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No point posting if you are going to be pedantic tamborine, I'm well aware of the breed I do read and have had dogs for many years, the point is Jet played very well with other dogs before being attacked.
chrissa1 I bought him a nylon muzzle which he chewed off very quickly, I had bought it at a pet shop where I spent quite a while trying to find the right one needless to say the staff weren't very helpful. The vet does not advise a muzzle, he said it's definitely a behavioural problem.
Thanks divegirl, I'll get one of these leads and give it a go along with everything else you have advised.