Better treatments take the headache out of this annoying and sometimes dangerous behavior

by Kenneth L. Marcella, D.V.M.

HEADSHAKING is a very frustrating condition that can turn a valued equine athlete into an unridable nuisance. It is such a noticeable behavior, and it has so many possible causes and potential treatments, that it remains a confusing affliction to most horse owners, despite the fact that it is written about frequently in equine publications.

New information about headshaking horses seems to come to light often, however, and it is an area of almost constant research leading to the development of new products and treatment strategies. An article in the Equine Veterinary Journal in 1987 described headshaking as "a widely recognized entity but one of the most poorly understood conditions affecting the riding horse." As recently as 1992, Robert Cook, D.V.M., Ph.D., then a leading researcher at the Tufts University School of Veterinary Medicine, summarized the status of equine headshaking by stating: "Prognosis poor, condition incurable." These descriptions need to be somewhat updated now, as the body of information concerning headshaking in the horse has begun to yield some important findings and our knowledge of this still frustrating condition is much greater than ever.

Different severity

Equine headshaking is a specific, patterned behavior where the horse tosses or shakes its head while being ridden or exercised. Affected horses usually are adults of either sex. All breeds have been reported to be affected. Though headshaking signs are worse when being ridden or lunged, they sometimes can be seen at pasture and at play. Stress increases signs; tossing of the head is virtually absent at rest.

Affected horses will shake or toss their heads in a side-to-side motion, a circular motion, or an up-and-down motion where they jerk the nose, sometimes very violently, toward the chest.

Headshakers are graded from 1 to 5, based on the severity of the motion:

•Grade 1 horses show mild and intermittent facial muscle-twitching and mild headshaking; these horses still can be ridden;

•Grade 2 horses show increased intensity of these signs and more frequency of their occurrence;

•Grade 3 horses are difficult to control;

•Grade 4 horses are unridable; and

•Grade 5 horses are dangerous.

A headshaking horse tends to act as if it is experiencing some type of facial or nasal irritation. It often will strike at its nose with a foot, or it may rub and scratch its nose on a foreleg, post, tree, or even the ground. Some horses may sneeze, cough, or have nasal secretions and tearing from the eyes. These horses often try to avoid light, wind, or warmth on their noses. They may be spooky or uncomfortable going from dark barns to bright light, and they may show increased headshaking behavior on dusty days with increased pollen or particulate matter in the air, such as smog, burning leaves, or other organic material and dust.

Headshaking tends to be seasonal in some horses and starts in the spring, stabilizes through the summer, and resolves in the fall. Many of these horses will show signs only during daylight and can be worked normally at night.

No clear-cut cause

So many owners have had problems with their headshaking horses that a National Equine Headshaking Database was started in 1998 by veterinary researchers at the School of Agriculture at De Monfort University in Leicester, England. This database now contains information on thousands of headshaking horses, which allows scientists to draw some important conclusions about the condition.

Researchers at De Monfort now know that more than 60 equine diseases or conditions can cause horses to show headshaking signs. This means that diagnosing the specific cause of an individual horse's shaking problem is very difficult. Information in the database and studies of large numbers of horses identified as headshakers also show that even if the treating veterinarian diligently checks for all these diseases, no definitive diagnosis for headshaking will be found in most cases. Clinicians have a requirement to look for a possible cause, nonetheless.

Veterinary clinicians will perform a complete physical examination, including radiographing the head, examining the eyes and mouth, and performing an endoscopic examination of the sinuses and nostrils. If there is any apparent problem causing the headshaking, it can then be addressed and resolved. The list of different causes of equine headshaking includes:

•Infections of the sinuses or the nostrils;

•Diseases of the guttural pouches (the blind-ended openings in the sides of the horse's throat);

•Dental disease (infected teeth and gums or simply sharp, imbalanced teeth that can interfere with the bit);

•Nasal tumors or polyps;

•Seizure disorders;

•Behavioral conditions that resemble obsessive-compulsive disorders;

•Ear mites and ear infections;

•Diseases of the eye; and

•Allergic responses to flies, insects, dust, pollen, and mites.

Light sensitivity

More recent research information has increased the likelihood that a headshaking horse may be a photic headshaker. These horses are extremely sensitive to light. It is believed that horses in this group have an inherited light sensitivity of the fifth cranial nerve. This nerve is called the trigeminal nerve, and it provides sensation to the nostrils and parts of the nose and face. Extreme sensitivity to light stimulates the trigeminal nerve, making the horse's nose itch or twitch. In response, the horse rubs its nose or shakes its head.

This condition also exists in humans where it is called vasomotor rhinitis or trigeminal neuralgia. Interestingly, in humans, this facial pain and muscle twitching in response to light is seen predominantly in women over 50, but most people can relate to the sensation of nasal irritation and the resultant sneeze caused by gazing into the sun. Intensify that response greatly, and you have the basis for photic headshaking in the horse.

Possible solutions

Researchers at Pennsylvania State University investigated the use of perineural anesthesia as a possible treatment for photic headshaking. In this study, 20 headshaking horses were identified and clinically examined. Radiographs of the head, endoscopic examination of the nostrils and throat, eye examination, dental examination, and evaluation of these horses at rest and after exercise revealed no cause for headshaking.

An anesthetic drug then was injected into a specific part of the trigeminal nerve called the posterior ethmoidal branch (PET). Of the 20 horses evaluated, 17 showed 90% to 100% improvement. When these same researchers used a long-acting irritant solution designed to damage and disrupt the PET, they found, "Headshaking was completely abolished, in some cases with effects lasting for six to nine months."

Other treatments for photic and various types of headshaking involve both high-tech and low-tech approaches.

Sometimes headshaking is related to the rider or to ill-fitting tack. Switching riders can reveal a rider-related cause for the problem, and lunging the horse with and without tack should reveal if tack is a possible cause of headshaking. If headshaking occurs under tack, the owner should carefully inspect the fit of each item.

Many horses show a reduction in headshaking when wearing a nose guard. This can be as simple as a piece of pantyhose tied to the noseband of the halter or as complex as a commercially made, contoured, net mask that attaches to the noseband and covers the lower part of the horse's nostrils and mouth.

Various styles of masks are produced for headshaking horses by Equilibrium Products of England. Working with researchers at De Monfort University, these nose nets have been designed specifically to treat horses with this behavior. Tests have shown a 25% reduction in headshaking in some horses wearing these nose nets.

Some masks for headshakers have been equipped with a dark-tinted, acrylic shield much like ski or motorcycle goggles. Studies by John Madigan, D.V.M., M.S., and his research group at the University of California at Davis showed that dark-tinted contact lenses also achieved a high rate of success in controlling headshaking. Various companies produce tinted contact lenses for horses that need a long-term solution for headshaking.

Madigan also suggested blindfolding and lunging an affected horse to determine if it is a photic headshaker. Horses affected by bright light will show an immediate cessation of headshaking once they are blindfolded, and most horses tolerate lunging with a dark blindfold.

Another well-established treatment for the successful management of headshaking is the use of a combination of cryproheptadine (Periactin) and carbamazepine (Tegretol). Cryproheptadine is an antihistamine and an agent that blocks various nerve-conducting substances (neurotransmitters) in the brain. Carbamazepine is an anticonvulsive drug that affects how sodium is moved from cell to cell. Use of a combination of these drugs showed an 80% to 100% improvement within three to four days of beginning treatment.

Pennsylvania State University's study concluded: "Equine headshaking might be a trigeminal disorder consisting of neuralgia (damage to the nerve) in which differing trigger mechanisms may explain the variety of headshaking movements." Those trigger factors may be categorized as pollen, dust, and various allergens. Drugs that slow or reduce the trigeminal nerve's ability to respond to these triggers will lessen or eliminate headshaking.

Some scientists hope genetic research will identify a gene marker for a predisposition to headshaking and that this information will be instrumental in reducing the incidence in the equine population. But that day is far off. For now, there are many more ways to help headshaking horses than just five years ago. New developments in this rapidly changing area of equine research continue to appear.

In every headshaking study during the past three years, a few horses did not respond or show improvement, but most horses were helped. The use of masks and contact lenses allows many horses to return to functional use, and drug combinations now are helping other horses to resume athletic competition.

Kenneth L. Marcella, D.V.M., is a practicing veterinarian in Canton, Georgia.