My cat scratches herself raw

Question
Hello.  I have a cat that is approximately 9 years old. (I'm guessing, because my husband adopted her when she was abandoned in an apartment building 3 and 1/2 years ago... our vet told us apx how old she thought she was.)  After about 6 months she started scratching the area between her eye and her ear.  We used to think that she may have mites in her ears, but she never has.  The vet would give her a steroid shot every once in a while (depo) and that used to help for about 3 months, and then she'd go back to scratching it again.  And when I say "scratching," I mean scratching it until it is raw and bloody.  Then it will start to scab over, but she will scratch the scab off and it will be bloody and raw again.  After some time, the shot would become less effective, only lasting about a month.  About a year ago,  It doesn't bleed, but it has become very thick, inflamed-looking skin covering a large portion of the outside of she started, not scratching, but licking her right leg raw. (along with scratching the area above her eye)  Since Puddin weighs almost 17 lbs., our vet doesn't want continue to give her these shots for fear of developing diabetis.  We have had tests run... a certain bacteria came up negative, another test's lab results showed it was most likely caused by some kind of allergy.  We are currently in the process of changing her food to the z/d diet, but I am afraid to hope it will really be the 'magic cure.'

Have you ever heard of anything like this before?  If it were a food allergy, wouldn't it itch all over her skin, and not just in one or two isolated areas?  Any advice or ideas would be greatly appreciated.

Answer
Given her history of abandonment and no positive results to any testing it is possible that she has psychogenic alopecia.  This is in effect an overgrooming, often stimulated by stress, boredom or change of environment which becomes a compulsive behavior very difficult to correct.  It as will allergies often is shown in places that your cat can reach to scratch or lick itself.  If the food change does not turn out beneficial I would ask your veterinarian about the possibility of this being her problem.

If she is indeed one with psychogenic alopecia it may help to provide structured play supplemented with interactive toys and interesting stimulus (such as cat videos or a fish tank) while you are not a home.  If none of these help anti-anxiety medications can be attempted.

Please let me know if I can be of any other assistance.  Best of luck to you and your feline friend.