Without food, a horse’s expectation of life is measured in weeks; without water, in days; without air, in seconds. The purpose of this article is to provide an illustrated sequel to a previous article “Horse Sports’ Option: Ban or be Banned” (Cook 2024). A series of diagrams will illustrate how a horse breathes; explain why the bit handicaps this fundamental need; and provide the basis for recommending a simple step to a future for equestrian sport.

To understand how bit-free breathing improves a horse’s quality of life, promotes performance, and reduces accidents, we need to ask ourselves the question “How does a free-roaming horse in the wild breathe and balance when running? Five requirements (laws of nature) are apparent, four for breathing and one for balancing: 

  1. The horse is a nose-breather and cannot mouth-breathe.
  2. The horse runs with a closed mouth and sealed lips.
  3. The mouth is empty, not just of bits and tongue straps, but of air itself. Prior to running, a horse seals its lips, swallows, and creates a negative atmospheric pressure in its mouth that tethers the soft palate and keeps the throat airway open.  
  4. The soft palate clings to the root of the tongue and is ‘buttoned-up’ to the voice box. 
  5. The horse stays balanced by choosing its head/neck position from moment to moment.  

The bit method of human-to-horse communication denies a horse all five requirements. It is physiologically abnormal for a horse to have one or more foreign bodies in its mouth (Cook and Strasser 2003). Bit-ridden and bit-driven horses experience pain (Mellor 2020a,b) and shortage of breath (Mellor and Beausoleil 2017). 

Two precepts of a law of physics governing the flow of gases in tubes (Poiseuille’s law) are relevant:  

  1. Halving the radius of a tube increases resistance to flow 16-fold. Small constrictions have large effects. 
  2. Pressure gradients in a tube increase as distance from the site of a constriction increases. Physical damage to lung tissue occurs when a significant pressure difference develops between negative pressure in the air sacs of a horse’s lung and positive atmospheric pressure. The term for this barometric tissue damage is barotrauma.  

Bit-induced pain and the handicapping of a horse’s ability to breathe and balance explains why, in racing particularly, a horse may stumble, fall, incur a catastrophic accident, or die a sudden death.  

Breathing is a ‘suck-blow’ process. Inspiration generates negative pressure; expiration generates positive pressure. Bit-induced constriction of the throat airway causes abnormally strong negative pressures to develop in the air sacs of the lung, hence the universal scourge of the racehorse, commonly referred to as ‘bleeding.’ Exercise-induced pulmonary hemorrhage (EIPH) in the horse is analogous to negative pressure pulmonary edema (NPPE) in man, a respiratory emergency caused by airway obstruction (Cook 2016, Mellor and Beausoleil, 2017). 

Edema of the lung is a condition in which the air sacs of the lung become flooded with heavily blood-stained fluid. In a ‘waterlogged’ lung, oxygen exchange is impeded. Shortage of breath and physical exhaustion quickly follow, hence falls, fractures and dislocations. The #1 precept of Poiseuille’s law explains why the bit-ridden horse is vulnerable to being strangled. The #2 precept explains why obstruction of the throat airway in the horse, a long-necked animal, causes a potentially fatal disease of the lungs. It also explains why the most severe deformity of the windpipe occurs at the level of the first rib (Fig.13) and why the most severe barometric bruising and bleeding occurs at the lung’s posterior end (Fig. 18). The bitted horse is demonstrably prone to trigeminal neuralgia (‘headshaking’) and, because like all mammals it has a trigemino-cardiac reflex (Cook 2022), it is also prone to heart attacks. Entwined with the mouth’s sensory nerve, the Trigeminal, are branches of the Vagus nerve that control heart function. In the heat of a race, with adrenaline surging, the pain of one or more bits and a tongue-tie may trigger this reflex and cause cardiac arrest.   

There are many moving parts in a horse’s mouth and throat. A series of illustrations will show how they are configured for running in the horse at liberty, and how the relationships are disturbed (i.e., rendered pathophysiological) by a ring bit (i.e., two bits) and a tongue strap. Let us start with the naming of the parts (Figure 1). 

Figure1. Anatomy of the respiratory and digestive tracts. Because a bit is in place, the lips are unsealed, and air is present in the digestive tract. The spacing of the parts clarifies the anatomy but is physiologically abnormal. Key: white = bone; red = cartilage; and brown = soft tissue. 

Figure1. Anatomy of the respiratory and digestive tracts. Because a bit is in place, the lips are unsealed, and air is present in the digestive tract. The spacing of the parts clarifies the anatomy but is physiologically abnormal. Key: white = bone; red = cartilage; and brown = soft tissue.

The respiratory tract comprises the two nostrils and nasal cavities, the nasopharynx (throat), larynx (voice box), and trachea (windpipe). The digestive tract comprises the lips and the oral cavity (mouth); the oropharynx (throat) – the tonsillar area lying under the front section of the soft palate; and the lingual pharynx (not depicted in Figure 1 but see Figure 4), i.e., two food channels, one on each side of the larynx, that connect the oropharynx to the oesophageal pharynx, the first part of the oesophagus (gullet). 

The bit method of rider/horse communication:

  • Hinders the first requirement (the ability to breathe freely)
  • Denies the second and third requirements (sealed lips and an oral vacuum)
  • Denies the fourth requirement (the tethering of the soft palate to the root of an immobile tongue and its air-tight ‘buttoning-up’ to the larynx)
  • Interferes with the fifth requirement (the ability to balance by free choice of head/neck position, i.e., coercive bit-induced poll flexion, as in the ‘rating’ of a racehorse, the poll flexion of a showjumper, and the hyperflexion of a dressage horse).
  • In the racehorse, doomed attempts to prevent bit-induced tongue movement by tying the tongue to the lower jaw adds a further level of pain and distress.
  • Separately, the effect of each negated requirement leads to obstruction of the throat airway. Collectively, the effect can be life-threatening strangulation.

Unlike other sections of the respiratory tract, the throat’s boundary walls are mainly soft and unsupported by bone or cartilage. Because of this, the throat is especially vulnerable to obstruction. Language recognizes this vulnerability with words that specifically describe obstruction of the throat. In English, we have the verbs to choke, throttle or strangle. Significantly, each one is often followed by the phrase ‘to death.’ Lions inherently know that the quickest way to kill a zebra is by biting it in the throat. The horse is especially vulnerable to being strangled because another airspace, the guttural pouch, lies immediately above the roof of its throat (Fig.6). A common infectious disease of the horse, in which an abscess develops in a lymph node on the floor of the guttural pouch, is aptly named “Strangles.”

As already mentioned, before running, a horse at liberty seals its lips, swallows, and generates a negative atmospheric pressure in its mouth that is maintained throughout the run. Unless this and the other requirements are met, a horse is unable to breathe freely when needing to breathe rapidly and deeply. At the gallop, a horse breathes at the astonishing rate of two or more breaths a second. As breathing and striding are synchronized (one breath/one stride) the respiratory rate can be assessed by counting the stride rate. At liberty, the running horse has a relatively dry mouth. Salivation is in abeyance.

To understand how a horse breathes, it is also necessary to understand how it swallows.

Figure 2. Breathing and swallowing. Showing the switch-point changes for each function.  Key: AC = the paired arytenoid (‘flapper’) cartilages; E = epiglottis; LP = laryngopharynx (the food channel on each side of the larynx by which a liquid stream of masticated grass flows into the gullet during grazing, without interrupting breathing); NP = nasopharynx; OI = ostium intrapharyngium (the ‘buttonhole’ in the soft palate); OP = oropharynx; SP= soft palate.  For clarity of illustration, air is shown in the digestive tract. 

Figure 2. Breathing and swallowing. Showing the switch-point changes for each function.  Key: AC = the paired arytenoid (‘flapper’) cartilages; E = epiglottis; LP = laryngopharynx (the food channel on each side of the larynx by which a liquid stream of masticated grass flows into the gullet during grazing, without interrupting breathing); NP = nasopharynx; OI = ostium intrapharyngium (the ‘buttonhole’ in the soft palate); OP = oropharynx; SP= soft palate.  For clarity of illustration, air is shown in the digestive tract.

Figure 3. Swallowing. Showing the soft tissues of the throat in the first stage of swallowing, for comparison with Figure 1. With a bolus of food in position, the oropharynx is enlarged at the expense of the nasopharynx and the cartilages of the larynx close the entrance to the trachea. At the next stage, the lips will be sealed, and a swallowing reflex will propel the bolus into the gullet, evacuating air from the oral cavity/oropharynx and establishing their negative atmospheric pressure. 

Figure 3. Swallowing. Showing the soft tissues of the throat in the first stage of swallowing, for comparison with Figure 1. With a bolus of food in position, the oropharynx is enlarged at the expense of the nasopharynx and the cartilages of the larynx close the entrance to the trachea. At the next stage, the lips will be sealed, and a swallowing reflex will propel the bolus into the gullet, evacuating air from the oral cavity/oropharynx and establishing their negative atmospheric pressure. 

The throat serves two functions with diametrically opposed configurations (Figures 2 and 3).  

 Another ‘moving part’ is the larynx, a valve in the respiratory tract that must be open for breathing and closed for swallowing (Figure 4). A quirk of mammalian evolution determines that the nerves that control the laryngeal muscles (the paired recurrent laryngeal nerves) are of an extraordinary length, especially so for the left nerve. Both nerves arise as branches of the 10th cranial nerve, the Vagus nerve. But instead of branching off the Vagus as it courses alongside the larynx at the top of the neck, they do not branch until the Vagus is alongside the heart.  Consequently, each of the aptly named recurrent nerves must ‘recur’ up the entire length of the neck before they enter the larynx.  

Figure 4. Recurrent laryngeal neuropathy (RLN, laryngeal paralysis, ‘Roaring’). Two endoscopic views of a larynx, during fast exercise.  A: A healthy larynx, fully open on both sides. B: Left-sided recurrent laryngeal neuropathy. The paralyzed left flapper cartilage has failed to open at the gallop and is in a fully closed position, seriously obstructing the airway. 

Figure 4. Recurrent laryngeal neuropathy (RLN, laryngeal paralysis, ‘Roaring’). Two endoscopic views of a larynx, during fast exercise.  A: A healthy larynx, fully open on both sides. B: Left-sided recurrent laryngeal neuropathy. The paralyzed left flapper cartilage has failed to open at the gallop and is in a fully closed position, seriously obstructing the airway.

‘Roaring’ is common in tall horses (e.g., Thoroughbreds, Warmbloods and Clydesdales). Its cause is unknown. A theory that I propose for testing is that it may be caused by bit-induced shortage of oxygen (hypoxia) selectively damaging the long left recurrent laryngeal nerve, i.e., that ‘Roaring’ may be a hypoxic neuropathy (Cook 2018). Absence of evidence is not evidence of absence, but I have yet to hear of any horse, bit-free all its life, that has become a ‘Roarer’. If bit-free racing is introduced, my expectation is that this disease will become a rarity, as will dorsal displacement of the soft palate (Figure 5) and ‘bleeding’, two other common diseases currently categorized as of ‘unknown cause.’  

Figure 5. Dorsal displacement of the soft palate. Showing the soft palate’s ‘buttonhole’ in perspective. This common abnormality in the racehorse, often referred to by the acronym DDSP, describes the calamitous ‘unbuttoning’ of the soft palate from the larynx, i.e., a ‘dislocation’ of the nasopharynx from the larynx; a disruption of the airway. Its root cause is bit-induced loss of negative atmospheric pressure in the oral cavity and oropharynx. The configuration is a normal part of swallowing (Figs 2 and 3) but if it occurred in a race, the horse would suddenly be unable to breathe, i.e. would asphyxiate and fall. The jockey or driver may or may not hear the horse make a noise like a death rattle. The episode would be classified as a ‘sudden death.’ The Greek word asphyxia means ‘a ceasing to throb’ i.e., a ‘heart attack.’   


Figure 5. Dorsal displacement of the soft palate. Showing the soft palate’s ‘buttonhole’ in perspective. This common abnormality in the racehorse, often referred to by the acronym DDSP, describes the calamitous ‘unbuttoning’ of the soft palate from the larynx, i.e., a ‘dislocation’ of the nasopharynx from the larynx; a disruption of the airway. Its root cause is bit-induced loss of negative atmospheric pressure in the oral cavity and oropharynx. The configuration is a normal part of swallowing (Figs 2 and 3) but if it occurred in a race, the horse would suddenly be unable to breathe, i.e. would asphyxiate and fall. The jockey or driver may or may not hear the horse make a noise like a death rattle. The episode would be classified as a ‘sudden death.’ The Greek word asphyxia means ‘a ceasing to throb’ i.e., a ‘heart attack.’

When a horse is galloping, its elastic-sided soft palate ‘buttonhole’ should be embracing the laryngeal ‘button’ even more firmly than it does at rest. If the horse does not have recurrent laryngeal neuropathy, the larynx will be fully open. This secures the necessary air-tight seal at the junction of the throat and laryngeal airways. Unfortunately, some degree of left-sided recurrent laryngeal neuropathy in the Thoroughbred and the Warmblood is common, so this second airtight-seal cannot be relied upon.

When ridden or driven the respiratory and digestive tracts should be configured as in Figure 6.

Figure 6. The normal configuration of the soft palate for running. The oral cavity and oropharynx are reduced to a potential cavity. The lips are sealed. The red coloration in the oral cavity and elsewhere indicates the presence of a negative atmospheric pressure, i.e., a vacuum.  Strictly speaking, the body of the tongue should be depicted in contact with the hard palate and its root in contact with the front section of the soft palate but, to show the location of the vacuum, some space is necessary. At maximum speed, the jowl angle is increased. 

Figure 6. The normal configuration of the soft palate for running. The oral cavity and oropharynx are reduced to a potential cavity. The lips are sealed. The red coloration in the oral cavity and elsewhere indicates the presence of a negative atmospheric pressure, i.e., a vacuum.  Strictly speaking, the body of the tongue should be depicted in contact with the hard palate and its root in contact with the front section of the soft palate but, to show the location of the vacuum, some space is necessary. At maximum speed, the jowl angle is increased.

A bit breaks the lip seal and dissipates the negative pressure. As a result, the bit-ridden horse cannot breathe freely and, at fast paces, is throttled by its untethered soft palate (Figure 8). In the racehorse, bit-induced suffocation leads, in my opinion, to a cascade of events such as negative pressure pulmonary edema (‘bleeding’ and ‘waterlogging’ of the lung), stumbling, falls, catastrophic accidents and sudden death. 

 

 

 

Figure 8. Dorsal displacement of the soft palate. Showing how, with a bit in a horse’s mouth, the unsealed lips dissipate what should be a negative pressure in the oropharynx, resulting in the soft palate becoming unbuttoned from the larynx, i.e., it depicts the throttled condition of the airway in a racehorse with ‘dorsal displacement of the soft palate.’ The two red spots mark how the respiratory tract has become constricted at the junction of nasal cavity and nasopharynx. The two green spots mark the edges of the soft palate buttonhole (Figure 5) and another level of airway constriction at the junction of nasopharynx and larynx.

Figure 8. Dorsal displacement of the soft palate. Showing how, with a bit in a horse’s mouth, the unsealed lips dissipate what should be a negative pressure in the oropharynx, resulting in the soft palate becoming unbuttoned from the larynx, i.e., it depicts the throttled condition of the airway in a racehorse with ‘dorsal displacement of the soft palate.’ The two red spots mark how the respiratory tract has become constricted at the junction of nasal cavity and nasopharynx. The two green spots mark the edges of the soft palate buttonhole (Figure 5) and another level of airway constriction at the junction of nasopharynx and larynx.

 

Figure 9. Throttling. Showing how the respiratory part of the horse’s throat, i.e., its nasopharynx (the checkered section) becomes increasingly constricted as bit-induced poll flexion increases and other bit-induced complications occur. A: A healthy airway (no bit) with head and neck extended (Fig.6). B: With moderate bit-induced poll flexion, the size of the throat is decreased. Because of the loss of the oral vacuum, the soft palate is unstable and vulnerable to further elevation each time the horse takes a breath, i.e., the horse is throttled (Figure 11) C: The soft palate has become unbuttoned from the larynx, a problem in the racehorse called dorsal displacement of the soft palate (DDSP) or, in harness horse language, “choking-up.’ (Fig.8) D: As bit-induced poll flexion increases, the unstable soft palate undergoes further dynamic collapse at each labored inspiration (Fig. 11). The constriction becomes even more serious if the nasal plane drops ‘behind the vertical.’ (Fig 10).   

Figure 9. Throttling. Showing how the respiratory part of the horse’s throat, i.e., its nasopharynx (the checkered section) becomes increasingly constricted as bit-induced poll flexion increases and other bit-induced complications occur.  A: A healthy airway (no bit) with head and neck extended (Fig.6). B: With moderate bit-induced poll flexion, the size of the throat is decreased. Because of the loss of the oral vacuum, the soft palate is unstable and vulnerable to further elevation each time the horse takes a breath, i.e., the horse is throttled (Figure 11) C: The soft palate has become unbuttoned from the larynx, a problem in the racehorse called dorsal displacement of the soft palate (DDSP) or, in harness horse language, “choking-up.’ (Fig.8) D: As bit-induced poll flexion increases, the unstable soft palate undergoes further dynamic collapse at each labored inspiration (Fig. 11). The constriction becomes even more serious if the nasal plane drops ‘behind the vertical.’ (Fig 10).

 

 

Figure 10. Rollkur. Showing how hyperflexion in the bit-ridden horse puts a hairpin bend in the airway (blue). By itself this reduces airflow but, in addition, further constriction from dynamic collapse will occur during labored inspiration at each of the points marked x, due to unsealed lips and the increased negative pressure (Figure 11). 

Figure 10. Rollkur. Showing how hyperflexion in the bit-ridden horse puts a hairpin bend in the airway (blue). By itself this reduces airflow but, in addition, further constriction from dynamic collapse will occur during labored inspiration at each of the points marked x, due to unsealed lips and the increased negative pressure (Figure 11).

Figure 11. Throttling. Showing the dynamic collapse of the airway that occurs during inhalation at speed in any horse in which a bit unseals the lips and prevents the development of a negative pressure in the oral cavity and oropharynx. Note that the floor and most of the roof of the nasopharynx is of soft tissue, unsupported by bone or cartilage. Likewise, overground endoscopy shows that the soft parts of the larynx, the aryepiglottic folds and the vocal cords, can also develop dynamic collapse from side to side. 

Figure 11. Throttling. Showing the dynamic collapse of the airway that occurs during inhalation at speed in any horse in which a bit unseals the lips and prevents the development of a negative pressure in the oral cavity and oropharynx. Note that the floor and most of the roof of the nasopharynx is of soft tissue, unsupported by bone or cartilage. Likewise, overground endoscopy shows that the soft parts of the larynx, the aryepiglottic folds and the vocal cords, can also develop dynamic collapse from side to side.

Figure 12. Scabbard trachea. Cross-sections of a healthy and deformed windpipe. On the left, the cartilage ring of the normal windpipe is ‘C’-shaped, and the healthy airway is approximately circular in cross-section. On the right, due to airway constriction at the throat and/or voice box, increased suction forces during inhalation have caused permanent collapse. The soft and unsupported membranous roof of the airway has become sucked away from the tips of the cartilage ring and become stretched in width. In time, with repeated episodes of dynamic upper airway obstruction, the cartilage rings become permanently deformed and elliptical in cross-section. As distance from the site of obstruction increases, suction forces become more powerful (Poiseuille's law) so that the rings at the base of the neck become flattened in cross-section and ‘scabbard-like.’

Figure 12. Scabbard trachea. Cross-sections of a healthy and deformed windpipe. On the left, the cartilage ring of the normal windpipe is ‘C’-shaped, and the healthy airway is approximately circular in cross-section. On the right, due to airway constriction at the throat and/or voice box, increased suction forces during inhalation have caused permanent collapse. The soft and unsupported membranous roof of the airway has become sucked away from the tips of the cartilage ring and become stretched in width. In time, with repeated episodes of dynamic upper airway obstruction, the cartilage rings become permanently deformed and elliptical in cross-section. As distance from the site of obstruction increases, suction forces become more powerful (Poiseuille’s law) so that the rings at the base of the neck become flattened in cross-section and ‘scabbard-like.’

Bit-induced scabbard trachea deformity of the windpipe is common in racehorses. The constriction of the airway at this level and yet more dynamic collapse during inhalation will add further to the bruising and bleeding of the lungs in the racehorse (Fig.13).  

Figure 13. Scabbard trachea. Serial sections of a windpipe from a young racehorse. The dark structure on the extreme left is the larynx, followed by cross sections of the windpipe in the neck. Showing how repeated episodes of airway constriction cause a permanent ‘scabbard trachea’ deformity of the windpipe cartilages that are most marked at the base of the neck. 

Figure 13. Scabbard trachea. Serial sections of a windpipe from a young racehorse. The dark structure on the extreme left is the larynx, followed by cross sections of the windpipe in the neck. Showing how repeated episodes of airway constriction cause a permanent ‘scabbard trachea’ deformity of the windpipe cartilages that are most marked at the base of the neck.

In sum, the presence of a bit in the mouth of a horse is incompatible with a horse’s physiology (a law of nature) and is pathological. If the bit was not strapped in place, every horse would immediately let it fall out. 

At the level of the throat (the ‘pharyngeal’ regions), the digestive and respiratory tracts intersect (Fig.2). The throat has a dual function and, at different times, must be configured for either eating or exercising.  Think of the nasopharynx as a soft-walled and funnel-shaped tube that, during inhalation, converts the dual-carriage airflow from the nasal cavities, into a single-track air stream for being passed through the open laryngeal valve at the top of the windpipe. When grazing, a horse still needs to breathe but when galloping and taking two deep breaths every second, a horse should not be feeling the need to swallow. Switching from one function to another, from breathing to swallowing is clearly an act requiring appropriate timing, i.e., a running horse should not be swallowing.  

Unfortunately, the presence of a bit triggers inappropriate digestive system reflexes at exercise, like salivation, swallowing and elevation of the soft palate. Because of bit usage, a horse is vulnerable to being strangled. Like all mammals, the horse possesses a trigemino-cardiac reflex (Cook 2022). Bit-induced stimulation of this reflex in a racehorse could be a cause of cardiac arrest and sudden death. Another cause of sudden death is the bit-induced airway constriction at the level of the throat that, as explained by Poiseuille’s law, results in the lungs being bruised at each strangled breath by the abnormally powerful negative pressures of inspiration. As explained (Cook 2024), ‘bleeding’ and ‘waterlogging’ of the lung in the horse is analogous to a respiratory emergency in human medicine called negative pressure pulmonary edema (Cook 2016).  

The synchronization of the throat’s configuration, for either digestion or respiration, is easily upset and the consequences can be disastrous. Unfortunately, this is precisely what the bit does and what bit-mandated rules for most horse sports cause. Speed is the arbiter of success in horseracing and respiratory system competence is essential. Yet the bit seriously constricts the respiratory tract while inflicting intense pain in the mouth (Mellor 2020a, b) and negatively affecting a horse’s mental state (Mellor et al 2020). 

Figure 14. Epiglottal entrapment. Two diagrams show the normal and abnormal relationships of the epiglottis with the soft palate and the root of the tongue in a horse when running. The top diagram shows the normal relationship in a bit-free horse running at liberty. The vacuum pressure in the oral cavity keeps the front section of the soft palate firmly clasped to the root of the tongue, with the bottom edge of its ‘buttonhole’ well tucked-in at the base of the epiglottis (compare with Figure 6). Similarly, the top surface of the soft palate is in close contact with the underside of the epiglottis. The bottom diagram shows the abnormal relationship in a bit-ridden horse in which the buttonhole is no longer ‘buttoned-up’ to the larynx (compare with Figure 5). Because there is no oral vacuum, the loosely attached folds of mucous membrane on the underside of the epiglottis are exposed to the suction forces at each intake of air and get dragged in the direction of the lungs, forming a membranous hood over the apex of the epiglottis, i.e., epiglottal entrapment. This is a source of airway constriction at the entrance to the larynx (see Figure 15) and, at fast exercise, dorsal displacement of the soft palate is also likely to occur. [Errata: the legend “Body of” should read “Body of the thyroid cartilage”, i,e., the ‘Adam’s apple’ of the voice box in human anatomy] 

Figure 14. Epiglottal entrapment. Two diagrams show the normal and abnormal relationships of the epiglottis with the soft palate and the root of the tongue in a horse when running. The top diagram shows the normal relationship in a bit-free horse running at liberty. The vacuum pressure in the oral cavity keeps the front section of the soft palate firmly clasped to the root of the tongue, with the bottom edge of its ‘buttonhole’ well tucked-in at the base of the epiglottis (compare with Figure 6). Similarly, the top surface of the soft palate is in close contact with the underside of the epiglottis. The bottom diagram shows the abnormal relationship in a bit-ridden horse in which the buttonhole is no longer ‘buttoned-up’ to the larynx (compare with Figure 5). Because there is no oral vacuum, the loosely attached folds of mucous membrane on the underside of the epiglottis are exposed to the suction forces at each intake of air and get dragged in the direction of the lungs, forming a membranous hood over the apex of the epiglottis, i.e., epiglottal entrapment. This is a source of airway constriction at the entrance to the larynx (see Figure 15) and, at fast exercise, dorsal displacement of the soft palate is also likely to occur. [Errata: the legend “Body of” should read “Body of the thyroid cartilage”, i,e., the ‘Adam’s apple’ of the voice box in human anatomy]

Figure 15. Epiglottal entrapment. Showing how, at racing speeds, the entrapped epiglottis may become retroverted, further obstructing airflow. [insert bit]  

Figure 15. Epiglottal entrapment. Showing how, at racing speeds, the entrapped epiglottis may become retroverted, further obstructing airflow. 

A problem of supply not meeting demand 

The domesticated horse (bit-ridden or bit-driven) is expected to accommodate to the increased demand for oxygen when running by increasing the supply. The potential impact of some factors is currently overlooked. For example:  

  1. The work demands of the exercise (carrying weight or towing weight at speed) may exceed a horse’s inherent physiological capacity to meet the increased demand.  
  2. A bit-ridden or bit-driven horse is expected to meet this demand despite the bit reducing a horse’s ability to breathe and increasing the work of breathing.  
  3. The horse experiences intense pain in its mouth and, because of suffocation and rapidly developing pulmonary oedema (waterlogging of the lungs), may experience a sense of drowning and a heightened sense of fear. Undergoing these negative mental experiences, together with physical exhaustion from hypoxia, a racehorse may be unable to keep up with the herd and ‘stops trying.’  
  4. Like all mammals, a horse has a trigemino-cardiac reflex (Cook 2022), the dysfunctional outcome of which is vagal inhibition of the heart, i.e., a ‘heart attack.’  
  5. Collectively, there are several explanations for why a bit-ridden racehorse may stumble, fall, incur a catastrophic accident, or die a sudden death.  Consensus on the negative effect of the bit has yet to be reached within the veterinary profession (Harvey 2023).
Figure 16. Tongue movement causes movement of the larynx. The root of the tongue is anchored on the lingual process of the hyoid apparatus, a scaffolding that articulates with the base of the skull. As the larynx is anchored on the same scaffold, any movement of the tongue also moves the larynx, which causes air turbulence and reduced airflow. It also jeopardizes the continuity of the airway between the nasopharynx and the larynx.  

Figure 16. Tongue movement causes movement of the larynx. The root of the tongue is anchored on the lingual process of the hyoid apparatus, a scaffolding that articulates with the base of the skull. As the larynx is anchored on the same scaffold, any movement of the tongue also moves the larynx, which causes air turbulence and reduced airflow. It also jeopardizes the continuity of the airway between the nasopharynx and the larynx.

 

Figure 17. Retraction of the tongue. The tongue is a muscular hydrostat, i.e., a structure, like a squeeze ball, with the interesting property of retaining a constant volume, whatever its shape. A decrease of volume in one dimension causes a compensatory increase in at least one other dimension. Other animal hydrostats include the trunk of the elephant and the arms of the octopus. When a horse retracts the tip of its tongue to avoid the bit, the root of the tongue may bulge in the manner shown, causing an obstruction of the throat airway. The horse’s tongue is a sense organ, exquisitely sensitive to touch and taste. As a tool for manipulating food it is constantly on the move when a horse is eating but, when running, it should be immobile (Figure 16).  

Figure 17. Retraction of the tongue. The tongue is a muscular hydrostat, i.e., a structure, like a squeeze ball, with the interesting property of retaining a constant volume, whatever its shape. A decrease of volume in one dimension causes a compensatory increase in at least one other dimension. Other animal hydrostats include the trunk of the elephant and the arms of the octopus. When a horse retracts the tip of its tongue to avoid the bit, the root of the tongue may bulge in the manner shown, causing an obstruction of the throat airway. The horse’s tongue is a sense organ, exquisitely sensitive to touch and taste. As a tool for manipulating food it is constantly on the move when a horse is eating but, when running, it should be immobile (Figure 16).

Contravening a law of physics and several laws of nature, the bit introduces multiple abnormalities:  

  1. The lips cannot be sealed and kept sealed during exercise. 
  2. The negative pressure in the oral cavity cannot be established and maintained. 
  3. The head and neck may not be extended, so the boundary walls of the throat airway are flaccid and vulnerable to dynamic collapse at the gallop. 
  4. The soft palate is untethered from the root of the tongue (which is often mobile because of the pain of the bit) – hence soft palate instability, epiglottal entrapment and, even more serious, the suffocation due to dorsal displacement of the soft palate (Figure 6).
  5. The tongue and larynx are both suspended from the base of the skull by the scaffolding of the hyoid apparatus (Fig 16). As bit pain causes movement of the tongue when a horse is running, this in turn causes movement of the larynx and turbulence of airflow. 
Figure 18. Negative pressure pulmonary edema (‘bleeding’, EIPH) Showing how bit-induced dynamic collapse of the throat airway and poll flexion, by increasing the force of negative pressure in the lungs on inspiration, selectively causes the most barometric damage at the posterior end of the lung, furthest from the site of airway obstruction.

Figure 18. Negative pressure pulmonary edema (‘bleeding’, EIPH). Showing how bit-induced dynamic collapse of the throat airway and poll flexion, by increasing the force of negative pressure in the lungs on inspiration, selectively causes the most barometric damage at the posterior end of the lung, furthest from the site of airway obstruction.

Conclusions

To strap a bit in the mouth of a horse that is about to run is akin to strapping a muzzle on a horse that is about to eat. 

From the design of any bit (whether antique or modern) it is apparent that the purpose of the bit method of rider/horse communication is to cause pain. It has taken us a while to discover that pain does not control and that it is, in fact, the most common cause of a rider losing control. One thing that the pain of a bit does ‘teach’ a horse is a repertoire of conflict behaviours. A young horse quickly learns these.

Sixty-nine conflict behaviors have been identified to date (Cook and Kibler 2018). All of them lessen control and some of them endanger the life of both horse and rider (e.g., bolting, rearing, freezing) (Cook and Kibler 2022). From a group of 56 horses, the total number of conflict behaviors when bitted was 1643, when bit-free 202, a reduction of 94% (Cook 2013).

The pain-free touch of strap-on-skin provides a simpler, more accurate, and more effective signalling method, avoiding the serious side-effects of the within-mouth, bit-on-bone method, so often misunderstood by the horse. To recap on the handicaps that use of the bit imposes, a bit-ridden horse may experience:

  1. Acute mouth pain, nervousness, apprehension and fear.
  2. An inability to breathe freely because of unsealed lips, palatal instability, and restricted extension of the head and neck.
  3. Physical exhaustion from suffocation leading to falls.
  4. Acute chest pain from negative pressure pulmonary edema (‘bleeding’).

These negative affective mental experiences can be assessed using the Five Domains Model (Mellor et al 2020), a welfare assessment method now recommended by the International Federation of Horseracing Authorities. 

The word ‘surgery’ derives from a Greek word meaning ‘a working with the hands.’ Use of a bit is unquestionably a ‘working with the hands.’ It is a surgical intervention in a sensitive body cavity and should be recognised as invasive surgery without anaesthesia, using one or more blunt instruments. The intervention is carried out daily, throughout a horse’s working life.

Bit usage is a common cause of dental disease in the horse (Cook 2011). Illustrations of bit-induced damage to the teeth and the bars of the mouth are available (Cook 2023).  

Existing evidence predicts that if bit-free equestrian sport were to be permitted, several common diseases of the horse that are currently classified as being of unknown cause will be found to be caused by the bit. The proposed list includes exercise-induced pulmonary haemorrhage, catastrophic accidents, sudden death, recurrent laryngeal neuropathy, palatal instability, dorsal displacement of the soft palate, epiglottal entrapment, scabbard trachea, headshaking, and subtle gait abnormalities at ridden exercise (‘bridle lameness’). 

The problem of habituation and the power of dishabituation

A recently published book (Sharot and Sunstein, 2024) draws attention to the fact that our brains stop noticing things – good and bad – that are familiar, i.e., we become habituated. The 69 bit-induced pain behaviours are a striking example of this ‘not noticing.’ They are so common that owners often assume them to be ‘normal’ behaviors, a condition aptly named ‘bit blindness’ (Mellor 2020a, b). The sight of a bit in a horse’s mouth is so familiar that we are habituated to it and so fail to recognize it as a barbaric piece of equipment from the Iron Age. I count myself especially guilty of habituation. I had been a veterinarian for 46 years and had focused my research on diseases of the equine ear, nose and throat for 39 years before recognizing the bit as a painful and pathophysiological foreign body in the mouth of a horse (Cook 1998,1999). My dishabituation to the bit occurred on a day in 1997 when I supplied the owner of a 5-year-old, off-the-track Thoroughbred with the equipment to transition her horse from bit to bit-free. Having first observed the owner ride her horse with and without a bit, I too rode the horse in both modes. By his radically improved behaviour, the horse showed us both a ‘night and day’ difference.

Horse sports that retain the mandatory use of the bit or, as in racing, the required use of the bit as ‘standard practice, will be enforcing the very rules that cause a high rate of catastrophic accidents and sudden death, the continuation of which jeopardize the sport’s social license to operate. Current rules also block the bit-free trials that could lead to the problem’s solution.

RECOMMENDATION 

Administrators of Horse Sports that mandate or require use of the bit are urged to

  • Observe horses being transitioned from bit to bit-free.
  • Recognize the bit’s monstrosity and dishabituate to the bit.
  • Update the rules and provide a bit-free option.

This will enable comparative efficacy data to be compiled on performance, safety and welfare. Once bit-free improvements in speed, behavior, control, and handling become apparent, there will be no need to ban the bit. Use of the bit will be voluntarily discontinued and a major improvement in horse and rider/driver welfare will have been achieved. 

Declaration: Since 2016, the author has had no conflict of interest to declare. Since four mature school horses were first transitioned from bit to bit-free as part of an afternoon demonstration at a Certified Horsemanship Conference (Cook and Mills 2009), the process of transitioning has been described in detail several times on the internet, as for example at Going Bitless (2023).

REFERENCES 

  1. Cook, W.R (1998): “Use of the bit in horses.” Veterinary Record, 1998;142: 676.
  2. Cook, W.R. (1999): “The pathophysiology of bit control in the horse.” Journal of Equine Veterinary Science. 19, 3. 196-203 https://doi.org/10.1016/S0737-0806(99)80067-7
  3. Cook, W.R. (2011): “Damage by the bit to the equine interdental space and second lower premolar.” Equine Veterinary Education. https://doi.org/10.1111/j.2042-3292.2010.00167.x
  4. Cook, W.R. (2013): “A method for measuring bit-induced pain and distress in the ridden horse.” https://bitlessbridle.com/wp-content/uploads/2017/10/MEASURING-BIT-INDUCED-PAIN2013.pdf
  5. Cook, W.R. (2014): “A hypothetical etiological relationship between the horse’s bit, nasopharyngeal asphyxia and negative pressure pulmonary edema (bleeding).” Equine vet. educ. 26, 381-389 https://doi:10.1111/eve.12196  
  6. Cook, W.R. (2016) “Bit‐induced asphyxia in the racehorse as a cause of sudden death.” Equine vet. educ. 28, 405-409 https://doi.org/10.1111/eve.12455  
  7. Cook, W.R. (2018): “Fifty years of recurring struggles with recurrent laryngeal neuropathy.” Equine Veterinary Journal, volume 50, p870 doi:10.1111/evj.12981 Epub 2018 Jul 21  
  8. Cook, W.R. (2022): “Sudden death in the racehorse.”
  9. https://worldbitlessassociation.org/resources/sudden-death-in-the-racehorse/
  10. Cook, W.R. (2023): “Data and visual evidence for the bit-free debate.”
  11. Data and Visual evidence for the Bit-free debate – World Bitless Association
  12. Cook 2024 “Horse Sports’ Option: Ban or be Banned” Horses and People Magazine. https://horsesandpeople.com.au/horse-sports-options-to-ban-or-be-banned/
  13. Cook, W.R. and Strasser, H (2003): “Metal in the Mouth: The abusive effect of bitted bridles.” Sabine Kells, Qualicum Beach, BC, Canada. 
  14. Cook, W.R. and Mills, D.S. (2009). “Preliminary study of jointed snaffle bridle vs. crossunder bitless bridle: A quantified comparison of behaviour in four horses.”  Equine Veterinary Journal, 41 (8) 827-830 doi: 10.2746/042516409X47215
  15. Cook W.R. and Kibler, M. (2018): “Behavioural assessment of pain in 66 horses, with and without a bit.” Equine Veterinary Education. 31, 551-560 https://doi.org/10.1111/eve.12916 
  16. Cook W.R and Kibler, M (2022): “The effect of bit-induced pain in the horse on the feelings of riders about riding.” The effect of bit-induced pain in the horse on the feelings of riders about riding (2022) – World Bitless Association
  17. Going Bitless (2023): Going Bitless: The Ultimate Guide to Transitioning to Bitless Riding (theclassicalridingschool.com)
  18. Harvey, A. (2023): A Bit of a Problem in Equine Welfare: What is the Role of Veterinarians? Center for Veterinary Education, Control and Therapy Series, Number 6001. Issue 313, pp23-26
  19. Mellor, D.J. (2020a): “Mouth Pain in Horses, Physiological Foundations, Behavioural Indices, Welfare Implications, and a Suggested Solution.” Animals 2020 Mar 29;10(4):572. doi:0.3390/ani10040572. 
  20. Mellor, D.J. (2020b): “Bit Blindness.” Vet Script, New Zealand Veterinary Association, September, p32-34
  21. Mellor, D.J. and Beausoleil, N.J. (2017): “Equine welfare during exercise: An evaluation of breathing, breathlessness and bridles.” Animals. 7, 41 doi:10.3390/ani7060041 
  22. Mellor, D.J., Beausoleil, N.J., Littlewood, K.E., McLean, A.N., McGreevy, P.D., Jones, B. and Wilkins, C. (2020): “The 2020 Five Domains Model: Including Human–Animal Interactions in Assessments of Animal Welfare.” Animals 10 (10), 1870; doi:10.3390/ani10101870 
  23. Sharot,T and Sunstein, C.R. (2024). “Look Again: The power of noticing what was already there.” One Signal Publishers, New York, London, Toronto, Sydney, New Delhi.