I have male rescue pit bull (about 5 years old) about 60 pounds, that came to me from a shelter. At the time I was fostering another pit bull.
The rescue had a terrible cough when he came to me and promptly spread it to the other dog. I used colloidal silver ( a couple of capfuls) in each of their full water bowls along with about a teaspoon of 3% hydrogen peroxide. It cleared up within a month as I recall. I've had him a couple of years now and no recurrence. The foster has been returned to his owner and she has not had any further problems either.