What action is taken for any equine found at an event without the required negative test result?

For any equine found at an event without the required negative test result, the appropriate action is to isolate the equine. This measure is crucial for maintaining the health and safety of other horses at the event. Isolating the horse helps prevent the potential spread of any infectious diseases that could be present. It is a standard biosecurity protocol aimed at protecting the larger equine population from exposure to pathogens that may be harmful.

The other options do not appropriately address the health concerns associated with unwarranted exposure to an untested equine. Allowing the equine to compete would compromise the safety of all participants. Returning the horse to the owner without any precautions could also pose a risk if the equine is carrying an infectious disease. Offering the equine for sale does not align with health regulations and ignores the necessary precautions that should be taken when dealing with animals that have not undergone required health testing. Therefore, isolating the equine is the correct and responsible action in this scenario.