I was hoping to get some additional information from you, but, I didn't want you to go any longer without an answer.
There are various possibilities for your cat's symptoms. If she is itchy, but the vet said she doesn't have fleas, all it takes is the bite of one flea, in a 'flea-allergic' cat, to cause a system-wide reaction. It's possible this is what has happened and the flea is long gone. You can give her an over the counter antihistamine to help with the itching for the time being and discuss with you vet if an injection of an anti-itch medication might prove more useful. I prefer Chlor-trimeton, and you can give 1/4 of a 4 mg. tablet, twice a day, as needed. Here is more information:
In addition to fleas, there are other external parasites that are very tiny and can burrow under the skin, like certain types of mites, so discuss this with your vet, as well.
The excessive biting, licking and scratching may be due to an obsessive-compulsive condition called Feline Psychogenic Alopecia. The cause of this is difficult to determine, but sometimes, it's stress or due to changes in environment, boredom, and the vet usually treats it with a steroid shot and sometimes, antihistamines, to stop the itching and make the skin less sensitive. Here is more information:
If this behavior may be caused by anxiety or stress, Feliway, a cat-calming pheromone will help.
Food allergies are possible, so try to feed mostly high quality canned food containing no grains or artificial additives, preservatives or carageenan, as these are common allergens for cats. Sometimes, cats are allergic to the protein (meat) source in their food, so you can try a 'limited ingredient diet' with alternative protein and carb sources. These are made by various manufacturers, like Royal Canin, Natural Balance, Blue and many others.
Her weight loss may be due to stress or not eating as usual, because her physical symptoms are bothering her. Offer her some 'Cat Sure', a cat version of 'Ensure' for humans, to help replenish her vitamins and essential nutrients.
If a blood test and urinalysis were not done at the vet's, that would be a good next step, to make sure all her levels are within normal limits.
I hope things improve for your furry girl and the cause of her symptoms can be identified and treated appropriately!
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