Environmental contaminants such as dust and mold can cause allergies in your dog. So can his food although it takes time and can be difficult to diagnose.  Kim Horne, a veterinary technical at the University of Minnesota Veterinary Medical Center says that your vet will put them on a food elimination diet trial. That means that the dog will spend between eight and ten weeks on a new diet. He will eat protein and carbohydate sources that he hasn’t had before. Then he will go back to eating his regular diet and his responses will be monitored. If the dog is truly allergic to food, there will be an increase in the clinical signs. These signs can include itching and inflamed skin. If any of these signs appear, various foods will be tried until one of these doesn’t trigger the allergy symptoms.  This could take time so be patient and do not be quick to administer any kind of pet medication that could suppress the symptoms.