All horses shake their heads sometimes. While the skin on a horse’s flanks or abdomen can twitch to dislodge a fly, the skin on his face doesn’t have that ability. So, if a fly lands on your gelding’s face, he will instead shake his head to get rid of it. Horses shake their heads for other reasons too: your mare might find the position of the halter on her head annoying, so she shakes her head to move it away from a sensitive area. Your horse may also shake his head impatiently as you wait to enter the jumping ring – all perfectly normal. As a medical or behavioural condition, though, headshaking refers to a horse that repeatedly shakes his head, even when there is no apparent reason.

It appears to be involuntary, and the horse often appears to be in distress. The headshaking is usually an up-and-down movement – like nodding – and is often combined with sneezing, snorting, sometimes with extension or kicking of forelegs and sometimes rubbing the nose on the rider’s leg or on the ground. In many cases, the horse does not display this problem when at rest, but starts headshaking after the first 10 minutes or so of being ridden and tends to get worse the longer the exercise continues. While most cases of headshaking are related to exercise, some horses headshake when eating or at other times as well.

Frequently, the head-tossing motion is violent enough to throw both horse and rider off balance and to make it impossible to continue riding the horse. Headshaking is known to occur more frequently in spring and early summer and many cases disappear spontaneously in the fall and winter. Some horses do headshake all year round, however.

Dr. Fernando Marqués, professor of Large Animal Medicine at the Western College of Veterinary Medicine in Saskatoon, said, “We see cases only sporadically, as the disease is not very common. According to an Equine Health Survey published by Slater in 2013, the prevalence is 1 – 1.5 per cent.”

Headshaking has been reported in many countries around the world as well as Canada. It doesn’t, however, seem to exist among wild horses.

An Array of Possible Causes

What causes it? This is the big question, because identifying the underlying cause would lead to developing successful treatments. There have been many different theories over the years about possible causes; however, the current consensus is that there is no single cause.

In a minority of cases, said Dr. Marqués, this behaviour seems to be caused by an illness such as middle or inner ear infections, guttural pouch disease, dental problems, temporohyoid osteoarthropathy (progressive disease of the middle ear), ticks or mites in the ears, allergic rhinitis, eye disorders, equine protozoal myeloencephalitis (a neurological disease caused by a tiny parasite) and others. Some research suggests that overweight horses are more likely to become headshakers, although it is not yet understood why this might be a factor.
In some cases, Dr. Marqués added, it appears to be related to the use of inappropriate bridles and bits or to a rider who is causing the horse pain due to lack of skill.

One popular theory in the past was that this was entirely behavioural: the horse had figured out that by shaking his head, he could get the rider to dismount and give up on the exercise session. This led at times to the rider punishing the horse in an attempt to stop the headshaking, something veterinarians today do not recommend.

Currently, veterinarians believe that the abnormal movements of the head are caused by a reaction of the trigeminal nerve. This nerve has three branches on each side of the horse’s head. One goes under and around each eye, one goes down the horse’s cheek and one extends the length of the horse’s face to his nose.

“The bottom line is that the clinical signs of headshaking are now thought to be caused by trigeminal neuropathic pain,” said Dr. Marqués. Researchers Prickles, Madigan and Aleman suggested in 2014 that the condition be described as trigeminal-mediated headshaking, because their research suggests a hypersensitivity of that nerve leading to the abnormal behaviour.

Dr. Marqués added, however, that “the underlying mechanism remains elusive and it is probable that a variety of diseases or conditions could trigger this response in the trigeminal nerve.”

On the site www.headshakerinfo.org, created by University of California Davis professor Dr. John Madigan, the point is made that when autopsies are done after horses with headshaking die, the trigeminal nerve appears normal. This means that there is no actual physical damage to the nerve involved in headshaking; rather, the nerve function has become abnormal for some reason. This piece of information, according to Dr. Madigan, is, in reality, a positive discovery. Since the nerve remains undamaged, there is a possibility of finding a cure that would make the nerve function normally again.

A Seasonal Condition for Some

About 65 per cent of headshakers are affected seasonally (the others shake their heads year-round). These are the horses, as described earlier, that tend to be worse in spring, summer and early fall and improve over the winter. Sunny days are much worse for these animals than cloudy, rainy days. Often they will improve significantly if kept and ridden indoors. Since sunlight seems to be the root of the problem for these animals, they are described as “photic headshakers.”

Some of the year-round headshakers will react in response to certain sounds, or when eating carrots or fibrous hay. Some of these will do dramatically better if switched to a soaked, pelleted food. Equine naturopath Julie Massoni of Greenpet suggests that headshaking is often caused by excess potassium in the horse’s diet and seen most commonly in the spring and summer because of the high levels of potassium in freshly-growing green grass.

Because the underlying cause of any individual case of headshaking isn’t known, there is no specific treatment or cure, explained Dr. Marqués. Many researchers have studied different strategies and a number of commercial products have also been developed, but Dr. Marqués said the effectiveness of these tends to be poor and extremely variable. This makes sense when you consider the many possible factors involved in the development of headshaking.

Attempts at Prevention

There is currently nothing known that can prevent most cases of headshaking.

The first step if your horse begins headshaking would be to have your veterinarian check the animal thoroughly for any of the diseases which can cause this behaviour. Infections in the ears, dental problems and other conditions can be treated and this should eliminate the headshaking as well. You might also want to consider if your bit, bridle or riding technique could be a factor in causing pain or irritation for the horse, especially if it started soon after buying a new piece of equipment.

If no diseases or infections are diagnosed, it may help to track the environmental situations linked to your horse’s headshaking. Does it seem to be seasonal and related to going out into the sunlight? Does it get worse with exercise? Did it start after a change in feed? Keeping track of the conditions that seem to trigger headshaking will help you work with your veterinarian to decide which treatments might be helpful.

Attempts at Headshaking Treatment

The headshakerinfo.org website includes numerous treatment options, pointing out that “to date, there is not one treatment that works in all horses, and the results for each treatment vary widely with individual horses.”

Some options horse-owners can consider using without a prescription:

  • If your horse seems to headshake in response to light, you could keep the horse indoors during the day and ride in an indoor arena if possible. Some have suggested that light-blocking contact lenses could help, but a 2000 study by Newton et al. found these were not effective. UV-blocking sun shade fly masks do seem to help some horses.
  • Ear covers and fly fringes seem to help some horses who headshake during exercise.
  • One of the most effective treatments, according to research, is the ‘nose net’ – a mesh net covering the nostrils and muzzle. In a 2003 study by Mills et al., three types of net nose coverings were tested, and 75 per cent of the horse owners reported some improvement with each type. About 60 per cent felt the improvement was greater than 50 per cent. It’s not clear exactly why this helps.
  • Seasonal headshakers sometimes respond to treatment with melatonin. For best results, Dr. Madigan of the University of California Davis recommends giving 15–18mg daily (depending on the size of the horse) all year round.
  • Magnesium supplements may also be helpful for some horses as magnesium is important in nerve function.

Studies of chiropractic treatments and acupuncture have shown these to be generally ineffective in treating headshaking.

Another “alternative medicine” treatment is the Equiwinner Performance Patch, which is advertised as a cure not only for headshaking, but for exercise-induced pulmonary hemorrhage, tying-up (azoturia), and anhidrosis (inadequate sweating). The manufacturer suggests that the patch, which is completely sealed so that no ingredients are absorbed into the horse’s body, works by signalling the skin cells to release electrolytes that have been inappropriately stored. This will, the company website states, lower the horse’s blood pressure during exercise and thus stop the headshaking.

There is no research, however, to indicate that the patch works as described or to show its effectiveness in treating headshaking.

Pharmaceutical Intervention

If these simpler strategies have not proven effective, your veterinarian may decide to prescribe medication. Some of the commonly used drugs are:

Cyproheptadine: In one study by Madigan (2003), 43 out of the 61 horses tested showed at least moderate improvement in headshaking when given this antihistamine medication twice a day. This medication does have a number of side effects, including colic, sedation, dryness of the mucous membranes, lethargy and lack of appetite.

Carbamazepine: While one study (Newton et al., 2000) reported improvement in 88 per cent of horses given this medication used to treat seizures and nerve pain four times a day, other researchers were not able to achieve equally good results with this drug. Netwon did note that, for some horses, the benefit appeared to be short-lived.

According to other research, combining these two drugs may be a good option if the horse does not respond to either medication on its own: Dr. Marqués cited a 2000 study by Newton, which found an 80-100 per cent reduction in headshaking in 80 per cent of the horses who were treated with both carbamazepine and cyproheptadine. The study included only 20 horses, however. Since between 20 and 30 per cent of headshakers will simply stop the behaviour at some point, even with no treatment at all, it is preferable to have a control group to compare to the horses being given the medication. This study did not have a control group so the small numbers mean the results are less reliable.

The headshakerinfo.org site also mentions using hydroxyzine (an antihistamine), fluphenazine (an antipsychotic) and phenobarbital (an anti-seizure drug) in cases where the other drugs are not successful.

The Ultimate Prognosis

Dr. Marqués said the prognosis for headshakers is difficult to predict. Headshaking outcomes vary significantly among the affected horses. With some, treatments such as nose nets, eye-covering masks or medication will reduce headshaking enough that the horse is comfortable and often able to be ridden. Horse show competition may not be possible, however, since some of the medications will show up in drug tests, and the side effects of some of them may make the horse an unreliable performer.

In some cases, the headshaking is so persistent that the horse finds it difficult to eat or relax and is clearly in distress; for those animals, euthanasia may be the appropriate choice.
It should be encouraging to horse owners to know that researchers are continuing to study this condition, to look for new and more effective treatments, and to gain more understanding of this puzzling condition.