I have a 1-1/2 year old 50 lb dog that developed a bad cough over a two month period this summer, and began having trouble breathing on a regular basis. She was given a 2-week course of antibiotics which did nothing. Over time it seemed it became more like respiratory attacks rather than a chronic cold/cough. It seemed like asthma. Exercise seemed to trigger it and make it worse. The vet ran a bunch of tests about a month ago, and $600 later we had ruled out many things, like lungworm and pneumonia. They wanted to do a bronchoscopy next, but before I went that route (because it's expensive, requires anesthesia, and not necessarily definitive wrt diagnosis), I decided to see if there were dietary changes that might help, and decided to give this experiment one week to test.
I researched for a full day online regarding alternative/natural cures for asthma, in people and dogs, and as a result, decided on this protocol: mega doses of buffered, powder Vitamin C (right now, we're at 2000 mg per day, given in 4 separate doses for max absorption), 2 tsp raw, chopped garlic/day (1 tsp per 30# body weight is very safe in dogs), and salmon oil supplement plus canned wild salmon added to her food. She went from very ill to 95% better in about 3 days. She's been running like her crazy self the last 3 days, fully off leash and at full speed, and has only coughed twice and wheezed once. So, I'm sticking with this for now, and it marvels me how such simple nutritional changes can make such a difference in a creature's health.