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Dermatologic food reactions are non-seasonal reactions which occur following ingestion of one or more allergy causing substances in an animal’s food. The physical reaction is frequently excessive itchiness, with resultant excessive scratching at the skin.
While the pathogenesis of these reactions is not fully understood, immediate reactions and delayed reactions to food are thought to be due to a hypersensitive immune response. On the other hand, food intolerance is a non-immunologic idiosyncratic reaction due to the metabolic, toxic or pharmacologic effects of the offending ingredients. Since it is not easy to distinguish between immunologic and idiosyncratic reactions, any negative response to food is generally referred to as an adverse food reaction.
Symptoms and Types
Non-seasonal itchiness of any body location
Poor response to anti-inflammatory doses of glucocorticoids generally suggests food hypersensitivity
Crusts – dried serum or pus on the surface of a ruptured blister or pustule
Scale – flakes or plates of dead skin on the skin's surface
Self-induced baldness due to scratching
Abrasions/sores on the skin due to scratching
Leathery, thick, bark-like skin
Hyperpigmentation – darkening of the skin
Hives – swollen or inflamed bumps on the skin
Giant wheals (elongated marks) on the skin
Pyotraumatic dermatitis – infection of the skin wounds due to scratching excessively, and bacteria entering the wounds
Immune-mediated reactions – result of the ingestion and subsequent presentation of one or more glycoproteins (allergens) either before or after digestion; sensitization may occur as the food passes into the intestine, after the substance is absorbed, or both
Non-immune (food intolerance) reactions – result of ingestion of foods with high levels of histamine (an antigen known to cause immune hypersensitivity) or substances that induce histamine either directly or through histamine-releasing factors
It is speculated that in juvenile animals, intestinal parasites or intestinal infections may cause damage to the intestinal mucosa, resulting in the abnormal absorption of allergens and subsequent sensitization to some ingredients
Your veterinarian will perform a complete physical exam on your dog, including a dermatological exam. Non-food causes of dermatologic disease should be ruled out. Your veterinarian will order a blood chemical profile, a complete blood count, a urinalysis and an electrolyte panel to rule out other causes of disease. You will need to give a thorough history of your dog's health, onset of symptoms, and possible incidents that might have preceded this condition, especially regarding any changes in diet, and any new foods added to your dog's diet, even if temporary.
Food elimination diets are advised for dogs thought to be suffering from adverse food reactions. These diets typically include one protein source and one carbohydrate source to which the dog has had limited or no previous exposure to. A clinical improvement may be seen as soon as four weeks into the new diet, and maximum alleviation of clinical signs may be seen as late as thirteen weeks into the food elimination diet.
If your dog improves on the elimination diet, a challenge should be performed to confirm that the original diet was the cause of disease and to determine what ingredient in the original diet triggered the adverse reaction.
Challenge: feed your dog with the original diet. A return of the signs confirms that something in the diet is causing the signs. The challenge period should last until the signs return but no longer than ten days.
If the challenge confirms the presence of an adverse food reaction, the next step is to perform a provocation diet trial: going back to the elimination diet, begin by adding a single ingredient to the diet. After waiting a sufficient amount of time for the ingredient to prove either agreeable or adverse, if there is no physical reaction, move on to adding the next ingredient to your dog's diet. The provocation period for each new ingredient should last up to ten days, less if signs develop sooner (dogs usually develop signs within 1–2 days). Should symptoms of an adverse reaction develop, discontinue the last added ingredient and wait for the symptoms to subside before moving forward to the next ingredient.
The test ingredients for the provocation trials should include a full range of meats (beef, chicken, fish, pork, and lamb), a full range of grains (corn, wheat, soybean, and rice), eggs, and dairy products. The results of these trials will guide your selection of commercial foods, based on those that do not contain the offending substance(s).