Allergies are the most common cause of canine skin rashes. It is possible that your dog's nutrition, the environment, or parasites are to blame for any allergies he may have. A few flea bites on a dog that is allergic to flea saliva can cause itching and scratching for days or weeks after the bites have been removed. It's also called as 'flea allergy dermatitis' when the allergy is to fleas.