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It sounds like this is probably due to a collapsing trachea. These dogs will cough more when in hot/ humid weather or when they are exercising. They also cough more when being leashed with a collar as there is pressure on the trachea. These dogs do better in cool weather and with a harness when out for a walk.
Prednisone does help. I also like to use a cough suppressant as needed- Codeine or Hycodan. I don't use any diuretics unless it's coughing secondary to heart disease. I don't think the Lasix is needed here.
Here's a nice handout on the condition:
I hope this helps, let me know if you have any other questions.