Food allergies often cause skin disorders in dogs and cats. They can be complicated by inhalant allergies (atopy) that cause similar symptoms. In cats, food allergies may be an underlying trigger for inflammatory bowel disease. It is often thought that corn and soy, preservatives, and food colorings are responsible for food allergies; however, beef, dairy, and wheat are the most common food allergens for dogs. Beef, dairy, and fish are the most common sources of food allergy in cats. These ingredients are hard to avoid in most commercial dog and cat food, even in trace amounts. Typically, food allergies occur by exposure to an ingredient over a long period of time. Prepared hypoallergenic diets are available from veterinarians only, and all other sources of calories including treats and rawhides are eliminated during a food trial.
Other sources of skin problems must be ruled out or resolved before an effective response to hypoallergenic food trials will be seen. Fleas are the number one cause of pruritus (itchy skin) and alopecia (hair loss). They must be absolutely eliminated as a contributing factor. Sarcoptic mange causes insatiable itching and is often difficult to diagnose because the mites can be hard to find on skin tests. Inhalant allergies (atopy) may respond well to steroid therapy; whereas food allergies may respond poorly.
Food allergy induced itching can be differentiated from inhalant allergies in that it is non-seasonal. Usually, allergy symptoms caused by pollens and molds will wax and wane throughout the year. Any distribution of skin involvement may occur, but the ears, rump, limbs, axillae (arm pits) and groin are commonly affected. Cats may develop eosinophillic granuloma complex, crusty oozing lesions around the face and chin. The itch may involve primarily the paws and the ears. Ear infections may be persistent and recurring. Other symptoms of food allergies may include papules (pimple-like bumps), secondary pyoderma (skin infection), and hot spots. Cats may show symptoms of inflammatory bowel disease like chronic or intermittent diarrhea.
There are blood tests and intra-dermal testing methods of diagnosing food allergies. In most experts’ opinions, they are unreliable, and a feeding trial is considered the best diagnostic tool for food allergies.
For severely itchy and infected skin, an animal may be started on oral antibiotics and steroids. Ear infections would be treated appropriately with topical antibiotic / antifungal / anti-inflammatory medications. Measurement of the response to a food trial alone will diagnose and treat the allergy, however. It is important that if your pet is started on a hypoallergenic diet that it cannot eat anything at all except for that one particular food. Basted and impregnated toys and chews may also contain allergens and should be taken away from the pet.
There are two technologies upon which hypoallergenic diets are based: novel protein, novel carbohydrate formulas and hydrolyzed protein formulas. The first type of food avoids all common food ingredients associated with allergies. Instead, a novel protein source such as rabbit or salmon, and a novel carbohydrate source such as potatoes or peas is chosen. This technology assumes that the animal’s immune system has never been exposed to these ingredients. Misuse of novel diets and over-the-counter marketing has caused manufacturers to continually search for even more exotic meats and starches. Such is the case with lamb and rice diets which are ubiquitous these days. Alternatively, a hydrolyzed protein diet usually consists of a soy protein that has been pre-digested enzymatically to a point where the immune system does not recognize it as an allergen. The protein is still of nutritional value, and this type of diet has no potential for misuse. Switching between different over the counter foods is a haphazard method of curing food allergies and at best will achieve limited response.