Horses will occasionally give their head a shake for normal reasons. A flick of the neck can dislodge flies. The occasional shake can indicate discomfort from bits and other tack. A head and neck roll may be a body language expression to other horses.
But a small number of horses experience shaking to a pathological extent.
Head shakers, as they are known, repetitively nod their heads as if saying yes. It looks similar to how a horse might react to a fly up the nose. Shaking can also be accompanied by face rubbing, snorting, sneezing, appearing anxious and striking at the head with the front legs.
The shaking seems to be worse during exercise with more shakes occurring at faster gaits. At a lope, affected horses can shake more than 25 times a minute. It is disruptive and distracting during riding.
The shaking can also be dangerous, making some affected horses unable to be ridden safely.
In very severe cases, horses may even shake when standing at rest. This condition can compromise welfare and also decrease the value of affected horses, resulting in euthanasia.
Most studies of head shakers have identified that more geldings than mares are affected. The behaviour usually starts in mature horses; nine years old is the average age of onset.
The condition seems to have a seasonal component. Most affected horses are worse in the spring and summer compared to the winter and fall. And unfortunately for affected horses, it occurs year after year. Bright light also seems to be an important trigger, with horses becoming worse when outside during the day. The behaviour typically stops at night.
The exact cause remains a mystery but it appears to be an involuntary action rather than a learned behaviour.
The most popular theory to explain head shaking is that these horses are experiencing nerve pain, leading to tingling, itching and painful sensations in the face. The trigeminal nerve seems to be the culprit. It is one of the cranial nerves, originating directly in the brain and running along the side of the face. In rare cases, there is some type of lesion or abnormality associated with the nerve that explains why it misfires. But in the vast majority of horses, there is nothing visibly wrong.
A veterinary examination can rule out a physical cause of pain and any mass or other lesion that might impinge on the nerve. The trigeminal nerve can be temporarily blocked using lidocaine or a similar freezing injection. A nerve block in this area works similarly to the lower leg blocks used to diagnose lameness. If the head shaking is reduced or stops following the block, then it helps confirm that the problem is related to that nerve.
In a research setting, horses have been tested for how readily the nerve fires. These studies show that affected horses have sensitive trigeminal nerves. However, this test is not practical in routine veterinary practice; it is difficult to perform, requiring general anesthesia.
Because the exact cause remains unknown, a variety of treatments have been tried with limited success. These include antibiotics, pain medication (such as “bute”), antihistamines and steroids. Some owners reported improvement with the drug cyproheptadine.
Another approach that has some success is a nose net. Made of similar material as fly masks, the netting attaches to a nose band and drapes over the muzzle. The net is thought to work by providing alternative sensations through contact with the area, like how you rub your finger after whacking it with a hammer to reduce the pain. Surgery has been used in the past with some success but it is expensive and comes with serious risks.
This condition really has more questions than answers. What is the underlying cause of the nerve misfiring? What horses are at greatest risk of developing this condition? How can owners prevent it from happening? And ultimately, what is the most effective treatment?
Hopefully with new technology and research, we can get to the bottom of this enigmatic condition.
Dr. Jamie Rothenburger, DVM, MVetSc, PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine.