Your cat most likely is suffering from eosynophilic granuoma -also known as a rodent ulcer. These are extremely difficult to get rid of. Here is a link to a website for additional information on the problem your kitty is having. http://www.thepetcenter.com/gen/eulcer.html Please let me know if I can assist further.
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Lori McLaughlin
Your veterinarian should ALWAYS be consulted first and foremost when your pet's welfare is at stake.