my 4 year old border collie bitch has a bottom jaw which hangs loose when she is relaxed, she has not had an injury and it slowly seems to be getting worse.  I have been to my vets who has no idea what is causing it. It's open about an inch and doesnt appear to cause her any discomfort.  She did not have it as a pup, it has been like this for about 12 months.  She can close it, and even does on command now when I say close your mouth but then it will fall open again.

Anyone?

No real idea but do wonder if she finds it easier to breath through her mouth and that the problem, if there is one, is in the nose.

I used to have a horse that had a constantly loose, floppy, bottom jaw when he was relaxed, but never seen it in a dog, sorry