One of the least understood hypersensitivity disorders in either dogs or cats is food allergy. The reported prevalence varies greatly and many studies have not confirmed the diagnosis by dietary challenge. We lack good knowledge of the pathogenesis of these disorders in dogs and cats, and therefore have difficulty in making the diagnosis efficiently and correctly. Yet a food allergy is likely to be the easiest to control: We simply avoid the offending allergens.

There are many public misconceptions about food allergy in pets. Many people believe that a change in diet precipitates a food allergy, but the reality is that some exposure to a protein is most often required to break immune tolerance. A sudden food change might induce gastrointestinal upset, but not because it is allergenic. Another popular misconception is that feeding a "hypoallergenic" diet has a dominant effect! Thus, if one feeds such a diet, the treats, flavored medications, toys, and table scraps won't matter. Nothing could be further from the truth; it is important to emphasize to our clients that that helping a pet with food allergy requires not eating offending foods. A third possible misconception is that some foods are naturally hypoallergenic. This misconception has been exploited by the pet food industry, particularly those newcomers producing foods for over-the-counter sales. As soon as we find novel proteins that are useful in veterinary prescription diets, they are coopted by over-the-counter diets and then become useless for diet trials. A fourth misconception is that grain-free diets are hypoallergenic. These diets will only be useful for those food allergic pets that have a hypersensitivity to individual grains. Grains in pet diets are not necessarily inappropriate for most dogs and cats. A veterinary misconception is that food allergy is a disease that fails to response to steroids. While this is true in some cases, we have found that many cats and dogs with food allergy can experience significant relief from steroids and we often use them to provide some relief when we first start our diet trials.

Food allergy is only one type of adverse food reaction, and is characterized by involvement of the immune system in the pathogenesis of disease. Other adverse food reactions include food intolerances, pharmacologic reactions, and toxin-mediated reactions. Food allergies in people are divided into IgE-mediated and non-IgE-(cellular) mediated reactions. The best understood of these latter include eosinophil-mediated diseases of the gastrointestinal tract, but there are other inflammatory diseases of the gut that have been attributed to food hypersensitivity. Even in humans, food allergies are difficult to understand. There is increasing evidence, however, that abnormal immunological reactions to foods may be involved in a variety of inflammatory bowel diseases, including gluten enteropathy and Crohn's disease.

The major dermatologic sign of food allergy in cats, as well as dogs, is pruritus. The classic food allergic cat has intense pruritus and evidence of self-trauma around the head and neck, but cats can also have one of the triad of syndromes we often associate with allergic skin disease: hair-pulling/over-grooming, military dermatitis, and eosinophilic granuloma complex, particularly eosinophilic plaque. Our suspicions of food allergy are increased in pets with concurrent dermatologic and gastrointestinal disease, but either skin disease or gastrointestinal disease can occur as the sole organ involved. It is of interest to me that many cats are treated chronically for inflammatory bowel disease; it seems prudent to try to determine whether dietary management could help these patients, in order to avoid the use of chronic steroid therapy.

Diagnosis of food allergy can be difficult. Most veterinary dermatologists agree that the diet history and a diet trial remain the gold standard for diagnosis, but we don't agree on what constitutes the appropriate diet and how long it should be run. Furthermore, diet trials are dependent on the owner's ability to adhere to them, and to recognize and track the changes that occur when potentially allergenic foods are reintroduced into the diet. Some clients are more motivated than others to explore this option. The diagnosis of food allergy can be complicated when the pet has atopic dermatitis. It is often advocated to rule out food allergy first prior to diagnosis and treatment of atopic dermatitis. Yet pets with both disorders may not show sufficient improvement during the food trial for an owner to recognize improvement. It is not helpful therefore to make hard and fast rules about how to approach the diagnosis of these disorders. Each patient needs to be addressed individually.

The first step in diagnosing food allergy is a careful diet history. In areas with limited pollen seasons, seasonal pruritus rules out the need to pursue food allergy. For the vast majority of cats and dogs, food allergy is a nonseasonal disease. A careful food history is very important, but often hard to elicit. What we need to know is what proteins and carbohydrates the animal has eaten in the past. Most clients don't read the list of ingredients on the label and they may forget about treats, table scraps, flavored medications, and edible toys. Cats that go outdoors may be eating a variety of things that we will never discover. Many clients believe that a change in diet will precipitate food allergy but that simply is not the case. It is difficult to break gastrointestinal immune tolerance. Fairly long term exposure to the dietary allergen in question is the norm.

The second step in diagnosing food allergy is selecting the appropriate diet for the trial. There is no one diet that will work for all patients. Some veterinary dermatologists believe that the only appropriate hypoallergenic diet is a home-cooked diet; however, I am not one of them. My personal belief is that the advent of good quality, truly hypoallergenic diets has enabled us to make the diagnosis of food allergy successfully for a larger number of patients. There are two types of commercial diets currently available: limited ingredient novel protein diets and hydrolyzed diets. Picking a novel protein can be difficult because of our lack of knowledge about cross-reactive proteins in dogs and cats. There is immunologic evidence that IgE from beef-allergic humans can sometimes cross-react with venison and lamb, and that IgE from chicken-allergic humans can cross-react with duck. We don't have that data for animals, so it seems prudent to pick the most novel protein we can. This might be the rabbit and pea diet for cats, or the rabbit and potato or kangaroo and oat diet for dogs. Fish diets may not be novel enough anymore to use as elimination diets for either dogs or cats. Over-the-counter diets are not good choices for food trials. Recent studies have shown that these diets are contaminated by small amounts of chicken or beef or other dietary ingredients; thus, they are not truly limited ingredient diets and are poor choices for diagnosis, although for some patients they might be suitable for feeding after the diagnosis has been made. The alternative to the novel protein diet is the hydrolyzed diet. Currently we have a choice between hydrolyzed chicken and hydrolyzed soy. There are several extra elements to consider when we are trying to use a diet for cats with suspected food allergy. First, will they eat it? While we can bully most dogs into eating our choice, we can't take this approach for cats, and we don't want to try to starve them into submission, for fear of inducing hepatic lipidosis. Our strategy has been to offer 3–4 samples of each of the diets we stock and let the cat choose. Then we feed the one the cat will eat! The second situation is the multiple cat household and how best to insure that the patient eats only its hypoallergenic diet. Sometimes the easiest strategy is to feed all the cats the hypoallergenic diet. The third dilemma is the outside cat. There is probably no point in trying to do a diet trial for an outdoor cat, unless they can be kept indoors for the duration of the trial.

Convincing owners to be strict with the food trial can be difficult. Many clients enjoy feeding their cats and dogs treats and if we don't honor this need, the diet trial will likely fail. For dogs, it is easy to provide treats; in the USA, Select Care treats (Royal Canin) work well, as do Gentle Snackers (Purina). Sam's Yams are good substitutes for rawhide chews, and we have allowed carrots, green beans, apples, and bananas for dogs (if they have not been in the diet before). Most cats in our practice do not seem to get lots of treats; a few will actually eat banana. If they need a treat, perhaps canned rabbit/pea could be provided in small morsels. The difficulty for many cats comes when we need to control what flavors are put into their medications. It is important to talk to your compounding pharmacist about whether the flavorings used are natural or artificial. If natural, alternative ways to medicate the cat may be needed for the duration of the food trial.

Once the diet trial is accepted by the client, the next step is to decide how long to feed. My feeling is that if a cat or dog truly has a food allergy, we should see signs of improvement within 6–8 weeks. There may not be complete resolution of the signs but the response should be significant enough for the client to observe. If there is no improvement in that time, I believe that food allergy is much less likely, although we can't rule out the possibility that we chose the wrong diet! At this point, though, it makes sense to move on to other causes of pruritus. A strategy I like to employ is to start the diet and medicate with steroids to reduce itch for the first 4–6 weeks of the diet, then stop the steroids. If the patient relapses, then food allergy seems less likely to me.

The third step in diagnosing food allergy is to confirm it with food challenges. I believe this is very important. If we can determine what foods precipitate itch, we can get our patients back onto a commercial diet and simply avoid the offending protein. This gives the client and the pet more freedom with diet choices and selection of treats. It also avoids the dilemma of what to feed if he/she runs out of food at a time when the veterinary office is closed. Many clients will prefer not to go through this phase and continue to feed the elimination diet. It is their choice, but it has made it difficult for us to determine what foods have been associated with food allergy in cats. In one study, food allergy was diagnosed in 10 cats; the owners of 9 of them refused to perform any challenges to determine the cause. If clients have used a home-cooked diet and wish to continue cooking, it is best to make sure that the diet is complete and balanced by having it formulated by a veterinary nutritionist. These types of consults are readily available on line (https://secure.balanceit.com (VIN editor: Original link modified 9-23-15.) and https://www.petdiets.com).

Is there a role for skin testing or blood allergy testing in the diagnosis of food allergy? Skin testing has proved uniformly unreliable and none of the veterinary dermatologists will recommend this test. Blood allergy testing is available from several companies. Most veterinary dermatologists have found these tests unhelpful as well. Recently, though, Dr. Richard Halliwell has suggested that food allergy screens may be useful if positive. Foods against which IgE are directed should be avoided, but having a negative reaction does not make a food "safe." Certainly a great deal of data is available in the archives of the companies who run the tests, but none of these data have been published.

The treatment of food allergy is simple: avoidance of the foods that precipitate the disease. In general we find that our food allergic pets, whether they are cats or dogs, react to 1–3 proteins in their diet. It is quite easy to find diets that do not contain the offending ingredients in specialty pet food stores these days.

Many pets with food allergy have other allergies as well. Concurrent flea allergy and food allergy is quite common, and it is not unusual for food allergic pets to have concurrent atopic dermatitis/atopy. In a retrospective study performed in Dr. Linda Messinger's practice in Englewood Colorado, we discovered that half of our food allergic patients, whether cats or dogs, were also atopic. Determining the underlying cause of pruritus in cats and dogs, therefore, can be quite complicated and can take a considerable amount of time. We haven't even begun to explore the possibility that veterinary patients could have oral allergy syndrome as humans do. These are allergic patients that have known pollen allergies that can cross-react with certain foods. Typical examples include ragweed allergic people who can't eat cantaloupes, or birch allergic people who have trouble with apples.

I have become convinced that we cannot truly rule out a food-related disorder for all patients with one diet trial. We often find with atopic patients that the first food trial, performed before skin testing and immunotherapy, can be difficult to interpret. Once the diagnosis of atopic dermatitis has been made and immunotherapy started, it may be helpful to come back to food allergy again, particularly if immunotherapy is only partially successful. If one is convinced food allergy is involved and a hydrolyzed diet has been used initially with no success, it is always worth trying a novel protein diet.

References

1.  Cave NJ. Hydrolyzed protein diets for dogs and cats. Vet Clin North Am Small Anim Pract. 2006;36(6):1251–1268, vi.

2.  Gaschen FP, Merchant SR. Adverse food reactions in dogs and cats. Vet Clin North Am Small Anim Pract. 2011;41(2):361–379.

3.  Hardy JI, Hendricks A, Loeffler A, Chang YM, Verheyen KL, Garden OA, Bond R. Food-specific serum IgE and IgG reactivity in dogs with and without skin disease: lack of correlation between laboratories. Vet Dermatol. 2014.

4.  Hensel P. Nutrition and skin diseases in veterinary medicine. Clin Dermatol. 2010;28(6):686–693.

5.  Jackson HA. Diagnostic techniques in dermatology: the investigation and diagnosis of adverse food reactions in dogs and cats. Clin Tech Small Anim Pract. 2001;16(4):233–235.

6.  Mandigers P, German AJ. Dietary hypersensitivity in cats and dogs. Tijdschr Diergeneeskd. 2010;135(19):706–710.

7.  Olivry T, Bizikova P. A systematic review of the evidence of reduced allergenicity and clinical benefit of food hydrolysates in dogs with cutaneous adverse food reactions. Vet Dermatol. 2010;21(1):32–41.

8.  Verlinden A, Hesta M, Millet S, Janssens GP. Food allergy in dogs and cats: a review. Crit Rev Food Sci Nutr. 2006;46(3):259–273.