Just like humans, some dogs can develop allergies towards one or more ingredients (usually a protein source) of their usual diet. Generally the onset of the first symptoms is sudden, so that dogs unexpectedly become allergic to something they have always eaten without any apparent problems. That is why pet owners are not unlikely to get surprised or even incredulous when they hear from the veterinarian that their dog might have become allergic to one or more food ingredients that have never caused any allergic reactions to them before. However, along with a familiar predisposition, the conditio sine qua non of the development of dog allergies to a particular food protein is just the previous exposure of the animal to that food allergenic agent. In other words, hereditary predisposed dogs can become allergic to a certain food ingredient of the diet only if they have already eaten it at least once in their lifetime.
But, how do dog allergies to food develop? Well, food allergy in dogs is an immunologically mediated disease which develops as a hypersensitivity reaction to a specific protein ingredient (called a “food allergen”) found in dog food, which is recognised as foreign by the immune system of the pet patient. When a sensitive dog eats the incriminated food for the first time, his immune system reacts by producing specific antibodies (immunoglobulins E or IgE) against that food allergen, without however inducing the development of any observable adverse reactions. When this occurs, it is said that the dog gets sensitized to the specific food protein ingredient. Once sensitized, the dog’s immune system responds to subsequent intakes of the incriminated food by producing greater amounts of allergen-specific antibodies. These, in turn, stimulate particular cells to produce and secrete histamine, which is the substance ultimately responsible for the onset of allergic symptoms. Food proteins that can trigger such reactions can be both of animal origin (such as proteins found in beef, poultry, fish, eggs, milk and their derivative products) and of plant origin (such as proteins found in legumes, soy, wheat, maize and rice).
In sensitive dogs, allergic symptoms to food usually involve the skin and/or the gastrointestinal system. Skin symptoms typically include itching, excessive licking or chewing of a body part with consequent local hair loss, various kinds of dermatitis and even otitis (ear inflammation), while gastrointestinal symptoms, which can either coexist with skin symptoms or appear alone, usually include diarrhea and occasionally vomiting and lack of appetite. Fortunately anaphylaxis, which is the most severe hypersensitivity reaction among dog allergies to food, is very rare.
It is worthwhile to remember that, in dogs, true allergies to food are less frequent than one may think and that many adverse reactions to food are actually caused by a complex of mechanisms altogether known as food intolerance. Although food allergy and food intolerance usually cause similar symptoms and signs and are often confused with each other, they have actually different pathogenetic mechanisms and are triggered by different food ingredients. Food intolerance, in fact, is not an immunologically mediated disease and does not involve a process of immunological sensitization. In dogs, food intolerance often results from an inflammatory reaction to certain irritants such as food additives (such as colorants, flavouring compounds, antioxidants and preservatives) or from the effect of pharmacological substances naturally present in certain foods (classical examples are salycilate and hystamin intolerance). Despite these different pathogenetic mechanisms, however, food allergies and food intolerance share the same skin and/or gastrointestinal symptoms, as well as the same diagnostic and therapeutic approach.
Diagnosing Allergies in Dogs
The diagnosis of both dog allergies and dog intolerances to food is not simple. And there are at least two reasons for this. First, dog food (and especially commercial dog food) always contains several different ingredients that, even if present in small amounts, can act as food allergens or can cause food intolerance: so it is not easy to exactly identify which is the actual ingredient responsible for the onset of symptoms. Second, a simple blood test is not enough to diagnose food allergy or food intolerance in dogs, but a more complex approach is required. The diagnosis of food allergy (and also of food intolerance) is usually made on the basis of the response of the pet patient to a so-called hypoallergenic diet, which can be either commercial (containing protein sources known to be unlikely allergenic) or home-made (formulated with protein sources that the pet has never eaten before). With this approach, also called “elimination diet method”, dogs are fed with a commercial or home-made hypoallergenic diet for at least 6 to 8 weeks. If during this period they do not develop any adverse reactions to food, a diagnosis of food allergy or food intolerance can be made. On the contrary, if the dog doesn’t respond positively to the hypoallergenic diet, it is possible that food allergies or food intolerances are not the actual problem and other approaches are needed to detect the real underlying cause of skin or gastrointestinal symptoms.
Once a diagnosis of food allergy or food intolerance is made, in order to discover which is the food ingredient responsible for the onset of adverse reactions, it is necessary to gradually reintroduce possibly allergenic ingredients (once at a time) in their hypoallergenic diet. Of course, if symptoms appear after introducing a specific ingredient, that ingredient is the one to which dog is allergic or intolerant. Although diagnosis of dog allergies and dog intolerances is not simple, once diagnosed, both these conditions can be effectively treated with the simplest existing approach: avoid feeding one’s pet with the incriminated food!