What triggers a horse to flip his head uncontrollably, sometimes to the point of endangering him and his rider?

The amazing thing about horses is how such large, powerful animals can be exquisitely aware of the slightest sensation, such as a small insect on its back or face. Rippling of skin or an occasional head shake is a normal response to the tickling trigger of nerve endings. But, there are times when a horse can’t stop shaking or tossing its head to a seemingly inapparent sensation; such incessant behavior is known as headshaking.

Even when a human understands a source of discomfort or pain, it is hard to ignore it or stop natural aversion reflexes. In the case of headshaking, a horse doesn’t understand why his muzzle or face persistently feels a tingling or painful sensation. Headshaking behavior is considered to be caused by overactivity of branches of the trigeminal nerve that supply sensation to the face and muzzle. A horse’s behavioral reflex causes him to flip his head, snort or sneeze, rub his head, or take evasive action. Most headshaking horses (89% of them) flip their head vertically, according to research findings.

In general, the horse behaves like you might expect if a bee flew up his nose, making it difficult or dangerous for him to be ridden or handled.

Researchers agree this syndrome involves abnormal firing of the trigeminal nerve. Research (Knottenbelt, 2009) evaluated the response achieved by placing a coil into the infraorbital canal to put pressure on the infraorbital nerve, one branch of the trigeminal nerve. Continuous feedback induced by the coil was able to stop nerve firing, thereby corroborating the trigeminal nerve as a source of irritation