Thank you, Lisa.
As I am sure you can appreciate, feline skin disease can be one of the most frustrating conditions to try to address. When we have cats caught in a cycle of skin irritation and hair loss, we do have to consider a wide range of agents (ie mites, bacterial infection, fungal issues, behavioural overgrooming, allergies to dietary proteins/pollens, flea saliva, environmental allergens, etc). That all said, from your history, we can start ruling out issues and narrowing down the cause for Sparky's signs.
With her history in mind, if you are seeing her actively grooming then this tells us that her skin is irritated in this region. This makes stress overgrooming less likely right off the bat. Furthermore, if we are seeing no pustules, pimples, or crusting; bacterial or fungal infections are also less likely. And if she has had no diet changes, then we can also dietary protein allergies here. As well, since the skin looks thickened but not raw, I would also be less worried about mites at this point.
With this aside and considering the area she is targeting, this does make concerns of conditions of diffuse itchiness (ie flea saliva or pollen allergies) our top differentials. And I would note that because it is her back and because she isn't on any flea treatment, flea allergy dermatitis would be a serious consideration. Especially as it is the #1 most common culprit to cause these kinds of signs on the back. And just to note very sensitive feline allergy dermatitis kitties with just a single flea bite from a visiting flea can develop this (since they are allergic to the saliva and don't need to be covered or even carrying any fleas at this stage to be irritated as she is.).
Therefore, in order to address her situation, our first step is to treat for fleas. Ideally, we want to use a good quality flea treatment (ie Advantage II, Activyl, etc). And just to cover all bases, if you wanted to make sure mites weren't an issue here they you can perhaps consider Stronghold, Revolution or Advocate; since they also will cover mites and let us just rule them out here. (These are available OTC from your vet as long as they have a recent weight for her).
Furthermore, since the bite only starts signs and this is otherwise an allergic reaction, we also need to address that for her to. To do so, you can start her on an antihistamine. Most commonly we tend to use Piriton (Chlorphenamine) at a dose of 2-4 mg every 8 hours. Or you can use Cetirizine at a dose of 5mg once daily. Ideally, start at the lower side of that dose since it can cause drowsiness for kitties. And of course, do check with her vet before use if she has any ongoing health issues or is on any other meds. The aim of these is to soothe that allergy to stop the drive for overgrooming this area. And just to note, if she was severely itchy, then sometime we do need to put these kitties on a short course of steroids from their vet to halt this skin allergy.
Finally, we do need to limit her ability to directly traumatize the skin (since this leads to risk of secondary infection which will just make her even more itchy). Therefore, you can consider fitting her with a buster collar to protect her from grooming the area. Alternatively (and often better tolerated) would be to put a baby t-shirt (with an elasticated waist) or onesie on her to protect her from overgrooming her back. Of course, it will depend just where on her back this is to determine which would be the best option for her.
Overall, these would be my suspicions for Sparky's current signs. So, do treat for fleas but also consider an antihistamine and blocking her access to the area. As long as she settles, we are happy. But if her signs are severe, then we may need her vet to start her on a short course of steroids to halt this skin reaction, soothe this irritation, and get her settled so that her hair can regrow.
I hope this information is helpful.
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All the best,
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