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Introduction
There is not one, sole cause of headshaking!
Headshaking is a 'presenting sign' of disease. It is a sign that something is wrong and/or irritating the horse, probably in the head area. Headshaking has been reported in horses suffering from nearly 60 diseases and conditions, including:
- Respiratory problems, e.g. guttural pouch mycosis, vasomotor rhinitis, sinusitis
- Eye problems
- Central neurological problems and diseases e.g. EPM
- Hormonal problems
- Ear mite infestations
- Harvest mite infestations
- Middle ear problems
- Temporohyoid arthropathy - requires a radiograph for diagnosis
- Back pain
- Teeth problems
You should consult your vet to rule these out initially.
Also Rule Out:
'Nodders' - In the past, the distinction between those that are 'classic' headshakers and those that are 'nodders' or 'head bobbers' has perhaps been unclear. Now it is generally considered that 'nodders' are horses that only shake when stabled or when 'bored' or frustrated. Their head movements are thought to be more rhythmical than those of a classic headshaker's and they do not usually show the other associated symptoms like nose rubbing. 'Nodders' may respond to changes in management such as being in a more natural, herd environment when not being ridden or by being given more interesting riding lessons.
Riding and tack problems - In these cases the horse may be totally symptom-free except when being ridden and may not show the additional signs such as nose rubbing. Possible causes include;
1. Rider's hands jerking the bit in the horse's mouth
2. Back problems - get a good equine chiropractor to check out jaw, poll, neck, back, pelvis
3. Teeth problems, including needing to be rasped - the horse may have sharp edges to its teeth which are making its mouth sore particularly at the very back of the mouth. Make sure your equine dentist is properly qualified and does a thorough job.
4. Lameness
5. Badly fitting saddle or bridle causing discomfort
Okay - I've checked him out and found none of the above...................
In this case, your horse may be an idiopathic headshaker (which means the cause is not known). In this case the headshaking is treated very much like a disease in itself, there being no other apparent signs of disease apart from the headshaking.
Idiopathic headshaking
The symptoms described in Clinical signs commonly refer to horses with what is known as an idiopathicheadshaking problem. This means that the vet has examined the horse and cannot find anything that might be causing the headshaking. However idiopathic headshaking still has many likely potential causes and these could be interrelated, i.e. the cause within in one horse is 'multifactorial'. The mechanisms causing the headshaking are probably something we can't easily see, e.g. allergic reactions or nerve sensitivity. However, do not give up - just because you don't have a name for what is wrong with your horse doesn't mean you can't try to find out more.
Here are two main theories for the cause of idiopathic headshaking. They can both produce the same symptoms as in Clinical Signs. They are:
Allergic rhinitis
What is happening?
Allergens such as pollen, oilseed rape volatiles, dust, etc get into the horse's nose. An allergic reaction occurs in the mucus membranes that line the inside of its nasal passages. Inflammation and creation of mucus stimulates irritant nerve receptors in the nasal passages, which irritate the horse, and so it throws its head around, snorts and rubs its nose.
How do I know?
Ask your vet. They may be able to test your horse's nasal fluid for signs of an allergic reaction. They may also be able to prescribe drugs for this condition. However, as response to typical drugs for this condition is often poor, many vets believe that allergic rhinitis is a relatively rare cause of headshaking and that nerve pain (see below) might be a more likely explanation.
Nerve pain
What is happening?
Nerve pain has long been blamed for the cause of headshaking, and, as long ago as the late 1800s vets were surgically severing the nerves in an affected horse's nose in an attempt to cure the problem. There are many sources for potential nerve pain in the horse's head but the vets think that branches of the trigeminal nerve which conducts sensation from the muzzle and face to the brain may be the main source, particularly the Infraorbital nerve which is a branch of the maxillary. See Figure 1 below. For some reason, parts of this nerve may be damaged and 'fire' inappropriately causing painful sensations in a particular area, i.e. the nerves are telling the horse that it is being hurt in this area when in reality it is not. In humans this is called Trigeminal neuralgia or Tic doloureux. This problem can explain all the symptoms listed in Clinical signs including the tick-like head movement and signs of nasal irritation because the horse may be trying to remove the sensations by rubbing. The location of the nerve malfunction is difficult to identify but may explain the differential effect of various triggers between horses. Sunlight may suggest a malfunction in nerves nearer the eye, whilst irritation caused by particles or airflow may suggest a malfunction in nerves supplying the nasal passages. Sensitivity to touch may imply that more peripheral nerves are affected. Unfortunately the cause of Trigeminal neuralgia in humans is often a mystery although it can be caused by pressure from an enlarged blood vessel ‘squashing’ the nerve or direct damage to the nerve as a result of a previous injury. Exercise may therefore trigger this misfiring due to an increase in blood pressure, similarly particles inside the nose will increase the blood flow to the nasal passages. However, Dr Robert Cook has suggested that the pressure of the bit in the horse’s mouth might directly cause this neuralgia. Unfortunately this is difficult to prove because removal of the bit might not instantly cure an established neuralgia. See work by Cook in Literature or www.bitlessbridle.com
Fig. 1 The trigeminal nerve distribution in the horse’s head. The three branches of the trigeminal are shown (ophthalmic (1 st), maxillary (2 nd) and mandibular (3 rd)). Adapted from Goody, P (2000) Horse anatomy: a pictorial approach to equine structure 2 nd Ed. J.A. Allen Publishers, London. From Taylor, K. (2004) PhD Thesis, De Montfort University, Leicester.
Other kinds of nerve pain can cause headshaking such as:
Post-viral neuralgia from unnoticed, previous viral infections , possibly EHV-1. (e.g. post herpetic neuralgia, similar to post-shingles pain in humans). This may cause a general hypersensitivity around the muzzle or other areas. Horses which may be suffering from this kind of nerve pain may be very sensitive to light touch such as rain, snowflakes, etc.
Myofascial pain - These are bands of hypersensitive tissue around the facial area that cause pain when touched
How do I know?
Your vet may be able to perform temporary nerve blocks to see if an anaesthetic can temporarily stop the headshaking. This might not always work as the correct nerves may not have been anaesthetized. Vets use this procedure to see if an operation to sever or kill the faulty nerves is likely to be successful. In many cases it is but it remains an unpopular choice amongst owners.
Your vet might also give your horse some drugs that are indicated for these kinds of conditions in humans. Most popular ones include cyproheptadine (periactin) or tegretol (carbemazepine). These drugs are not without some side-effects though and should always be given under the strict guidance of your vet.
Photic headshaking
Photic headshaking is a popular description for headshaking in horses that appears to be stimulated by exposure to sunlight. It is not an explanation. There are a number of theories but they all involve the nervous system so Nerve pain still applies to this type of headshaking. Sneezing in response to light is a harmless phenomenon seen in some humans (photic sneezing) and something similar might be going on here. It is currently thought that hormonal imbalances may make the horse more susceptible to referred stimulation of the trigeminal nerve from the optic nerve. This is thought to be a common cause of headshaking particularly in US horses but appears to be less common in the UK, see Work at Lincoln University. See also work by Madigan in Literature and Other sites
© Katy Taylor, De Montfort University, Leicester, UK