Headshaking is a behaviour displayed by horses, where the horse continuously shakes its head vertically and/or horizontally. In the 1980s it was considered a bad behaviour, but instead it turned out to be a painful medical condition. Minor head tossing is quite common in most horses as a response to insects and airborne irritants, however headshaking is a separate, severe issue. The headshaking needs veterinary attention if it occurs spontaneously with no sign of any external stimuli that may cause the motion. The behaviour is most commonly described as a vertical flicking of the head, as if an insect has flown up the animal's nostrils. Many horses also show nasal irritation and engage in rubbing or striking the muzzle off of stationary objects, such as walls and fences.
The behaviour is most commonly displayed while the horse is being led, lunged, or ridden, and typically subsides while the horse is at rest. This makes riding a horse with the syndrome dangerous, as they may begin to violently toss their heads and thus, throw off the rider's balance. Because of its prevalence during exercise, many riders incorrectly attribute headshaking to a behavioural problem or to poorly fitted tack. Although other causes have been noted, the most common cause of headshaking is the neurological disorder known as trigeminal neuralgia. It has been found that many factors, such as environment, sex, and breed, contribute to the prevalence of the behaviour. Many treatments have been proposed to help control the symptoms of the condition, but a solution to cure the behaviour has yet to be found.
Causes
Headshaking may be caused by a variety of medical issues, such as ear ticks or mites, dental diseases, airway abnormalities, skull trauma, sinusitis, etc. However, the most common and well-researched cause is trigeminal neuralgia, a neurological condition that affects the animal's trigeminal nerve and results in abnormal firing. The somatosensory- evoked potential was tested and is in healthy horses established at 10mA, while affected horses show a result of 5mA. Research shows that the infraorbital nerve branch of affected horses is sensitized and has an increased threshold compared with unaffected horses. The branch, therefore, has a lower firing threshold and can be triggered by various, even minor stimuli. But some seasonally affected horses seemed to have a normal threshold, when tested out of season. If the clinical symptoms are in fact connected to the threshold of the nerve, it holds the possibility of a reversible condition. The maxillary branch of the trigeminal nerve plays a major role in facial sensation, thus causing the horse to feel intense pain in the face and muzzle area. It was distinguished that there is no difference in the involvement of the left or right branch from the trigeminal nerve. The horse's pain is expressed in its obsessive headshaking and evasive behaviour. The condition is known as "trigeminal-mediated headshaking" or "facial pain syndrome".
Diagnostic The veterinarian has to observe the horse during exercise, at rest and under the influence of different environments and possible triggers. It sometimes can be helpful to have video materials of the owner and observe the horse on different days, to distinguish certain triggers. After the inspection, the horse has to be examined for other causes of the headshaking not related to the trigeminal- nerve. Additional diagnostics should be considered as well. These include:
MRI X-Rays CT Endoscopy Even though there can be a variety of different causes, 98% of the affected horses in need of veterinary care turned out to have no connection to external causes, leading to a connection to the trigeminal-nerve.
Influences on prevalence
Environmental triggers Research has shown that trigeminal-mediated headshaking may be triggered by various external factors, such as temperature, season, time of day, wind, light, and many others. Approximately 1.4% of horses are affected by headshaking and of those with the condition, 64% are affected seasonally. Light availability seems to play the biggest role in triggering the behaviour. Affected horses tend to experience more symptoms when outdoors on sunny days. Therefore, headshaking occurs most often during the spring and summer months when sunlight is more prevalent. This side of the syndrome is termed "photic headshaking" and is thought to occur due to the close proximity of the optic and trigeminal nerves. The symptoms of photic headshakers lessen when indoors and during the nighttime. Headshaking appears to occur in horses all over the globe, but variations in the symptoms can be based on region. In the USA, 91% of affected horses were found to experience increased headshaking during the spring and early summer. While in the UK, only 39% of affected horses displayed worsened symptoms during only the spring and summer months, with more horses being affected during the spring, summer and fall. Other individuals have been found to be triggered by the wind conditions, rain conditions, and the presence of loud sounds, although the reasoning behind these influences are currently unknown.
Age and sex Headshaking seems to be an acquired condition that develops during the prime of the horse's life. It affects a wide age range, but the majority of horses develop the behaviour after reaching maturity, usually between 6-10 years old. A larger percentage of geldings appear to be affected by the condition when compared to mares and stallions. In some studies, as high as 90% of headshakers were found to be geldings. It is thought that geldings may be more susceptible to headshaking, as they do not have testosterone feedback, therefore during the spring when horses typically mate, the gelding's gonadotropins will be incredibly heightened. The gonadotropins alter the sensitivity of the trigeminal nerve, making it unstable and more susceptible to stimuli.
Exercise Exercise can also influence the intensity of headshaking behaviour. Intense or more frequent exercise provides more stimulation to the brain causing increased firing of the trigeminal nerve. The increased airflow over the muzzle and nostrils while riding or lunging may also play a factor.
Breed and discipline
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