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Hello and thanks for trusting me to help you and Harvey today. I am a licensed veterinarian with over 25 years experience and would be happy work with you.
It sounds as if fleas could be a very definite possibility, but just because the other pets are not displaying the same scabs doesn't mean they are affected as well, just that they are likely not allergic to flea bites as Harvey may be. It would be advantageous to make sure ALL pets are treated with an effective flea prevention product safe for use in cats like Frontline or Advantage Multi. If Harvey is scratching, he may benefit from a short course of steroids to break the itch cycle and help to resolve the scabs and dermatitis.
If you are treating for fleas and new lesions continue to appear, it is possible that he may still have allergies, but to something else in his environment and not fleas. It is also possible that it could be due to something else like a bacterial or a yeast infection. Your veterinarian can perform testing of the skin to see if that is the case.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.