Sue McDonnell, PhD, Certified Applied Animal Behaviorist, was preparing a horse for long-distance transport. Getting the horse from her home base at the University of Pennsylvania's New Bolton Center to his final destination involved a jostling trailer ride to the airport, then a cross-country flight. Compound that with separation from his stablemates, a disruption of his normal feeding schedule, and having to adapt to his new environment, and the horse was under considerable stress.

"Every time we put a horse on a trailer, I'm amazed at how well they do and how much we expect from them," said McDonnell, who is the founding head of the Equine Behavior Program at the University of Pennsylvania School of Veterinary Medicine. The horse on the move is exposed to nearly every factor recognized as a potential stressor for horses. Each one has the potential to make him vulnerable to a myriad of physical ailments, from respiratory infections to gastric ulcers.

But horses needn't be in extreme circumstances such as long distance travel to experience stress. Extreme hot and cold climate conditions, training regimens, dietary or feeding schedule variations, general daily routine disruptions, injuries, and changes in social interactions with other horses and humans can all represent equine stressors