A sting in the tail

I have a six-year-old Jack Russell Terrier who is obsessed with his tail. He has an exceptionally long and fluffy tail which curls over his back. He has never liked his tail since he was a puppy, and seems to view it as competition to himself. Now and again he would have a go at it, then he would forget about it for months and then start again for a few days. During these times, he did not severely attack it, drawing blood etc but within the past few months he has started to really attack it and draw blood. We have had to start putting on a muzzle to protect the tail. However, as soon as we take off the muzzle, he starts on the tail again. Any suggestions?

Ann Gillies, by email