Cat that's scratching her neck raw

my 10 year old Persian cat has been itching for the past month. She had the same issue 4 years ago, but I thought it was solved with a diet change since it stopped as soon as I put her on Hill's Prescription Diet z/d which is for sensitive skin and stomach. However, it has come back. I checked to see if they changed the ingredient in the food, which they did and I changed her diet to the Hill's Prescription Diet d/d and its not going away. I've taken her to the vet and he said that it could be allergies or an ear infection which was treated but the skin irritation is still prevalent since the treatment. We've given her antihistamine and all the other medication that my vet recommended...nothing is working. She's scratching her skin raw to the point where it won't stop bleeding. She's become very antisocial and aggressive towards me and just lays under the treadmill all day. She eats normally however but isn't herself otherwise. We have another cat which is also a Persian and he is fine. I don't know what to do...

Hello - and I'm sorry that you have this frustrating situation.  It is often said that the most likely reason for cats to scratch is fleas and indeed, an itchy cat should be flead regularly with something that treats the environment as well as the cat, because even when fleas didn't cause it, they can exacerbate it.   Allergies are also possible and, while we tend to think of cats as being allergic to 'this' or 'that,' the numbers of things that a cat can be allergic to can increase over time.  However.....

Self mutilation can be even more serious - for example, an inability to deal with pain e.g. toothache or even elsewhere in the body, or even as a sign of skin cancer or a neurological problem.  So, while this seems 'only' skin deep, it may not be.

Furthermore, to anthropormorphically (thinking of an animal as a human) imagine damaging one's own skin like that, some level of pain does seem likely.  So, as the problem is continuing, repeat visits to the vet will be necessary.  It is worth being aware that tests may be needed before a diagnosis is found and that ideally, treatment may be two-fold:  a) giving drugs etc to keep skin disease under control and b) doing further investigations to try to pinpoint the cause, or to rule out some possibilities.