Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm really sorry to hear about Mandy's excessive grooming.
The most common reason for these symptoms is flea allergy dermatitis. Flea allergy dermatitis is horrible. In allergic cats it only takes one bite a month for them to itch like crazy. If you don't see any fleas you might not have a huge population. If you have other pets who aren't itchy they may not be allergic. I recommend flea control year round in an allergic cat. Many over the counter products have a problem with fleas being resistant and some can be down right dangerous. I never recommend any products that Hartz or Sargent's make. I recommend Advantage II or Frontline Plus every 3 to 4 weeks or the new Seresto flea collar, which lasts for 6-8 months. All pets in the house must be treated as if you don't the nonallergic ones serve as a flea reservoir for the allergic one and you'll never solve your problem.
Inhaled allergies (to pollens, dust mites or mold spores) are another possibility for itchy skin.
To control allergic symptoms you can try:
1) Benadryl (diphenhydramine only, the combination products with decongestants and acetaminophen are toxic to cats) at 1mg to 2mg per pound every 8 hours. That's a half of 25mg tablet every 8 hours for a cat that is 8 to 15 pounds.
2) Chlorpheniramine at 4mg twice daily.
3) Zyrtec (Cetirizine hydrochloride) at a dose of 5 mg per cat given orally every 24 hours. Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because cats cannot tolerate decongestants.
Most cats respond best to either Zyrtec or Chlorpheniramine, but if not try Benadryl.
Combined with high doses of omega-3 fatty acids antihistamines work to relieve her itchiness. I find 3V caps or Derm Caps to be excellent omega 3 fatty acid supplements. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give hwe 20mg of EPA per pound of body weight per day. For example an 8 pound cat could take 160mg of EPA per day. Antihistamines and omega-3 fatty acids work together synergistically, better than either one alone. These should help reduce the itch. Be aware that antihistamines can cause drowsiness or hyperactivity which should resolve with continued use.Food
allergy is another reason for excess grooming if she seems to be itchy year round. Blood tests are very unreliable to diagnose a food allergy. The best way is to feed a true hypoallergenic food and see if she improves so you might wish to try a true hypoallergenic food for 8 weeks or so. My suggestions are Hills Z/D or Royal Canin Duck and Green peas. She will need to eat only that food, no other treats or foods during her food trial. The over the counter low allergen foods aren't usually restrictive enough to diagnose a food allergy.
If there are other cats in the home that she doesn't get along with or outdoors that upset her perhaps this is stress related. If so a product called Feliway, which is a synthetic version of a calming feline pheromone, may help. You can purchase this from petstores or online.
If she goes outside other parasites like Cheyletiella, demodex or sarcoptic mange are possible causes as well.
For now I would start with continued flea control, an antihistamine and omega 3 fatty acids. If she's not significantly better in 2 weeks then it's time for another veterinary examination with sedation before hand to look further for problems and/or a hypoallergenic food trial.
Please let me know if you have further questions by replying to this post.