Now what do I feed? Identifying food hypersensitivity in dogs and cats
A look at the most common dermatologic signs and consequences.
Food hypersensitivity, food intolerance and other adverse reactions to food (ARF) cause a myriad of effects on several different systems of the body, with the integument and digestive systems being most commonly affected. This two-part article series provides insight into how ARF affects the skin in dogs and cats and how to definitely diagnose and manage patients with food allergies.
Most common culprits
Because food hypersensitivity can be the manifestation of a type I, III or IV hypersensitivity reaction, the onset of clinical signs can range from minutes to days after ingestion of the offending allergen. In people, the allergen usually has a molecular weight above 12,000 daltons, although this has not been confirmed in domestic animals, where the offending allergen may be smaller.
A number of studies published over the years have listed the most common food allergens in dogs and cats. Summarizing these reports has led many dermatologists to conclude that animals have the potential to become allergic to any food stuff to which they are exposed, especially proteins. In a 1996 report in dogs in the United States, the most common allergens were beef, chicken, chicken egg, cow milk, wheat, soy and corn.1 In this report, 80 percent of the dogs reacted to just one or two items, although there are reports of dogs allergic to as many as nine food items. Additional published reports list fish, rice and potato as foods known to cause adverse reactions. The food items most commonly known to cause ARF in cats include chicken, fish and dairy products. A few minutes spent reading the ingredient labels of most commercial cat foods will show these are the most common ingredients used in formulating the diets.
One common misconception of clients and many veterinarians is that food allergy is more likely to develop only after a recent diet change. In fact, when food allergies develop, the offending allergen has often been fed for more than two years, and some patients will eat the same protein for many years before the allergy develops.
Further complicating the workup of a patient with a suspected food allergy is the recognition that some patients will have cross reactions between related food ingredients. This phenomenon is well recognized in human medicine as well. Examples include patients allergic to chicken who will not tolerate duck or turkey. Some patients allergic to beef will cross-react or show clinical signs when exposed to other ruminants, such as lamb or venison. Fortunately, not all patients with food allergies will have cross reactions, but some will.
Food hypersensitivity in dogs
No age or sex predisposition is known to exist regarding the development of food allergy in dogs, but as many as 50 percent of patients with food allergies may exhibit clinical signs at less than 1 year of age. The so-called allergic breeds such as Cocker spaniels, Springer spaniels, Labrador retrievers, miniature schnauzers, Shar Peis, West Highland white terriers, Wheaten terriers, German shepherds and Golden retrievers may have a higher rate of food allergy. I have seen a higher rate of food allergy in three dog breeds—German shepherds, Rhodesian ridgebacks and Shar Peis.
Clinical signs vary, but nonseasonal pruritus, otitis and dermatitis are frequently seen. Sometimes the clinical signs are as simple (or vague) as recurring pyoderma or a nondescript keratinization disorder (seborrhea). Food allergy should always be considered in any patient with recurring urticaria, and eosinophilic vasculitis has also been associated with ARF.
In general, the clinical signs of food allergy are nonseasonal, although they could be episodic if they are due to sporadic treat administration. It is also possible for the effects of a food allergy to be low or subclinical (below a pruritic threshold) and only occur with the addition of environmental allergens will the patient flare.
Any dog with a nonseasonal pruritic dermatosis should have food allergy ruled out as a contributing cause of the skin disease. Several other clues may raise your index of suspicion that a patient is suffering from a food allergy. One is the pattern of skin disease. Food allergies are known to commonly affect the "ears and rears" of the patient. Another potentially useful clue is the response to corticosteroids. Atopic dermatitis is usually responsive to corticosteroids at anti-inflammatory doses. While some patients with a food allergy will be very responsive to corticosteroids, some will not, and when the pruritus is not corticosteroid-responsive, food allergy should be considered.
Nearly half my patients have gastrointestinal manifestations of their ARF. Dramatic gastrointestinal signs include vomiting and diarrhea, but they may be as subtle as flatulence or frequent (more than twice a day) bowel movements.2 Rarely reported clinical signs of ARF include seizures and respiratory signs such as bronchitis, rhinitis and chronic obstructive pulmonary disease,3 all of which I have seen.
Feline food allergy
No age or sex predisposition has been reported or recognized in cats with food allergies. One study in cats with food allergies reported the offending allergen had been fed for more than two years on average.4 Siamese and Siamese crosses may be predisposed to food hypersensitivity. The classic or hallmark clinical sign of food allergy in cats is pruritus, especially of the head. Cats may also exhibit self-induced alopecia or any manifestation of the eosinophilic granuloma complex.
Definite diagnosis of an ARF requies an elimination diet trial. Part 2 of this two-part series will cover how to conduct a successful trial and help patients get rid of the itch once and for all.
Dr. Lewis sees dermatology patients in California, Azrizona, Nebraska, New Mexico and Washington. In 1991, he established Dermatology for Animals, PC.
1. Jeffers JG, Meyer EK, Sosis EJ. Responses of dogs with food allergies to single-ingredient dietary provocation. J Am Vet Med Assoc 1996;209(3):608-611.
2. Paterson S. Food hypersensitivity in 20 dogs with skin and gastrointestinal signs. J Small Anim Pract 1995;36(12):529-534.
3. White SD. Food allergy in dogs. Compend Contin Educ Pract Vet 1998;20:261-268.
4. White SD, Sequoia D. Food hypersensitivity in cats: 14 cases (1982-1987). J Am Vet Med Assoc 1989;194(5):692-695.