AHeadshaking in horses has been observed by horsemen and veterinarians for nearly 100 years. Several causes have been suggested, including middle ear disorders, ear mites, cranial nerve disorder, guttural pouch mycosis, and vasomotor rhinitis. But treatment has had little effect on this problem. In a study recently conducted at the University of California, Davis, it was discovered that headshaking was the result of a pathophysiologic mechanism similar to the photic sneeze in humans. Optic-trigeminal summation was suggested as the means by which the horse develops facial neuropathic pain in response to light stimulation.

This condition is characterized by shaking of the head side to side, up and down, acting like a bee has gone up the nose, snorting or sneezing with headshaking, and rubbing the nose on objects or on the ground while moving. The most common onset of headshaking is in spring and early summer when sunlight becomes more intense. These symptoms are usually enhanced when the horse is worked or exposed to direct sunlight. Outdoor behavior of horses which suffer from this condition suggests that the horses seek to diminish their exposure to sunlight naturally. For example, the horse will seek shady areas, or the horse may put his face right under the tail of other horses, such as following the horse in front of him on a trial ride so closely that his face is under the leader’s tail.

It is thought that stress created by sunlight and exercise may trigger a response of the central nervous system that causes the headshaking response in affected horses. With these stimuli, the horse experiences a tingling sensation or inappropriate stimulation of the sensory branches of the trigeminal nerve in the muzzle area. It is also thought that alterations in blood flow could explain the sudden nasal rubbing, snorting, and flipping of the nose. The pain felt by the horse is referred to as neuropathic pain, and may be persistent or intermittent