Adverse reactions to food can be divided into two categories, those involving the immune system and those that do not. Just like flea bite dermatitis, food intolerance is an adverse reaction that does not involve the immune system. Symptoms are limited to the gastrointestinal tract, most often seen as vomiting and/or diarrhea. True food hypersensitivity, like flea allergic dermatitis, is an immune-mediated reaction to molecules found in food.
Food hypersensitivity, manifested as skin disease, is the least common of the allergic causes of dermatitis, but it is the most often suspected. The job of the intestinal tract is to digest (breakdown) food items into basic molecules that can be absorbed through the intestinal tract. The immune system rarely sees molecules large enough to allow a reaction from the intact intestine. However, in predisposed individuals, over time a small amount of leakage can induce the immune system to create antibodies specific to molecules of food. This takes time, and most companion animals are fed a diet for over two years before they react (in other words, it is less likely that allergy resulted from a recent diet change).
Symptoms of food hypersensitivity can affect the gastrointestinal tract and/or the skin. Vomiting and diarrhea are common with food hypersensitivity that affects the gastrointestinal tract; itching is seen with the skin. The pattern of itching is similar to that of atopy; the face, neck, feet, armpits, and chest are most often affected. Persistent ear infection, with or without itching, is also common.
The only way to diagnose food allergy is through a restricted-ingredient food trial; serum allergy tests are not accurate for the diagnosis of food allergy. Ideally, a home-cooked diet of one protein source and one carbohydrate source to which the animal has never been exposed is fed. This prevents any possible reaction to changes in the diet from processing and storage. Commercial diets containing novel (unusual) ingredients are available by prescription from your veterinarian. These diets are meant for the diagnosis of food allergy; lifetime feeding is rarely needed. Food trials should be conducted for at least 8 to 10 weeks. During the feeding trial, all snacks, treats, chews, rawhides, bones, and table scraps must be withheld. Following the trial, the original food and treats should be fed to note whether or not itching resumes. If it does not, then food allergy is not the cause of the patients itching.