Thanks so much for the additional information. I'm sorry not to have answered you sooner, but I've just now logged on to the website.
I also want to apologize for my long answer but there's just no "short" way for me to answer your question.
The scabby skin sounds like Miliary Dermatitis which is what happens when a cat overgroom or licks their body excessively (because the tongue is so rough). Sometimes these areas can become infected and when they do, then I dispense antibiotics.
There are several different reasons why a cat can become itchy to the point where they overgroom, the hair can become thin (or even bald) and Miliary Dermatitis can develop. Most of these conditions are "controlled" but not cured, unfortunately. This is very similar to allergic type reactions in humans and it may be helpful to think of Bella's problem in this way.
1. Fleas. You may or may not see fleas since they can hop off and on a cat's body and they can be ingested from all the grooming.Cats can become allergic to the flea saliva and literally one or two fleas can drive them crazy. And, the areas of their bodies that they tend to lick a lot are exactly what you describe....the spine in front of the tail. So, I would want to rule this problem out just to be safe even though you use a flea product on her. Most of these products are not very effective at repelling the fleas; they basically work best by killing the flea as it feeds. And, since it's the saliva that triggers the allergic reaction, the damage has already been done even though that particular flea may soon die.
This is a link that discusses this problem in great detail: Link
I have seen some cats behave in some strange ways when fleas are the underlying problem...they can be more nervous and skittish...some will even start hiding and not engaging with their families. But what they almost universally have in common is that the area in front of their tails is affected and they are a little sensitive in that area.
No question, fleas are a huge nuisance and a frustrating problem to eliminate especially if a cat goes outdoors where they can pick up more fleas. Treating the house and all pets is essential in trying to get this problem under control.
2. Food allergies.
These types of allergies are usually to ingredients in the food that a cat has been eating for a while--it's not usually new food. Changing to a hypoallergenic diet may be helpful such as grain-free, or Z/D (available from your vet) or Natural BaXXXXX XXXXXmited Ingredients, Nature's Variety Instincts line, Evo Duck or Venison, Nature's Variety Frozen Raw Medallions (I recommend that they be zapped in the microwave for 10-15 seconds on each side).
If this is the sole reason why Bella is itchy, then a change in diet should significantly help her. It can often take a month or more to see improvement in the skin though.
Most cats with food allergies will not be 100% responsive to steroids though so if she was, then this is not likely to be her only problem. Cats can have multiple allergies , so it might be worth considering a change in diet to eliminate this from the list of possible reasons for her behavior.
3. Problems with the spine can cause it; this is a more likely if the over-grooming is in one specific area on the back or tail such as you describe. There may be pain or discomfort in this area of which you are unaware but Bella is reacting to it by licking there.
This is going to be a harder problem to diagnose without trying pain drugs such as Buprenex.
4. Pollen allergies such as dust mites, mold, grasses, trees, etc. These cats can lick just about anywhere on their bodies with this problem.They may or may not respond to antihistamines (see below) but they almost always respond to steroids which can help with the diagnosis.
These types of problems are usually seasonal in nature, very similar to humans. Again, control versus cure is what is most commonly seen since allergies of this type are a fact of life for humans, dogs and cats alike.
5. Cats with hyperthyroidism can develop excessive licking and hair loss due to the licking although it's uncommon that Bella's skin problems would be due entirely to an overactive thyroid but it could be a contributing factor. Most, but not all, of cats with this condition will have other symptoms such as weight loss but a good appetite, increased activity level, sometimes vomiting. It would take a blood test to rule in (or out) this condition.
We have many good treatment options for this disease so I actually consider it a good condition to have for an older cat.
6 Cheyletiella which is an external parasite that can be very itchy.( LINK) It's also known as Walking Dandruff (although you might not actually see the dandruff move in all cases).
It's not always possible to find this mite when looked for under the microscope but if suspected, then I treat with Revolution every 2 weeks for 3 treatments although there are other treatment options (such as Ivermectin).
All pets in the household should be treated at the same time since this can be a contagious condition.
The link I sent above also discusses treatment of the environment which will be necessary to prevent reinfection.
You do have a few over the counter treatment options that you might consider for Bella in an attempt to make her less itchy. I'd also consider changing her food as I mentioned above.
Antihistamines can help in some cases.
Benadryl at a dose of 1/2 of a 25 mg tablet given twice daily or
Chlorpheniramine at a dose of 2-4 mg twice daiy or
Claritin at a dose of 2.5-5 mg twice daily or
Zyrtec 5 mg/cat once or twice a day
It is important to ensure that the formulations used contain ONLY the antihistamine and are not combination products (e.g. Claritin-D contains pseudoephedrine, which could cause very significant adverse effects in a cat).
Sedation is a common side effect with these kinds of drugs.
I hope this helps and gives some ideas to consider as to why Bella is having these recurrent problems with her skin. Deb