Hello,
There are many causes of itching.
Itching can be caused by:
1. Ectoparasites: fleas, scabies, demodex, lice, etc.
2. Allergies such as: Allergies to food, fleabites, environmental allergens (grass, ragweed, dust-mites etc, or bacteria).
3. Bacterial or Fungal skin diseases
4. Hormonal problems, Immune-mediated problems and behavioral problems.
Your dog may have allergies or scabies, or a combination of entities. Although there has been no change in the environment, allergies (atopy) can occur anytime up to approximately 3 years of age. They can develop seemingly out of the blue. Food allergies can develop at any age. A dog can be allergic to virtually anything:
any food, or anything in the environment.
Has your veterinarian performed a skin scraping or other skin tests to check for scabies, skin yeast, or ringworm? Scabies is very hard to find. A course of the topical medication Revolution could be used as a therapeutic trial. It is to be used in place of your heartworm and flea preventative every two weeks for 3 treatments. Your dog must be heartworm negative, of course, to use this product. If the itching stops soon after Revolution is placed on but returns when the next treatment is due, then your dog probably did have scabies and this is the right treatment. It may have to be continued for 6 treatments.
If you suspect that your dog has allergies than it would help if the allergen were identified. Once identified with a blood or skin test your dog can be desensitized against the allergen, or allergens. Antihistamines are only effective in dogs if you are very lucky. They are much more effective in humans.
If you suspect that your dog has food allergies this can be a little more difficult to diagnose. There is no reliable blood test for food allergies. The most reliable way to diagnose food allergies is to put your dog on a novel diet, available at your veterinarians such as venison and potato, duck and potato, rabbit and potato, Z/D diet, to name a few, for 12 weeks with no other treats. If your dog improves in the 12 weeks than you slowly reintroduce different foods. When your dog gets itchy again you will know which food he is allergic to, and you can eliminate it. Some dogs have to be on special foods for life. Some of the raw meat and vegetable diets can help these dogs.
Some dogs have a combination of allergies. They can have food and environmental allergies, such as seasonal allergies. In these cases the food allergies must be controlled with special diet, and for the environmental allergies controlled with desensitization, steroids, or a newer drug called Atopica.
I could continue on this subject for days. Try some of the things I have mentioned here and let me know how it goes. I have discussed the most obvious causes for itching. Please let me know if I can help you further and will be glad to discuss the more uncommon causes.
But thank you for the answer. It gives me a good starting point and some directions in which to look. I'll let you know if I find out anything.