Food allergy (FA) is defined as “all immune-mediated reactions following food intake”, in contrast with food intolerance (FI), which is non-immune-mediated.

The clinical consequences of food allergies are seen both in the digestive tract and on the skin, being non-seasonal pruritus the most common manifestation.

About 10% to 15% of dogs with food allergies have gastrointestinal problems, and about half of affected dogs have a nonseasonal pruritic dermatitis.

Adverse food reactions (food sensitivity) are divided into two categories:

1. Immunological reactions (IgE-mediated) => immediate and intermediate hypersensitivity to food (anaphylaxis).
2. Non-immunological reactions (non-IgE mediated) => delayed hypersensitivity to food

The wall of the digestive tract is the largest surface of the body exposed to the environment. The GI tract has to differentiate between nutrients on the one hand and potential harmful substances (bacteria, viruses, parasites) on the other hand, which have to be tolerated and expelled (immunity) respectively. The Gut Associated Lymphoid Tissue (GALT) accomplishes this double function.

Four mechanisms ensure the conflicting functions of tolerance and exclusion of antigens:

• Mucosal barrier
• Regulation of the immune response
• Elimination of antigens
• Tolerance of antigens reaching the mucosa.

Impairment of this GI defense predisposes patients to food allergy.

Although in most clinical cases, the exact etiology is not well understood, type I, III, and IV hypersensitivity reactions are the most likely immunologic mechanisms. Food allergens are (glyco-)proteins with a molecular weight from 10–70kDa and are resistant to treatment with heat, acid, and proteases.

Potential food allergens include meat (beef, chicken, pork, horse, lamb and fish), eggs, milk, rice, soy, wheat, corn and other cereals.  In most cases, the pruritus is due to one or two allergens. Some studies have reported that one-third to two-thirds of dogs are allergic to more than one food.

Up to now, an extensive food trial is the only way to diagnose FA. Unfortunately, the correct performance of the different phases of this test (elimination diet, challenge and provocation testing) is a very time-consuming activity. For a successful outcome of the food trial, the choice of an appropriate elimination diet is of great importance.

Complete or partial improvement of pruritus and / or gastrointestinal disturbance during the elimination diet is the main criterion for suspected food allergy.  The diagnosis is confirmed by the recurrence of clinical signs when a food administered previously is reintroduced in the diet.

By definition, a hypoallergenic diet should contain ingredients to which the dog has not had prior exposure. Dogs rarely respond to the elimination diet during the first week, they usually need several weeks on a hypoallergenic diet to diagnose a food allergy.