I'm new to this forum but wanted to put something about coughing on for other people who may be looking as I was.
My 12 year old Sheltie developed a chronic cough 9 months ago. He did not act as though her were ill and was not off his food. He was xrayed and had all the blood tests done which showed nothing. He was put on a bronchial dilator but this did not seem to do anything. The cough subsided eventually but didn't go away entirely. It returned with a vengeance a few months later and continued to worsen. More X-rays were done and a lung wash and tube down the throat to look at the trachea wall.
Lung wash was clear. Heart is fine. No bacteria. Trachea very calcified due to the length of time he has been coughing. Steroids were prescribed along with Corvental D, another bronchial dilator. This time the cough responded and was improved 60%. As I was concerned about long term effect of steroids the vet suggested we try an inhaler. Sav has taken to this very well :-) its a bit like the sort of contraption used for a child who can not breath in deeply. A tube with a muzzle cup of soft rubber. The puffer is expelled into the tube and then sav has to take 4-5 breaths. You can tell when this is happening as there is a flap which moves when a breath is taken in and out. The inhaler itself has to be bought from a chemist as there is no animal version yet. It's expensive! But it's so worth it. The cough is now 90% improved and sav knows he gets a treat after the inhaler. I still have no definite diagnosis but thought if anyone was wondering whether these things work it's worth a try!  If anyone else has any other advice I'd be glad to hear it.