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While grains are one possible cause for allergic reactions. Beef, dairy, wheat, egg, chicken, lamb, soy, pork, rabbit, and fish are common causes of allergies in dogs. Most owners put dogs on adult food at about 1 year of age, so that would be about the right timing for a food allergy. You might compare the puppy food your dog was on with the adult food you switched him to and the current food and look for common ingredients. Usually when you switch foods, you go to a limited ingredient food such as venison and potato and one the dog has not been on before. Even after the switch, any skin dermatitis present needs to be cleared as well.
Staph dermatitis and yeast dermatitis are skin conditions generally seen with allergies. Staph usually occurs on the lower regions of your pet and tends to have small pimple type bumps. Shampoo containing Chlorhexiderm and/or Oatmeal can help with this condition. Yeast typically shows as a greasy area that has a sweet musty odor. Sometimes the skin can become inflamed, darker and thickened due to itching. Yeast likes areas such as between toes, armpits and ears. Selsun Blue Shampoo can help with Yeast dermatitis. When shampooing, lather and leave on 15 minutes before rinsing. These shampoos are not meant to be a cure, just a relief until your pet can see a Vet. Benadryl can be given to your dog, the dose is up to 2mg per pound every 8 hours for itching due to allergies.
Seborrhea is also seen with allergies and you can read about this here:
You can read about allergies here: http://www.vcahospitals.com/main/pet-health-information/article/animal-health/allergy-general-in-dogs/428
Here is a site that goes over itching in dogs.
You might supplement your dog's diet with Omega n3 fatty acids. You can do this using fish oil capsules giving 180mg per 10 pounds of weight. This might help if dry skin is also an issue.
He is fairly young so you might want to consider allergy testing since it can also be an environmental allergen such as a grass, cleaning supply or even laundry detergent. At least then you would know for sure what your dog needs to avoid to solve his skin issues. I am assuming the vet did a skin scraping to rule out mange.
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