Itchy dogs probably account for about 25% of the dogs I see in practice. It's a very common problem that you think you have have a simple solution. If all these dogs have this problem, then why can't we figure out what the problem is? Well, actually, most of the time we can. It just takes patience and money. Most of my clients only come in the exam room with one of them....
First, let's look at what diseases will make a dog itch. Basically, you can divide most itchy dogs into 4 broad categories: 1) Fleas (and other external parasites), 2) Food Allergies, 3)Metabolic Diseases, and 4) Atopy (or inhalant allergies).
We'll leave fleas out of the discussion for now. Since I've beat the flea topic to death lately, let's start with food allergies. Many dogs I see with food allergies tend to have itchy feet and ears. However, they can present with any type of scratching. The good news about food allergy is that in theory it's easy to fix. Switching the dog over to a new protein source, or a specialty diet, such as Hill's Z/D should help the dog. However, switching the dog over to a protein source that the dog has never seen before can be difficult. Many dog foods that say "Lamb and Rice" on the label also contain other protein sources. I often tell people not to open the bag before they bring it to my clinic for me to read. This allows them to return the food if its not right. (Of course, it would be easier if they just bought the right food from me, but some people really think I'm trying to overcharge them. I promise most practices make so little off of dog food, they would rather it was easier for you to buy it somewhere else.)
Although rare in my experience, metabolic diseases such as thyroid and adrenal gland disease can cause itching. Most of these dogs also have other clinical signs, but many times the owners (or their veterinarians) don't recognize them. In dog's that are unresponsive to initial therapy, I always recommend a full blood panel.
The last area is the most broad, least understood and the hardest to treat: Atopy. The term atopy refers to inhalant allergies. In most cases, this is going to require a referral to a veterinary dermatologist.
Tomorrow we will discuss potential treatments. As always, though, you should speak with your veterinarian about your dog's specific condition.