cat licking bald spots, cortisone doesnt help

Question
Dear Dt. Gotthelf,
I have a 2 year old British Shorthair cat.  For the past year, she has been
licking herself bald in certain spots.  I am a Veterinary Tech and can give her
terrific care but my Vet and I are baffled.  Cortisone shots help for a few days
(depo-medrol 1cc) and then she gets a scab in that injection site.  She turns
her nose up to hypoallergenic food.
She was fine for her first 10 months of life, it seems that when she went into
her first heat, this started.  She had a kitten and then we spayed her.  She has
kept up the behavior and looks terrible.
Otherwise, she is a healthy eater, she's playful and does all normal cat things.  
Do you have any suggestions?
She lived in my home until last July at which point I gave her to my sister
because I felt that she wanted to be an only cat.  It broke my heart to give her
away, but I only want what's best for her.  I have 3 other cats in my home.  
She is now in a one-cat household with tons of love and affection and no
other animals around.  She has become more friendly in this household and
seems to love being the only cat.  Can you offer any advice?

Answer
If she does not respond to Depo, then try chlorpheniramine 2 mg twice a day.  If after 2 weeks, there is no response, then I would advise a skin biopsy of an active lesional area to see if there is an organic skin disease present.  Ringworm, allergic disease, deep fungi and bacteria may be present.  In some cats, we are finding demodex mites, too.