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Hello, this is Dr. Bruce and I would like to help you with Rio.
I'm sorry to hear that she is having this issue.
There are several possible causes to hair loss on a dog. The location on the body of the hair loss can sometimes give us a clue as to the underlying cause. Certainly, as your vet has mentioned, allergies can lead to hair loss. Typically, flea allergies and food allergies show hair loss towards the back end (rump, butt, and hind legs) like you describe. I agree with your vet that a flea preventative is key. I prefer using comfortis, however. This is a pill that is given once per month and prevents the fleas from biting.
Food allergies are also a possibility. It is very important to realize that when a food allergy is a concern, that there is no blood or skin test that can diagnose the food allergy. The only way to tell is to put the dog on a hypoallergenic diet for 8-10 weeks minimum. In addition, the dog CANNOT eat anything else for that period of time. No treats, no chewies, and no oral heartworm preventatives. Food allergies are typically caused by proteins in the food, and it is necessary to use a protein that the dog has never been exposed to in their life. So the food that Rio would need to eat is either composed of new proteins (kangaroo, duck, ostrich, etc) or is treated with steam (hydrolyzed) as with Hill's z/d diet.
There can be other possible causes of hair loss that should be investigated as well before assuming an allergy is the problem. A skin scrape needs to be done by the vet to rule out mange (microscopic mites in the skin). A fungal culture should be performed to rule out ringworm (a potentially contagious fungal infection). Generalized bloodwork should be run to look for any underlying condition that can cause skin problems (cushing's disease, thyroid disease). In some cases, if there is no obvious solution, and the trial using the new diet and flea preventative doesn't help, it may be necessary to sedate her and take a few small skin biopsy samples for microscopic evaluation.
In the mean time, I would recommend giving Rio a bath in a medicated dog shampoo 1-2 times per week, give her Benadryl (25-50mg) 2-3 times per day as needed to stop the scratching, and add an omega-3 supplement to her food to help improve the skin.
I hope this helps. If you would like to continue this chat, please reply to this post and I will get back to you as soon as possible.