand why these diets are not equivalent to a diet trial. Studies comparing labeled ingredients in commercial diets with ELISA or PCR results assessing unlabeled protein content have shown that many OTC diets labeled as new or limited ingredients contain ingredients not listed on the label. Potential exposure to unlisted ingredients makes OTC commercial diets an unreliable option for elimination diet trials. To ensure accurate results, prescription novel or hydrolyzed diets are recommended to minimize unexpected sources of protein that could skew the results of the diet trial.
Hydrolyzed or novel protein diet preference
A novel protein is obtained from an animal that has not been exposed before, and therefore, the animal is unlikely to have any prior sensitivity to that protein.
Prescription hypoallergenic diets have historically focused on novel proteins such as rabbit, kangaroo, and venison. Hydrolyzed diets contain proteins that have been size-fractionated to reduce the allergenicity of the molecules. Many of these are considered more palatable than novel protein diets; This can help the patient and the patient's family adapt to the process. In addition, according to many case studies, allergenicity is greatly reduced in hydrolyzed diets, and some patients who are allergic to the main protein (i.e. chicken) react to hydrolyzed versions of the main protein. Despite their disadvantages, hydrolyzed diets tend to be preferred over the new protein.
Saliva and hair test
Unfortunately, many relatives of patients want to avoid intensive diet trials that can take a long time to get results; they are attracted to the easy and quick answers offered by saliva and hair tests. Saliva and hair tests are very affordable, easy to apply, fast-resulting tests, and are used in different parts of the world.
However, many studies, including a study conducted in 2019, have revealed that many companies offering these tests cannot confirm the accuracy of their tests, and therefore can be an unreliable test for food allergies. Additionally, studies evaluating the accuracy of these tests have provided samples from allergic pets, nonallergic pets, or sham controls (i.e., water for saliva and fake fur for animal hair) and have had similar variable positive results between groups.
As a result, hair and saliva tests should not be considered a valid substitute for an elimination diet trial due to the lack of reliability of the results.
Serologic tests
Serologic testing measures allergen-specific IgE levels in blood samples. This test is widely used in human medicine for the diagnosis of food-specific allergens. However, serologic testing in dogs and cats has been found to have low reliability, low reproducibility, and high variability in accuracy. One of the major obstacles to this testing method is the unknown role of IgE in the pathomechanisms of CAFR, and the presence or level of IgE does not accurately predict food allergies in dogs.
In addition, when serum results are tested with food challenge tests, there is significant variability in positive results and subsequent cutaneous reactions upon challenge with food-specific allergens. All of this supports the variable accuracy of these tests. As with most dermatology cases, cutaneous adverse food reactions are a lifelong condition, but when properly diagnosed, can be easily managed with diet alone, making the dietary challenge a very important diagnostic test.
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