Mouth Pain in Horses: Physiological Foundations, Behavioural Indices, Welfare Implications, and a Suggested Solution

Abstract

A proposition addressed here is that, although bitted horses are viewed by many equestrians as being largely free of bit-related mouth pain, it seems likely that most behavioural signs of such pain are simply not recognised. Background information is provided on the following: the major features of pain generation and experience; cerebrocortical involvement in the conscious experience of pain by mammals; the numerous other subjective experiences mammals can have; adjunct physiological responses to pain; some general feature of behavioural responses to pain; and the neural bases of sensations generated within the mouth. Mouth pain in horses is then discussed. The areas considered exclude dental disease, but they include the stimulation of pain receptors by bits in the interdental space, the tongue, the commissures of the mouth, and the buccal mucosa. Compression, laceration, inflammation, impeded tissue blood flow, and tissue stretching are evaluated as noxious stimuli. The high pain sensitivity of the interdental space is described, as are likely increases in pain sensitivity due to repeated bit contact with bruises, cuts, tears, and/or ulcers wherever they are located in the mouth. Behavioural indices of mouth pain are then identified by contrasting the behaviours of horses when wearing bitted bridles, when changed from bitted to bit-free bridles, and when free-roaming unbitted in the wild. Observed indicative behaviours involve mouth movements, head-neck position, and facial expression (“pain face”), as well as characteristic body movements and gait. The welfare impacts of bit-related pain include the noxiousness of the pain itself as well as likely anxiety when anticipating the pain and fear whilst experiencing it, especially if the pain is severe. In addition, particular mouth behaviours impede airflow within the air passages of the upper respiratory system, effects that, in their turn, adversely affect the air passages in the lungs. Here, they increase airflow resistance and decrease alveolar gas exchange, giving rise to suffocating experiences of breathlessness. In addition, breathlessness is a likely consequence of the low jowl angles commonly maintained during dressage. If severe, as with pain, the prospect of breathlessness is likely to give rise to anxiety and the direct experience of breathlessness to fear. The related components of welfare compromise therefore likely involve pain, breathlessness, anxiety, and fear. Finally, a 12-point strategy is proposed to give greater impetus to a wider adoption of bit-free bridles in order to avoid bit-induced mouth pain.

Application of the Ridden Horse Pain Ethogram to Elite Dressage Horses Competing in World Cup Grand Prix Competitions

“…It was previously documented that there was a greater frequency of mouth opening when horses were ridden compared with being lunged in a bridle, suggesting that it was related to the horse being ridden [ 24 ]. There is ongoing debate about bits and their potential to cause pain and/or injury [ 26 ]. Many of the behaviours suggested to reflect bit pain [ 25 ] were not observed in the current study, and, specifically, there was a low frequency of occurrence of the tongue being out and no evidence of the tongue being over the bit.…”

Bit-Related Lesions in Event Horses After a Cross-Country Test

“…Horses wearing either thin (n = 13, OR 4.3, CI 1. [3][4][5][6][7][8][9][10][11][12][13][14] or thick (n = 17, OR 4.5, CI 1.2-17) bits had a higher risk of moderate/severe lesion status than horses wearing middle-sized (14-17 mm) bits (n = 80) (P = 0.01). In Model II sex (P = 0.2) and breed The results of the logistic regression analysis for the Model I containing breed, sex, and bit thickness.…”

“…Horse welfare in competitive equestrian sports is under increasing scrutiny ( 1 , 2 ), with attention being directed among others to bit-related lesions, which affect horse welfare by potentially causing anxiety, fear, and pain ( 3 ). Oral tissues have a strong somatosensory innervation ( 4 , 5 ).…”

Rein Tension Signals Elicit Different Behavioral Responses When Comparing Bitted Bridle and Halter

“…It should be borne in mind that when escalating pressure signals are used as a means of communication, there is always a risk of causing the horse discomfort, pain, and even physical injury (25). Bridles with bits and bitless alternatives both press on sensitive structures of the horse's head and mouth when rein tension is applied (26). Further, mouth injuries connected to use of bridles are common in horses (27,28).…”