Author: Bruce Novotny, DVM
Editor: Lisa Freeman, DVM, PhD, Diplomate, ACVN

How common are food allergies?
No one knows for sure how many pets have food allergies, but several numbers are frequently cited in the veterinary literature. Veterinary dermatologists suggest that adverse food reactions account for 1% to 6% of all dermatoses (skin diseases) in general practice and that food is the cause of 10% to 20% of allergic responses in dogs and cats. Food allergy is probably the third most common hypersensitivity skin disease in dogs and cats after flea allergy and atopy.

Food allergies affect the gastrointestinal tract, but we do not know how often. Food sensitivity probably is involved in some cases of the two most common causes of inflammatory bowel disease and chronic vomiting and diarrhea in cats and dogs—lymphoplasmacytic enteritis and eosinophilic enteritis.

What are the signs of food allergy?
The signs of a food allergy range from gastrointestinal (GI) signs such as vomiting and diarrhea to dermatologic (skin) reactions such as itching (pruritus), bumps (papules) and redness (erythema). In dogs, adverse food reactions typically occur as nonseasonal pruritic dermatitis, occasionally accompanied by GI signs. The pruritus varies in severity, and the lesions are often indistinguishable from those seen with atopy. One-fourth of dogs with adverse food reactions have lesions only around their ears. In cats, head and neck pruritus is a particularly common manifestation of food allergy.

Every part of the GI tract can be damaged by food allergies. The signs usually are the result of stomach and small bowel dysfunction, but inflammation of the large intestine (colitis) can occur. Vomiting and diarrhea are prominent features.

Gastrointestinal disturbances occur as well in 10% to 15% of dogs and cats with cutaneous manifestations of food sensitivity.

Can other allergies occur simultaneously with food allergies?
Yes. About 20% to 30% of dogs with suspected adverse food reactions have another allergic disease, such as flea-allergy dermatitis or atopy. Concurrent flea-allergy dermatitis or atopy may occur in as many as 30% of cats with suspected adverse food reactions.

Some veterinary dermatologists suspect that the effects of allergies are additive. That is, an animal that has both a flea allergy and a food allergy may reach some "allergen threshold" in which signs become apparent. However, in the presence of only one allergy, the pet may not have any signs. This hypothesis implies that controlling even one of multiple allergies may reduce or eliminate the signs.

How old are pets when food allergies occur?
Food allergy often occurs suddenly after the pet has consumed the culprit food for months or years. Affected pets can be as young as 2 to 6 months. In one study, almost half of the cats who developed a food allergy did so by 2 years of age.

Are adverse reactions to food and food allergies the same thing?
An adverse reaction to food is any abnormal response to an ingested food or food additive that usually is harmless. There are two general types: (1) allergies and (2) intolerances. In a true food allergy (hypersensitivity), the body's immune system overreacts to a food that is otherwise harmless. That is, food allergies have an immunologic basis. The pathogenic mechanisms that lead to a food allergy include interaction of the food with a biologic amplification system that leads to inflammation and clinical signs. On the other hand, a food intolerance is a nonimmune reaction that can have a variety of causes. For example, milk intolerance is caused by a deficiency of lactase, which breaks down milk sugar (lactose). The lactose thus remains in the intestine to be fermented by bacteria, causing gas and diarrhea. Another type of intolerance is overreaction of the blood vessels to vasoactive amines in food or abnormal responses to food additives.

What causes food allergies?
Food allergens are almost exclusively medium-size to large proteins, which are large enough to be interpreted by the animal's immune system as "foreign." Most food allergens are partially resistant to heat and digestion. Therefore, they are not broken down by cooking or the gastrointestinal tract and retain their antigens.

What proteins most frequently cause food allergies?
Ten scientific studies have shown that certain proteins in beef, dairy products and wheat account for two-thirds of the reported cases of cutaneous food allergy in dogs. Beef, dairy products or fish account for almost 90% of the reported cases of cutaneous food allergy in cats. The veterinary literature does not say which proteins are most commonly involved in GI food allergies.

How do food allergies occur?
Food allergies are an unfortunate consequence of protein survival in the GI tract. Remember, once foreign protein has been hydrolyzed to small peptides and amino acids (normal digestion), it is no longer able to provoke an immune response. However, food protein that escapes digestion yet is absorbed may retain enough "foreignness" to generate an immune response. The body builds antibodies, which remain to attack the protein whenever it appears again. One result of this immune response is the development of obvious signs of food allergy. The mucosal lining of the GI tract does not keep out all large molecules. Approximately 0.02% of the daily dietary protein intake is absorbed intact. Some observers claim that as much as 2% of fed protein may be absorbed. In the 70-pound German shepherd dog, these numbers mean that as much as 40 lb of protein may be absorbed over the animal's lifetime. If it is not dealt with by the body, this large protein burden may be recognized by the immune system as foreign and therefore cause food allergy.