Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 15 years of experience. I look forward to working with you.
This sounds quite frustrating! But unfortunately this is not an uncommon scenario. I assume your veterinarian performed skin scrapings to look for mites (demodex, sarcoptes) and cytology (bacteria, yeast) initially. If not, then this is a good next step with your veterinarian.
With regard to the question if this could be mange, the answer is it could be sarcoptic mange, but not demodectic (unless your vet did NOT do a skin scraping and then that means demodectic mange is still possible and would get much much worse on prednisone). Sarcoptic mange, on the other hand, is very tough to find on a skin scraping but causes profound itching to the point the prednisone might have improved her a bit but not entirely. Your veterinarian could make a decision to empirically treat for this (most commonly we use Revolution/selamectin done in 3 doses 2 weeks apart but ivermectin injections is another possibility).
However, I am less suspicious this is mange and more suspicious this may be allergic skin disease. And, whatever she is allergic to is still out there. What happens with allergies (food allergy, environmental allergy) is that the dog gets a secondary infection (bacteria, yeast) which improves on the initial medication (hopefully she was treated for 3 weeks minimum; if not then wasn't treated long enough as it takes weeks to clear this) and then once off the allergy kicks in again. There may just be something out there this time of the year and one more round of treatment will get her through the time period. Or, if not then reassessment to ensure no mites is important (or just empirically treat) and then discussion of figuring out the underlying allergy with your vet is needed (allergy testing, referral to dermatologist, or just medication like Apoquel or Atopica).
The benadryl is good and I would continue this until you can get her in next week to be seen by her veterinarian. If you cannot get her in tomorrow, then please call your vet to let him/her know what is going on and see if (s)he wants to start any medication until you have her seen again.
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