Headshaking in horses - not that simple

Headshaking in horses is a complex subject with a number of etiologies and an array of possible diagnostic procedures which are employed, depending on the signs and symptoms that the horse presents with. Some of the signs to be aware of include (Roberts, 2019);

· Frequent/violent flicks or jerks of the head, often vertical

· Snorting

· Sneezing

· Nose rubbing or shaking

· Signs of stress or reluctance when ridden

· Difficult to handle

· Head shy or reactive when head/face/upper cervical area approached or touched

· Apparent sensitivity to light

Causes of Head Shaking

Many texts on equine headshaking will discuss Trigeminal Mediated Head Shaking (TMHS), and with very justified reasons – this is now a much better understood condition which was historically referred to as Idiopathic (without identifiable cause) Head Shaking. Sadly, many people automatically jump to the conclusion that this is a voluntary behaviour and that the horse is being ‘difficult’, ‘fidgety’ etc (many other descriptive words are used).

However, whilst I will discuss TMHS in detail, there are several other possible causes for the signs listed above (Reed, Bayly & Sellon, 2018);

Ear Disorders

Ear mites

Otitis (Internal/medius/external)

Ear Ticks

Mass Lesions

Foreign Bodies

Ear Mites can cause headshaking

Psoroptes cuniculi (ear mites) can infest the outer ear and the external ear canal. Dry, flaky skin is characteristic, as well as head shaking and trying to rub the ear(s). They can affect other areas of the head.

Not as common in horses as in other animals, but this condition can cause extreme discomfort, as demonstrated by headshaking, weaving, trying to rub the ear, general malaise. Causes include: atopic dermatitis, black fly bites, insect hypersensitivity, ticks, psoporptic mange, vasculitis and less commonly, food hypersensitivity (Marsella, Senter and Funiciello, 2022)

Cranial Nerve Dysfunction

Oral Cavity Disorders

Disorders of the Eye

Upper Airway Disorders

Disorders of the Skull

Neurological Abnormalities

Cervical Pain

Behavioural

Objection to Rider and/or Tack

Ears back, Eyes & Facial Muscles Tense, Mouth Open - A Horse in Discomfort

Consistant or frequent flexion of the head and neck without periods of muscle relaxation/extension can cause the horse to try to fight the posture, which may also take the form of headshaking as he tries to free his body from such contraction

Another very important factor to rule out is musculoskeletal pain and/or discomfort. This is where a MSK Therapist may be beneficial prior to exploring veterinary diagnostics, working in conjunction with saddle/bridle/bit fitters, farriers/trimmers and other professionals to ensure that the whole horse has been considered. A British study in 2019 by Thomson, Chan and Dyson highlights the necessity to rule out musculoskeletal pain as a cause of headshaking. Of six horses studied, five showed a dramatic improvement during the study. From the authors’ observations, all horses were seen to head shake during ridden work and two displayed it during lunging. All had various MSK issues and most had multiple sources of lameness. Two of the horses had tight tree points on the saddle. When this was corrected, and the pain from lameness reduced with the use of analgesics, there was a dramatic resolution to the head shaking under all circumstances.

An evaluation by an MSK Therapist is an important element in diagnosing the potential cause(s) of head shaking.

The horse with no improvement also displayed signs when at rest and it was subsequently concluded that he was suffering from TMHS. Trigeminal Mediated Head Shaking was ruled out in the other five as there was no sneezing, snorting, rubbing, seasonal worsening or other clinical signs. The authors highlight the importance of step-by-step diagnostics before assuming that head shaking is rooted in trigeminal issues (Thomson, Chan and Dyson, 2019).

Prevalence

The prevalence of horses with conditions which cause head shaking in the UK is estimated at 20,000 in a population of 1 million. A study by Ross et al (2017) found a median age of 6 – 10 years, and 63-71% prevalence in geldings. There doesn’t appear to be a predisposition based on breed. Perhaps surprisingly, leisure horses in relatively light work were overly represented in a study of 254 horses at 91%. I also find it surprising that racehorses were extremely underrepresented in a study of 100, with only 2% affected (Pickles, Madigan and Aleman, 2014).

Similar signs and symptoms in humans can point to herpes and trigeminal nerve root de-myelination but neither are found to be the cause in horses (Roberts, 2019). Other studies consider the similarities with Trigeminal Neuralgia – a condition so painful and distressing that it has been dubbed ‘The Suicide Disease’. This is hugely concerning as it gives us an idea of the level of pain that affected horses may be suffering with (World Horse Welfare, 2021). However, other texts suggest that this is not comparable, as trigeminal neuralgia normally affects sufferers unilaterally, which suggests an abnormal conduction issue. This is not found to be the case with TMHS specifically, which could be viewed as a positive (Roberts, 2019). That said, I would still suggest that we need to consider pain levels and the fact that they are likely severe – we know that horses avoid showing signs of weakness and pain due to their prey animal instincts, so to display such overt signs could be, in my view, an indicator of severity.

The Influence of Tack

As mentioned, tack is thought to be a potential cause of headshaking, with some texts suggesting the bit in particular. Cook (2003) suggests that pain messages travel along the branches of the trigeminal nerve, so the brain is directly stimulated by mouth pain. The prevalence in geldings may tie in with this due to the proximity of the bit to the male canines. In some horses, wolf teeth may not erupt until well into adulthood, so potential interference with the bit is not reported (some mares also grow these teeth). Sensations in the mandibular branch of the trigeminal nerve may explain ear and/or tongue sensitivity (and therefore mouthiness). Pain referred to the maxillary branch could be behind nasal discomfort and that referred to the ophthalmic branch could relate to the light sensitivity. In addition, stimulation of the lacrimal gland could explain tearing and nasal discharge (Cook, 2003).

One significant indicator of whether tack is causing or exacerbating head shaking is observation of when the behaviour is displayed. If it is limited to ridden work, or lunging, then tack interference may well be a differential diagnosis. Trialling different riders is also important to see if it solely happens with a particular person and hence may be due to their riding/handling. The fact that tack may be contributing to pain/sensation behind head shaking then compromises some horses further as their handlers try to quell the behaviour with the use of auxiliary aids and/or a harsher bit (Francis, 2014).

In a survey of 679 people who had ever owned a head shaker, a significant majority reported that head shaking was worse when ridden (Marlin, 2021). This is part of an exploration as to links between poll and nasal pressure and head shaking. Making these observations is certainly crucial in examining the root cause(s) of the condition, and in some cases, a resolution may be found simply by having tack re-fitted.

We must consider, therefore, our influence over the wellbeing of the Trigeminal Nerve collection – its functions are crucial and many, and its location puts it at huge risk from outside factors such as bridles, bits and of course, handling styles. Pressure from noseband tightness on the infraorbital branch is a cause for concern (and therefore from auxiliary aids connecting to the noseband, such as a standing martingale). As discussed, referred interference from the bit via the mandibular branch is an area of interest. In addition, pressure on the bursa between the nuchal ligament, atlas and axis cannot be withstood and resultant collections of fluid and inflammation are noted as another cause of head shaking (Lesté-Lasserre, 2021).

Trigeminal Mediated Head Shaking (TMHS)

True TMHS is a neuropathic condition causing facial pain, sensations such as itching, burning and tingling, according to John Madigan who is a Professor of Veterinary Medicine and University of California, Davis and a leading expert in the subject. He suggests that exercise may increase nerve activity with breathing rate, nasal contractions and sensory innervation increased. He also feels, however, that risks may increase with horses who are laid off, have gained weight and those who don’t do aerobic exercise. He has been responsible for trialling some of the potential treatments that have shown poor/no improvements (Francis, 2014). TMHS is considered to be caused by the trigeminal nerve being hypersensitive and having a low threshold for firing rate.

Recent research suggests a link between the gut microbiome and headshaking in some cases

Signs of TMHS tend to include violent, vertical head jerks along with apparent nasal irritation.It is estimated that 1% of the horse population may suffer with the condition to a degree of severity which requires veterinary attention.The infraorbital branch of the trigeminal nerve is found to be sensitized in affected horses, which raises the question – could this be reversed? It is thought that it is a functional abnormality of the nerve, which leads researchers to think that the nerve could be desensitized. This needs more research, but holds an element of hope (Roberts, 2019).

In some affected horses, it seems that there is a seasonal factor involved, with many having worsening symptoms in the spring/summer. An explanation for this is the fact that there is a potential rise in gonadotrophins such as Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) due to a decrease in melatonin (due to lighter and longer hours of daylight). The rise in these hormones seems to affect the trigeminal ganglia, making the nerve fire at a higher rate in response to normal sensory information (Francis, 2014; Sheldon et al, 2019).

Relatively recent research has highlighted a potential link between the gut microbiome and head shaking in horses, with Cyanobacteria Nostoc being present in the microbiome of horses with headshaking when compared with healthy horses. Late in 2022, Equibiome released this update on the connection between the gut and headshaking;

“From 54 horses, 12 with severe symptoms and the rest with mild, compared to 75 horses without symptoms, the following pattern was found which the AI picked up with 100% accuracy. The gut microbiome profile of a head shaker: Lower Alpha Diversity Higher levels of pathogens (disease-causing bacteria). Higher levels of clostridia – a large family of well -studied pathogens, linked to colitis, diarrhea, liver disease, lockjaw (tetanus), and botulism. Has been named as a possible pathogen in grass sickness. Higher levels of a toxin producing bacteria called cyanobacteria. Nonexistent levels of Gemmatimonadota microbes one of the most common bacteria found in unpolluted water and healthy soil, easily killed off in treated water and unhealthy/compacted soil.” (Equibiome, 2022)

There is still much research needed into TMHS as a specific condition, and it is true also that there may be multiple causes which lead to the same presenting signs (Roberts, 2019). There are various grading systems that have been suggested to indicate the severity of the condition; Talbot (2013) – the effects of signs on the horse’s behaviour, Roberts (2014) – the utility of the horse to the owner and Newton et al (2000) – a combination.

In ‘true’ TMHS, the nerve fires at 10x lower threshold than normal. At variable frequency, the horse may suffer from pins and needles and other neuropathies, including pain, of course. This variable nature may explain, in some cases, the inconsistent appearance of the behaviour.

Diagnosis

Due to the complexity of the condition, diagnosis of the cause of head shaking is a multi-pronged approach. Depending on the presenting signs, the following explorations may be carried out (Roberts, 2021);

· Clinical exam

· Upper respiratory endoscopy (including guttural pouch)

· Ophthalmic exam

· Oral exam

· CT or MRI scan

· Radiography

· Blood/urine samples (however, there is a lack of knowledge as to the significance)

· Nerve blocks (however, whilst these confirm pain, they don’t pinpoint cause. A negative result cannot rule out particular causes, and placement accuracy is also a factor)

Weather may be relevant

Is it worse with tack?

Observing a horse carefully under a number of different situations/conditions may provide clues

Observation and record keeping are also important. These may demonstrate particular patterns, for example (Sussex Equine Hospital, 2017);

· Observation of frequency in ridden, lunging and resting states

· Weather conditions – sun, wind, rain, overcast

· Location of horse – stabled or turned out (I would also add to this that level of environmental stimuli may be noted as if the behaviour has a sensory aetiology, or if it is rooted in stress, this may be revealing)

· Pollen count

As discussed, MSK pain or discomfort should also be ruled out, tack fit should be thoroughly examined and as a potential contributory factor to the former, farrier/trimmer consultation.

Normally, if no other conditions or causes can be found, a diagnosis of Trigeminal Mediated Head Shaking may be made, further investigating seasonality, frequency and severity – grading is between 1 and 5 where 1 is manageable and 5 is a horse in notable distress and is, at least some of the time, uncontrollable (Sussex Equine Hospital, 2017).

Treatment

There have been a number of explorations into the treatment options for headshaking, and results are very variable between horses. I would say this may be due to the number of potential causes and the fact that an individual may have more than one contributing to the condition. It would also depend on the diagnosis, as conditions such as those in the table above have their own specific treatments. In terms of TMHS there are some which are said to be successful to some degree in studies reported by Roberts (2019);

Δ Nose Net – this is a cheap and non-invasive treatment which is safe and in studies has provided 70% relief in 25% of cases. The success of this option would depend on whether allergens, irritants, air flow, air temperature or other nasal stimulation played a part.

Δ Pharmaceuticals – these have varied results. Anticonvulsants such as Gabapentin and Carbemezapine have been trialled with some success. Serotonin is involved in afferent (sensory) signalling, and so seratonin antagonist drugs such as Cyproheptadine are sometimes prescribed alongside Carbemezapine. The downsides to pharmaceuticals are that they are banned in competition, may cause drowsiness and potentially have other side effects depending on the patient.

Δ Homeopathy gained positive reports in 93% of owners who’s horses were trialled

Δ Surgery – only 3 of 19 cases were reported successful

Δ Caudal ablation of the infraorbital branch of the trigeminal nerve had 50% positive results in a sample size of 57 horses. However, 20% of these relapsed. Side effects of caudal ablation include nose rubbing. Four horses had to be put to sleep during these trials due to the severity.

Δ Equipens Neuromodulation – Percutaneous Electrical Nerve Stimulation (PENS). 168 horses with TMHS were trialled with a 9% complication rate but most of these were mild. Remission levels were high at 53% of 136 which showed a positive response. Electroacupuncture followed EquiPENS, but this had a low success rate. The neuromodulation process is thought to reset the firing rate and patten of nerve impulses.

Δ Supplementation of magnesium and boron have had relatively positive results when provided in tandem as boron increases the absorption of magnesium, which has been found to be lower in horses with head shaking. Studies into this treatment have their limitations, particularly in sample size, however, is worth further exploration. Even magnesium supplemented without boron yielded more positive results than the control diet. Magnesium can have a neuroprotective effect which may potentially dampen neuropathic pain signals (Sheldon et al, 2019).

Nose nets may be beneficial in cases where nasal or nasal cavity stimulation is the cause, or a contributory factor. These are useful in altering the air temperature and reducing allergens. As they are legal in competition and can be tried wet, are non-invasive and cheap, these are worth trialling. Tinted contact lenses may be useful if the horse is found to be photosensitive (Roberts, 2021). Wearing a fringe may, it is suggested, have a similar effect to TENS whereby different afferent nerves are sensitized, decreasing firing of the currently hypersensitive nerve(s) (Francis, 2014).

Conversely, there are a number of treatments which have been trialled with little or no success. These include; antihistamines, antimicrobials, corticosteroids, NSAIDs, melatonin, chiropractic treatment, feed supplements, gonadotrophin-releasing-hormone vaccine, pulsed high dose dexamethasone (as the trigeminal nerve is not commonly caused by allergies, this was deemed unsuccessful) and acupuncture.

As discussed, discomfort and pain caused by an MSK issue should be ruled out, and may actually answer the problem. If headshaking is more prominent or limited to exercise, for example when asked to flex and collect, we could be looking at a back pain issue or a subtle lameness (Francis, 2014). Even if MSK issues aren’t the root cause of headshaking, I would suggest that it would be beneficial, as the cause itself as well as the effects are likely to initiate secondary MSK tension/issues.

If it is concluded that headshaking is out of over-excitement, increased turnout time (if not already 24/7) and/or long-reining may be of use, but must be done correctly and only in cases where it would not contraindicate other issues that the horse may have (Francis, 2014).

In a webinar presented by Veronica Roberts “TMHS in Horses – What, Who, Why and What to do about it”, some interesting points were raised. One was the potential of potassium as a trigger, but there is no evidential detail about this as yet as studies are currently unpublished. She also brought up the point of horses who only head shake in particular environments, which is something that should be closely observed as this may lead to some identifiable triggers (Roberts, 2021).

Watching horses who headshake under saddle or on the lunge shows variable potential causes, which is not a surprise given the number which exist. However, I have watched some where I would say tack and/or rider are having a large influence over the horse’s actions in terms of head shaking.

In summary, there are so many possible causes for head shaking in horses, from serious neurological issues to those which are simply fixed by tack fit and many things in between. The key is that it is not automatically put down to a behavioural issue, but that it is thoroughly investigated – which is partly a process of elimination, but also the involvement of a number of equine professionals.

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