"Food allergies generally account for 20 percent of the causes of itching and scratching in dogs," state Drs. Foster and Smith in their article "Food Allergies and Food Intolerance" on PetEducation.com. Because the symptoms of food allergies are so similar to the indicators of other dog allergies, veterinarians often have a difficult time establishing what is causing the inflammation and irritation. Once other causes have been eliminated, the animal's diet is often determined to be the root of the problem.
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Causes
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In canine food allergies, the animal's immune system causes specific cells---called mast cells---in the dog's intestinal tract to release antibodies, which fight allergens in the diet. Typical allergens include proteins found in beef, dairy products and wheat. Less common allergens in dog food include chicken, soy, eggs, pork, lamb and preservatives. When the allergic dog ingests these proteins, inflammatory responses including persistent gastrointestinal problems and skin signs can develop.
Food allergies can occur in dogs of any age; however, animals less than six months of age and animals six years or older are particularly vulnerable. According to Dr. Rosanna Marsalla in her article "Food Allergies in Dogs" at PetPlace.com, most allergic dogs develop aversions to diets that they have been fed for more than two years. She states, "Food allergy can develop in response to virtually any ingredient of the diet."
Signs and Symptoms
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Most dogs presenting with allergies show signs of scratching and hair loss on the face, feet, forelegs and ears due to the pruritic (itching), inflammatory nature of the disease. They can develop dermatitis and skin infections and often have recurrent ear infections. Hot spots, called moist dermatitis, may occur on the animal's tail, anus and under his armpits. They may also have diarrhea or an increase number of bowel movements. Veterinarians will commonly watch for symptoms that occur consistently year round in order to make a correct diagnosis of food allergies.
Diagnosis and Treatment
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Because the symptoms for food allergies are common to other diseases including mange, yeast or bacterial infections, seasonal air-borne allergies and fleabite allergies, veterinarians will need to rule out or treat these other diseases before making a diagnosis of food allergies. Scratch abrasions and ear infections are usually be treated with systemic and topical antibiotics. Veterinarians will commonly recommend a food trial be performed once all other causes for the scratching, itching and hair loss have been ruled out.
Food trials commonly consist of feeding the dog a carbohydrate and a highly digestible protein to which he has not been previously exposed. The trial will usually last for twelve weeks, allowing the veterinarian time to judge the efficacy of the new food and alleviate any symptoms. If the itching and scratching are still going on at the end of the three months, a new protein will be introduced. Client compliance is important; the dog is not allowed any other food than what is prescribed during the trial and some veterinarians may recommend supplements to balance the diet and add nutrients to aid in skin care.
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