An adverse, immunologic reaction to a normally harmless food protein is abnormal. It is unknown why some animals develop allergies or adverse reactions to food proteins are others do not. Cutaneous (chronic itching) and gastrointestinal symptoms (chronic and recurrent vomiting and/or diarrhea) are common symptoms associated with food adverse reactions. In some studies the incidence has been reported as high as 57% of cats and 20 to 40% of dogs with chronic itch or GI symptoms can be attributed to “food allergy”. Reactions to dietary allergens can be classified as food allergy (an immune-mediated adverse reaction to a food antigen), adverse food reaction (an abnormal response to a dietary constituent) or food intolerance (food poisoning or an idiosyncratic or metabolic reaction to a food). In clinical practice the term “food allergy” is commonly used to refer to food adverse reactions (“FAR”).
In most cases the classic symptoms that dogs and cats experience as a results of an immune based reaction to dietary allergens start at a young age (less than six to nine months of age) and tend to exhibit a non-seasonal occurrence. However food allergy can develop at any time throughout their life since diet influences the type of food allergy and because an animal cannot become allergic to an item that it has never been exposed to before. Since most of our pets eat a variety of treats and food items and their diet is constantly changing they can develop food adverse reactions at any age. Additionally dietary allergies (by definition) are related to previous exposure to the allergen so it is important to remember that they can develop the reaction to a food allergen that “they have always eaten before”.
With cutaneous signs they can itch around the face, ears, ventral abdomen, paws, axillary, inguinal and perianal regions. Sometimes the skin changes can be quite severe and they can develop crusting, ulcerations and severe redness and mimic immune-mediated skin disease. Gastrointestinal symptoms can involve vomiting, diarrhea, increased frequency of bowel movements, straining to defecate, excessive “scooting” and/or intermittent blood or mucus in their stool. There is a genetic predisposition with some dog breeds being overrepresented (Retrievers and German Shepherds).
It is important to have ruled out other conditions as well and a through history and diagnostic work up (if appropriate) is always warranted with chronic conditions. The symptoms of chronic itching can be related to environmental allergies (atopy) and there is cross-reactivity meaning that many patients that have atopy can also suffer from food adverse cutaneous reactions (as high as 20%). Contact hypersensitivity, secondary bacterial (pyoderma) and yeast (Malasezzia) dermatitis or otitis (ear infections), parasitic hypersensitivities (like flea allergy) and parasites (mites) can also contribute to chronic skin disease. Intestinal parasites, infiltrative gastrointestinal disease, inflammatory bowel disease, gastrointestinal cancer and many other GI conditions can cause chronic GI symptoms.
Dietary trials are the gold standard for diagnosis: we feed them a diet that contains a single novel protein and a single (preferably novel) carbohydrate for a minimum of 12 weeks and if improvement is noted, the food allergy/intolerance should be confirmed by challenging the pet with the original diet. Hydrolyzed protein diets that are commercially available (by prescription only) are also a great option. The hydrolyzed protein diets basically means that the protein allergen has been “hydrolyzed” or broken down to a molecular size that is so small that is considered non allergenic (typically chicken or soy based). This is due to the fact that the majority (98%) of the food adverse reactions are due to protein allergens. Therefore a “grain-free” diet (if it contains the same protein your pet is currently eating or has been exposed to before) is not a true limited antigen diet! Additionally there is a risk of cross contamination during the manufacturing and packaging in many of the over the counter commercially available diets so you must read the fine print in the label. Dietary trials need to be discussed with your pet’s doctor. They are familiar with your pet’s medical history and trained in nutrition and are always a better source of information than the Dr.Google or the retail sales representative at the pet store. A thorough history is extremely important for selection of the diet as well as proper feeding recommendations. Treats, chewable parasite preventatives and supplements should also be discussed. We cannot do a dietary trial on a “free-roaming” pet since we need complete control of what they are eating. Everyone in the family must be committed to the trial since even if it is “just a little piece of chicken” can invalidate their response to the dietary trial. A good analogy I relate to my clients is peanut allergy in people: you only need to eat one peanut to have a severe reaction! Finally, if you choose to cook for your pet you must ensure that it is nutritionally balanced and provides adequate macro and micronutrients which means it should be done only with an appropriate nutritional consult.