I have a 5 year old GSP. He is normally a very healthy vibrate working detection K9. But for the the first time in 5 years, during the spring to summer weather change and now in the fall weather change he has developed a dry hacking cough (not kennel cough). In the spring it required the common 10-day antibotic treatment to get rid of it. He has no other signs of problems, blood work is all excellent, good appetite, still wants to work everyday, he still has the energy level of a puppy,just this damn cough.

I feed a raw diet with supplements, vitamin b-complex, c,e,flax oil, etc. The vet basically says it is possiblly caused by the tempeture changes in our area during those two seasonal changes. We range from 40's at night to 80's in the day time. He does sleep inside the house overnight which could add a different kinda weather due to the AC/heater possibly being on during those times. Also, this is sinus and hayfever/ceder fever country.

Does anybody else experience these problems? Which supplements could or should I increase in his diet for stronger immune fighting? Any Suggestions

I wouldn't neccessarily take a dietary course of action for something like this. You really don't know WHAT's causing it. It could be anything from a physical disorder (such as a heart condition), to parasites, to seasonal allergies or some other type of allergy. From what I understand, if you really want to know the cause, you'll have to go through a process of elimination of the most common causes, from physical ailments to parasitic or bacterial or viral infections, to allergies.....there are just so many possible explanations, and not all of these have anything to do with the immune system.
Probably what I would do if this were my dog, is I would rule out any potentially life threatening conditions first and foremost, then go from there.
Since your dog isn't experiencing fatigue, a heart problem most likely isn't behind it. However, that doesn't rule out parasites; have you had a stool sample taken?
Since it seems to occur seasonally, I would tend to agree with your vet, or perhaps consider allergies of some sort. Those things would not be affected by antibiotics. Antibiotics should only be prescribed for bacterial infections.

Just a thought; have you tried keeping a humidifier going at night?

Hope this helps, just thinking out loud, so to speak.....

Thanks, J Parker, good reply. I didn't mention that in the spring we did a chest x-ray series to look at the heart/lung area. Looked good. Did a rectal test, I suppose for parasites/worms etc. and blood test. Everything appeared in the normal range.

You hit all the right things to check. I know alot of humans are having sinus/allery infections now but he has never been affected by these things in years past.

I am leaning toward sometype of bacterial situation as you mentioned but I am not sure how to prevent it. The humidifier or dehumidifier maybe a great option when he is in the house. I am thinking about an air purifier for the family as well so that may assist too. Thanks for the reply.