At this time of year, vets report a dramatic increase in the number of horses showing signs of “headshaking”. This is a significant neurological disease, which can be compared with various causes of facial pain in other species, including man.
The abnormal movements range from the faintest “tic” to a severe and distressing jerking of the head. Most shake up and down, but a few show side-to-side or rotatory head movements. The key point is that it is involuntary.
Voluntary head tossing, demonstrated by some stallions in an excited manner, is natural “showing-off” behaviour, not headshaking.
Some cases are tolerable, some are manageable and a few are treatable. Once started, the disease is best considered incurable, though it may be manageable. Given that horses are usually in considerable pain during episodes, euthanasia may even be justifiable.
In the past, headshaking was considered a behavioural issue, often blamed on poor tack or horsemanship. We now know that this is not only wrong, but is also a cruel and unnecessary approach to a seriously ill horse.
What causes it?
Damage to part of a major sensory nerve to the skin of the face (including the muzzle and ears), the eyes, teeth, sinuses and nasal cavity is the likely cause of headshaking.
Stimuli triggering one or more of the sensory nerve endings (touch, heat/cold, moisture/dryness, air movement or particles etc) send messages to the brain, where the higher centres “make decisions” on how to respond.
A trivial signal such as a single pollen grain in the nose may be ignored — the brain knows it is there, but the signal is below the threshold that triggers a pain response. However, a massive stimulus, such as pepper up the nose, will evoke a painful reflex (involuntary) response, such as sneezing, snorting and rubbing of the face and nose.
Our hypothesis is that a reduction in the threshold of perception in one or more of the nerves triggers a disproportionate response, as if it were a strong stimulus such as an electric fence. A persistent or repeated stimulus will induce multiple or persistent responses. The horse perceives this as pain and becomes increasingly distressed and difficult to calm down.
Most cases have an abrupt onset; this may be the first time the specific nerve endings have been stimulated appropriately or the damage has become sufficient to be triggered.
Inevitably, the first occasion is a traumatic and distressing event, and often signals progressive deterioration. A tiny proportion of horses become headshakers — but the condition is so distressing it can become a major welfare issue.
What signs do affected horses show?
The signs of headshaking are variable and are loosely categorised into reflex responses — such as the twitching, plus ear and eyelid flicks — and pain responses — rubbing, sweating and rearing or striking at the face.
- Up/down involuntary twitching of the head — movement range varies from mild to severe.
- High or low head carriage.
- Snorting or high-blowing, sometimes coupled with nasal discharges and behavioural seeking of nostril or muzzle protection (such as in a stable corner, under the tail of another horse, a water bucket, etc).
- Nostril clamping (closing) after exercise may suggest the trigger zone is inside the nasal cavity — affected horses clamp their nostril closed because they know that if they breath normally there will be excruciating pain.
- Face rubbing is a common sign that probably corresponds to areas of perceived pain or numbness, possibly linked to severely damaged parts of the nerve.
- Distracted behaviour may be seen, such as loss of stride, stopping, rearing and striking at the face during exercise, even at a canter/gallop.
True headshaking signs are shown in different circumstances, such as free exercise or lungeing, and with different riders. Some horses can “override” the pain when distracted, for example by showing them a fence to jump.
Classification of the disease
Headshakers can be classified according to the extent of seasonality, the presence or lack of intermittency of signs and their severity.
Many headshakers show the signs only during a particular time of year, most commonly spring and summer. Seasonality can be explained by the occurrence of trigger factors such as pollen, insects, dust, warmth, cold, rain or wind. Light has been suggested, but this is now largely discredited, as sunshine also accompanies many other factors that are more likely trigger factors.
Most headshaking horses show signs under defined conditions such as exercise, bright light, rain, wind, cold, warmth, on particular parts of a ride, for example, in shade, or when exposed to pollen or insects. Almost every case we have dealt with has had a different set of triggering environmental factors. The presence or absence of the specific trigger factor is a plausible explanation for this variation.
Some cases are very mild (grade one), where the movement of the head is slight and almost imperceptible. The horse is not unduly distressed and both horse and rider may ignore it. Grade five headshaker horses are severely distressed and may be uncontrollable. Any attempt to restrain the horse is resented and even touching the face can trigger extreme resentment. It may rub its face so badly that this causes sores and bleeding. These cases cannot be ridden safely.
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Can headshaking be treated?
The short answer is sometimes. 25% of horses respond positively to a nose net, due to the ‘gate theory’ of nerve stimulation. It acts as pressure on the tingling or painful nerves, and this increased stimulation takes away the pain.
If horses don’t respond to a nose net, they can undergo a full diagnostic test at either the Royal Veterinary College or Bristol University, which specialize in headshaking cases. Bristol offers owners a flat rate for all diagnostics, which means if a horse needs more than one CT scan, for instance, the owner doesn’t pay for the subsequent scans.
Once the horse is diagnosed as a trigeminal-mediated headshaker, it can be treated with PENS, electrical stimulation of the trigeminal nerve. In a paper for Veterinary Medicine: Research and Reports, it was reported that, “Remission of headshaking following the initial course occurred in 53% (72 out of 136) of horses. Median length of time recorded in remission was 9.5 weeks (range 2 days to 156 weeks ongoing). Where signs recurred, most horses went back into remission following future procedures usually for longer time than the previous procedure.”
Various drugs, acupuncture, supplements, and homeopathic remedies have been tried with no scientifically proven success. However, owner-reported diagnostics indicate that owners think these work, sometimes, so if not harmful per se, they won’t hurt (but most of their effectiveness may be down to the placebo effect).
Researchers at the University of California, Davis reported that headshaking diminished by 29% in six geldings that were given an IV of magnesium sulfate. They don’t know why magnesium reduced neuropathic pain in the subjects, but it’s worthwhile following this new line of inquiry and seeing where it leads.
Researchers at Bristol University are conducting a study of amino acids that stabilise nerve membranes in people, rats, and cats. Amino acids have less side-effects than conventional pharmaceuticals and are competition-legal. They are still looking for research subjects for this trial, so horse owners with trigeminal-mediated headshakers interested in participating should contact the Equine Centre at the university.
Can headshaking be prevented?
It is hard to prevent the underlying pathology when we have little idea what it is. There have been many suggestions, ranging from maintaining a high dietary magnesium intake, vitamin B complex supplements and vaccination against equine herpes virus.
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