Additionally, although food reactions occur as a non-seasonal dermatitis, many instances of atopy will similarly demonstrate no seasonal pattern. One factor that may be helpful for differentiating these two conditions is the observation that many dogs with food reactions will demonstrate skin lesions localized to the ears only. Therefore, dogs with recurrent ear infection even in the presence of bacterial or yeast infections should be considered suspect for a primary adverse food reaction. On average, adverse food reactions account for up to 10% to 30% of hypersensitive conditions in the dog, and of these, 80% of dogs with adverse food reactions will have a concurrent condition of atopy (refer to the previous section for more information on atopy).
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Also see "What Can I Do If I Think My Pet Has A Food Allergy":