Itchy Dog or Cat? It Could Be Food Allergy Dermatitus
Food allergy dermatitis, more precisely known as “cutaneous adverse food reaction,” is a chronic skin disease that affects both dogs and cats.
Reaction to certain foods is the third most common allergy in dogs, after flea allergy dermatitis and atopic dermatitis (hypersensitivity reactions to mold, pollen, dust, mites or other allergens in the environment), says Catherine Outerbridge, assistant professor of veterinary dermatology at UC Davis.
“Companion animals can suffer from dermatitis due to flea, atopic or food allergies alone, or in combination, such as flea and environmental allergies—sometimes all three,” says Dr. Outerbridge.
Dogs and cats with food allergy dermatitis, if continuously fed the same diet, do not have seasonal variation—they are usually itchy year round. Occasionally, dogs may have gastrointestinal signs as well, including diarrhea, vomiting or flatulence.
“While flea allergy dermatitis is also the most common allergy in cats,” says Dr. Outerbridge, “we see more cases with food as the sole allergy in cats than in the dog. By the time food allergy dermatitis is recognized, cats may have extremely itchy skin, especially around the head, face and neck area.”
“Since cat owners expect to see grooming behavior in their pets, it can be difficult to recognize mild itchiness versus normal grooming behavior,” she says.
“Many mildly affected cats with allergies may never be brought to the dermatologist. Dogs are not grooming animals, so any increased licking is usually interpreted by owners as a problem.”
Dogs with cutaneous adverse food reactions may itch anywhere, especially the face, ears and feet, says Dr. Outerbridge. “That is also true for pollen and mold allergy—the signs can look exactly the same.”
Both dogs and cats may scratch and chew affected areas until they inflict skin damage, and they may have recurrent skin or ear infections. The veterinary dermatologist must evaluate the animal and determine the nature of the allergy.
The onset of food allergy signs can appear suddenly, at any age, and continue as long as the offending food continues to be eaten. It might be assumed that a food allergy has resulted from a recent change in diet, yet it has been shown that dogs may regularly eat a food for at least two years before visible signs of an adverse reaction. (1) Also, rather than being caused by some exotic ingredient, reactions are generally elicited by common food ingredients—and those eaten most often by the animal.
Animals usually react to a protein source such as chicken, beef, soy or egg, but they can also react to a carbohydrate or preservative. Other possible dietary allergens include dairy products, poultry, lamb, fish, wheat, corn and rice. Dogs and cats may react differently to ingredients in their food in different geographic locations because of variability in available proteins, manufacturing techniques or additives. (1)
Food allergy dermatitis is diagnosed by means of an elimination diet. The animal is prescribed a homemade or commercial diet that contains a single protein and a single carbohydrate to which he or she has never been exposed, while all the other food elements are completely eliminated.
If the animal’s diet is variable, especially for cats that live outdoors and have access to mice, birds or other food, it may be more of a challenge to select ingredients for the elimination diet trial that are not likely to have
been consumed previously.
Any elimination diet must be adhered to strictly in order to diagnose the presence or absence of a cutaneous adverse food reaction in the animal. The animal must not be allowed to have treats such as raw hide chews, pig’s ear treats, other chew toys of animal origin or flavored medications and vitamin products—only the prescribed diet and water.
Once the offending food item is removed from the pet’s diet, it may take weeks until the animal has relief from the clinical signs. The testing period usually lasts 8–10 weeks.
If the signs improve, and food allergy is suspected as a result of the elimination diet, a “diet re-challenge” is then performed. The animal is returned to the original diet plus treats. If that animal is food allergic, the clinical signs should worsen within hours or days, confirming food allergy.
“But that doesn’t mean,” says Dr. Outerbridge, “that the animal doesn’t also have flea or environmental allergies.”
It is not yet known why some animals develop allergies to foods. Theories involve individual variability in absorption of protein breakdown products during digestion, or that in the gut of an allergic patient larger molecular weight proteins trigger allergic reactions or intolerance when they are detected by the immune system. That is why commercial hydrolyzed diets are sometimes prescribed to help diagnose food allergy. In hydrolyzed diets, common protein ingredients are altered to have a molecular weight below the presumed allergic threshold.
The good news is that if the pet is food allergic, veterinarian and client can then work together to discover exactly which foods are the culprits and eliminate those foods from the animal’s life.
Dr. Catherine Outerbridge is assistant professor of clinical dermatology at the William R. Pritchard Veterinary Medical Teaching Hospital. She is a diplomate of the American College of Veterinary Dermatology and a diplomate ofthe American College of Veterinary Internal Medicine in small animal internal medicine.
Food “allergy” or “hypersensitivity” results from an immunological (antibody-mediated) reaction to certain glycoprotein molecules generated during digestion. Immunological reactions likely involve genetic predisposition.
Food “intolerance” is a negative physiological response that can be induced by proteins, food additives, toxins or bacterial contamination.(2) Even if cutaneous adverse food reaction is caused by a true allergy, there are no reliable skin or blood tests currently available to make the distinction. In the case of either allergy or intolerance, an elimination diet is used to determine that an animal is reactive to certain foods.