by John E. Madigan, DVM, MS, Dipl. ACVIM
My horse is driving me crazy! Every time we go out to ride, he starts flipping his head. I've changed his bridle, put on gallons of fly wipe, and had his ears and eyes checked. What is his problem, and what else can I try?
Headshaking 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.
Also of note is that in humans it is thought that a central sensitization process following certain peripheral injuries can lead to central nervous system changes that permanently alter pain perception and thresholds, and this may hold true for equines as well.
Headshaking can manifest itself in all breeds and disciplines equally because it has a physiological basis and it can cause headaches for owners or riders. Horses which exhibit headshaking syndrome become unruly or even dangerous when they are worked, many times with the blame falling on the rider. It previously was thought that perhaps the bridle was illfitting or the rider was too busy with his or her hands. But now there is evidence that it is not the rider at all, but rather a response to the burning and tingling sensations stimulated by stress in the horse.
While nothing can be pinned down as to exactly why some horses develop headshaking and others do not, it is thought that horses which have been exposed to the EHV-1 virus are more at risk. With this theory, the herpes virus lies dormant in the horse's trigemina ganglia, then with heat and stress created by either direct sunlight or intense exercise, the virus becomes active again, this time affecting the central nervous system and producing the symptoms characterized by headshaking. More studies need to be conducted to determine if EHV-1 has a connection with headshaking, but evidence now available is pointing in that direction.
Once your horse has been diagnosed with headshaking, there are a few options available as treatments. While there is no "cure all" when it comes to treating this condition, there has been some success with cyproheptidine therapy with or without environmental protection from sunlight, or light blocking eye protectors which have been shown to provide moderate to great improvement in three-quarters of the cases which headshake. Cyproheptidine, which is an anti-seratonergic, anti-cohligenic, and anti-histamine drug, works effectively in most horses inhibiting the normal activity in the trigeminal nerve.
Because headshaking is most commonly brought on by intense sunlight and exercise, the reduction of sunlight exposure usually helps reduce the symptoms. For example, many owners have found that if they work their horses at night, the symptoms are all but gone, but if they work their horse during the day, the symptoms appear within a few minutes and worsen the longer the horse is worked. The horse should also be kept in as dark an environment as possible. So in addition to treatment with cyproheptidine therapy, you should include environmental protection that shields the horse from as much direct sunlight as possible.
Additionally, headshaking tends to be a seasonal problem in most horses. It's usually the same time each year that the horse begins to exhibit headshaking symptoms. A horse might only headshake from March to August, or in severe cases, he might headshake year round. One of the reasons headshaking shows up seasonally is that seratonin and melatonin changes occur with the changes of the seasons.
John E. Madigan, DVM, MS, Diplomate ACVIM, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis.
Reprinted from The Horse: Your Guide to Equine Health Care
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