Thank you for your question. My name is***** I have been a small animal veterinarian for 14 years and I'd like to help with your question.
Flea allergy is very common. Usually they will be very itchy (even more itchy that having fleas alone would cause) and they lose hair... mostly above their tail, but also on their back legs and sometimes in their arm pits.
If Abby does have flea allergy dermatitis, and it certainly sounds like she does, there are a number of things that you should be doing. First, your veterinarian should have given you something to control the itching... either a cortisone injection or an oral medication containing prednisone or apoquel. This is just to provide temporary relief while you do the second step, which is to get the fleas under control. Flea control can be more difficult than you would expect. Most product... especially the baths and sprays that you are using... do kill adult fleas. However, the adult fleas represent one 10% of the flea population in an environment. For every single adult flea there are 9 eggs, pupae and larvae. So killing the adults does very little to control flea exposure. You need to use a product that controls multiple stages of the flea life cycle. This mean you should use Advantage, Nextgard, Bravecto, or Frontline (my least favorite because I think there is some resistance). You also need to treat every single animal in the environment or the fleas will continue to use the non treated pet as a breeding ground.
These monthly products kill 3, but not all 4 stages of the flea life cycle so you have to treat all animals every 3-4 weeks for 3 months before you can completely control all the fleas in the environment. No product kills the pupae stage. You can try to treat this stage by frequent vacuuming and washing of any bedding that she uses, but it will take 3 months to completely get this stage out of the environment.
So, itch control, intense flea control are the most important. The third aspect is skin health... if the skin if dry, inflamed, flakey then it may help to put her on some fish oil and do some oatmeal baths to help the skin health.
I hope that helps, but if you have more question please let me know. If not, please don't forget to rate my answer.