Firstly - you need to make sure that your cat is definitely up to date with flea control. Make sure you are using a flea product appropriate for her weight. Even though you may not see any fleas on you girl doesn't mean there are one or two lurking and oftentimes this is all it takes for a cat with flea allergy dermatitis. In your girl's case it does sounds like she may have skin parasites or a fungal infection, or possibly even an underlying allergy of some sort with secondary infection.Flea allergies are avoided by strict flea control - every 3 - 4 weeks with Advantage or Advantage Multi or every 5 - 6 weeks with Frontline. You must also ensure all pets in the household are treated at the same time. You should also consider flea bombing the entire house to kill all larvae and eggs hiding in your carpet.
It is also possible your cat may have skin mites or lice as they commonly hair loss like this. These could be either Demodex (less itchy) or Sarcoptes mites (VERY itchy) and you should ensure a full Veterinary check up so that he or she can carry out a physical examination and possibly a skin scrape to check for mites and lice, as well as signs of fleas. There could well be something else going on here such as Demodectic mange which is a little harder to spot and requires on going treatment. The next step would be to double check for fungal disease (such as ringworm) and there are several tests for this including a UV light and a fungal culture. It could also be that your cat has a bad yeast problem. Again a full Vet check will be able to confirm or deny this.
Finally cats can become allergic to many natural ingredients in their diet. I would also recommend a food trial for your girl using a hypoallergenic food like Hill's Z/D ultra available from your Vet. Feeding ONLY this for 3 weeks will give you an idea of whether a food allergy is to blame here.
As I say, it really would be a good idea to have a full consult with a Vet to check all of the above. I think a skin scrape and microscopy is definitely a good next step. Best of luck with your girl and I hope this has been of help to you. Please reply if you need any more information on this.
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Dr M D Edwards