You can ask ALL attendees to present health certificates for the horses, but you can't single out one on a MAYBE you have a horse that MIGHT have come in contact with said disease. I know what you are speaking of it, it's here, and all shows here have been cancelled. No animals in or out right now but we have proof of the presence of the disease. By informing one person they are not welcome you opening up yourself to lawsuits and a lot of grief; by asking everyone for health certificates dated within the last 14 days you are giving everyone the chance to decided on their own, if they want to spend that money for the certificate and attend, or stay home.