This week's behaviour post is on head shaking. There are many possible causes to this which makes it such a complex problem.
There is a strong emphasis on careful examination of the individual horse when they are showing head shaking behaviours. Head shaking is a clinical condition but the problem is that it may be due to other causes such as unwillingness to accept the bit. It can also be due to stereotypy in some horses. Therefore caution must be taken when diagnosing it.
Head shaking may be accompanied by excessive snorting, nasal or ocular discharge, striking at the nose or face with the foreleg and frequent attempts to rub the nose or face on the foreleg or on objects or along the ground (Madigan and Bell, 2001). The majority of head shaking is seasonal, beginning in the spring and ending in the autumn.
There may be a seasonal link to it as well. It is said that there are 58 possible causes (Cook, 1980). Factors that commonly were found to cause it include Trigeminal neuraligia and allergic rhinitis and occasional factors include ear and harvest mites, guttural pouch mycosis and equine protozoal myeloencephalitits.
When looking at the Trigeminal neuraligia, this involves the nerves in the horse's head. There are three major branches. This would possible explain the positive effects a bitless bridle has on head shaking. Covering the eyes has also been found to help. This may be due to photic sensitivity similar to the photic sneeze seen in humans. This again relates the the nerves in the face.
When looking at allergic rhinitis a nose net is often used and has been found to help. There is speculation as to how this is working however. It may be acting as a filter or it may be reducing turbulence of air which would link back to it being caused by the nerves. Over 75% of horses were found to improve when using a nose net in one trial. It was most effective on vertical head shaking, nose flipping, shaking at exercise or when excited, shaking in the sun and nose rubbing behaviours.
"With wide acceptance that many horses with idiopathic headshaking are likely to be suffering from a trigeminal neuropathy, rather than being badly behaved. It enables us to move forward with applied research in neurophysiological and clinical science and also adds to the weight of information relating to the need for therapeutic relief of suffering in these unfortunate animals. This disease now requires research to determine its underlying pathophysiology in order to then allow targeted research into treatments" (Roberts, 2011).