He may be doing this because he actually has problem that he is trying to 'fix' but he is not hitting the right spot. Usually when a cat overgrooms, it's because something is bothering them in that area. On a rear leg, my guess is he has impacted anal glands and he is unable to reach the area and do anything to ease them. I would do two things. First I would get some Microtek Spray either from a local pet supply or order it online at
www.revivalanimal.com. This product is good because it will treat a great many skin conditions, so if there truly is a problem with the leg, this will help. Also, I would take him into the vet if this does not help so they can express his anal glands. Many times, this is causing discomfort and doing that will take care of it.
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Lori McLaughlin
Your veterinarian should ALWAYS be consulted first and foremost when your pet's welfare is at stake.