As mentioned in previous posts, you need to find something effective that eliminates fleas before they can breed, breaking the lifecycle. Once you have that product the dog has to go around stimulating all the pupae available to hatch - they either get on the dog and die, or they miss and die. Once all the pupae in the environment have been hatched, then you could take the dog off preventatives - will you know? No. Comfortis (Spinosad) is still great and getting good results. Panoramis (Spinosad and Milbemycin) is popular as it also worms the dog. I am not totally familiar with the new Flea and Tick products (Bravecto (Fluralaner) or Nexgard (Alfoxolaner)), they haven't been around long enough in a "flea season" to be able to tell yet. Probably excellent as they are a new compound family used for external parasites in dogs. In saying all of that - is it fleas? Certainly dogs get allergic to fleas. Specifically their saliva left under the skin. It is a true allergic reaction, not just irritant. Dogs can be allergic to a LOT of other things. If it is a long term issue and you are happy with flea control, you may need to go further with allergy testing.