Good Evening, and thanks for the question.
Sorry to hear about your ongoing problem with your horses. I can certainly understand your frustration, especially with seeing them have to endure this for so many months! I would think that whatever is occurring would have taken it's course by now. I've got a few ideas for you to consider, but I need a bit more information from you first, please.
1): When did this initially start, and did it start with just one of the horses?
2): Have any of them had a fever associated with the runny nose?
3): How are they all doing otherwise, as far as appetite, attitude, etc.?
4): Have there been any other type of antibiotics used besides the Penicillins?
5): What is there living environment like (stall with paddock, open pasture, etc.)?
Thanks, ***** *****'m looking forward to your reply.