Food Allergy in Dogs
Dr. Rosanna Marsalla
The term food allergy often is used to describe any adverse reaction to foods. Some reactions are mediated by the immune system (true allergy) whereas others are not (food intolerance). Food intolerance and true allergy cannot be reliably differentiated on a clinical basis, and this difficulty explains why a variety of clinical signs are attributed to what has broadly been called food allergy. Food-responsive dermatoses are uncommon in dogs with a reported incidence in small animals of one to 20 percent. Food allergy can occur in animals of any age, including young animals less than six months of age. Thus, food allergy should be considered in any itchy dog less than six months or older than six years of age.
Barriers in the gastrointestinal tract prevent adverse responses to ingested food in most individuals. These barriers include both physiologic and immunologic protective mechanisms. Abnormalities in the gastrointestinal defense mechanisms (mucosal barrier failure with increased antigen absorption, defective immunoregulation) may predispose your dog to the development of food allergies. Which of these mechanisms is important in the pathogenesis of food allergies in dogs presently is unknown.
The pathogenesis of food allergy in dogs has not been established. Type I hypersensitivity (mediated by a type of antibody called immunoglobulin E or IgE) may be involved. In this type of hypersensitivity reaction, mast cells in the intestinal tract degranulate and release inflammatory mediators.
Foods contain many proteins that can act as allergens. Allergenic foods tend to have high protein content. Complex proteins contain many sites that may act as antigens and thus are more likely to stimulate the immune system than are smaller proteins. Offending proteins must be large enough to link two IgE antibodies and trigger mast cell degranulation.
The foods most commonly incriminated as allergens in dogs are beef, dairy products and wheat and account for 66 percent of suspected cases of food allergy. Chicken, lamb, soy, eggs, pork, and preservatives account for only 22 percent of reported cases according to a recent survey of veterinarians in North America.
Reactions to food additives frequently are suspected, but little objective information supports this perception. Additional studies are needed to confirm the role of food additives in adverse reactions in dogs.
Related Symptoms or Ailments
Initially, food allergy causes non-seasonal pruritus. The onset of clinical signs usually is not related to a change in the diet. In fact, it is a common misconception that food allergy occurs shortly after changing an animal's diet. Most animals diagnosed with food allergy had developed allergy to a food that had been fed for many years often more than two years. Food allergy can develop in response to virtually any ingredient of the diet.
Clinical Signs of Food Allergy in Dogs
Muzzle, paws, armpits, groin, rump and ears are commonly affected areas of the body.
Localized or generalized pruritus (itchiness)
Otitis externa, or inflammation of the ear canals. Ear inflammation sometimes may be the only clinical sign of food allergy.
Recurrent moist dermatitis
Pododermatitis, which is inflammation of the paws including the skin between the digits
Recurrent skin infections including superficial bacterial infection (pyoderma) and yeast infection (Malassezia dermatitis)
A variety of primary and secondary lesions. These include papules (small red raised lesions), generalized redness of the skin, excoriations (moist lesions resulting from self-trauma), hyperpigmentation, epidermal collarettes ("bull's-eye" lesions), and seborrhea. The clinical signs and distribution of lesions may be indistinguishable from those found in dogs with atopy.
Diarrhea may occur in 10 percent of cases, but is not common.