I called the vet and he told me to come by and pick up some Uniprim (powdered feed-type antibiotic) to give him for 7 days, so I did, but that didn't even phase it. I finally ended up having to give him 25cc of penicillin every day for 10 days to get rid of it, and even then it started coming back a few days after I stopped the penicillin.
I mentioned all this to the guys down at the co-op and they recommended I give him some Aureomycin. It's a granulated tetracycline-based feed type antibiotic/growth hormone that's labeled cows, sheep and pigs, but they said they gave it to their horses any time they started getting a little snotty or coughing and it seemed to nip it in the bud. At that point I was willing to try anything. It wasn't that expensive either, ($17.95 for a 50# bag) compared to the Uniprim ($35 for a small container that lasts 7 days). You feed about a pound per day per horse and they seem to like it. It looks about like crushed graham crackers. I ended up having to give it to Whirl too because she started coming down with it after Goose and it kept her from getting a full-blown case. Kari and I split a bag and she used it on her bunch too and it worked on them.
I'd never heard of it before but sure was glad those guys at the co-op told me about it. It was a real life-saver so I thought I'd pass it along.
Thanks for the info. You feel so sorry for them, when they cough so much.