Horse headshaking or "the flicks" is defined as an abnormal behavior occurring when a horse shakes its head in the absence of obvious extraneous stimuli, and with such frequency and violence that it becomes difficult or dangerous to ride or appears to be distressed (Mair and Lane, 1993). Headshaking in horses is considered a natural behavioral pattern and part of their natural defense system when it is used to combat attacks by biting flies. This same behavior may also be seen in horses restrained prior to exercise; this is considered a normal sign of impatience and anticipation. In both these cases, the headshaking behavior is considered a normal physiological response (Mair and Lane, 1993)