Bits have the potential to disrupt the breathing mechanism in horses, a recently published review says.
Researchers David Mellor and Ngaio Beausoleil explore what they describe as breathing, breathlessness and bridles in their wide-ranging review of equine welfare during exercise.
The pair, from New Zealand’s Massey University, said horses showed clear signs of aversion to having a bit in their mouths, varying from it being a mild irritant to very painful.
“This in itself is a significant animal welfare issue that should be addressed,” they wrote in the journal Animals.
“A further major point is the potential for bits to disrupt the maintenance of negative pressure in the oropharynx [the middle of the throat], which apparently acts to prevent the soft palate from rising and obstructing the nasopharynx [the upper part of the throat behind the nose],” they said.
Untoward respiratory outcomes and poor athletic performance due to this and other obstructions were well established, they said, and pointed to the potential for affected animals to experience significant bouts of breathlessness.
“Bitless bridle use may reduce or eliminate such effects,” they suggest. However, they noted that direct comparisons of the cardiorespiratory dynamics and the extent of respiratory effects in horses wearing bitted and bitless bridles had not been conducted.
The pair suggested that studies be undertaken to directly compare cardiorespiratory responses to exercise in horses wearing bitless and bitted bridles.
The pair, whose review cited 164 scientific papers, said horses had superior athletic capabilities due largely to their exceptional cardiorespiratory responses during exercise.
“This,” they said, “has particular relevance to horses’ potential to experience breathlessness, especially when their athletic performance is reduced by impaired respiratory function.
However, the potential for breathlessness to occur in horses, who were usually ridden with bits, had yet to be evaluated in detail.
Horses engaged in strenuous exercise displayed physiological responses that approached the upper functional limits of key organ systems, they noted. Virtually every organ system was involved.
“Maximum athletic performance is therefore vulnerable to factors that diminish these functional capacities, and such impairment might also lead to horses experiencing unpleasant respiratory sensations, i.e., breathlessness.”
The researchers read existing literature on equine cardiorespiratory physiology and athletic performance to evaluate the potential for various types of breathlessness to occur in exercising horses.
They also explored the influence of management factors such as rein and bit use and of respiratory pathology on the likelihood and intensity of equine breathlessness occurring during exercise.
They said that, in ridden horses, rein use that reduced the jowl angle − sometimes markedly − and conditions that partially obstructed the nasopharynx and/or larynx, impaired airflow in the upper respiratory tract. This led to increased flow resistance.
Associated upper airway pressure changes, transmitted to the lower airways, may increase airflow resistance in the lower airways and impede respiratory gas exchange, they said.
Other potential consequences included decreases in respiratory minute volume (nasopharyngeal airflow), a worsening of the blood oxygen levels, carbon dioxide retention and abnormal acidity of the blood commonly observed in healthy horses during strenuous exercise.
“These and other factors are implicated in the potential for ridden horses to experience three forms of breathlessness — ‘unpleasant respiratory effort’, ‘air hunger’ and ‘chest tightness’ — which arise when there is a mismatch between a heightened ventilatory drive and the adequacy of the respiratory response.”
They continued: “It is not known to what extent, if at all, such mismatches would occur in strenuously exercising horses unhampered by low jowl angles or by pathophysiological changes at any level of the respiratory tract.”
However, different combinations of the three types of breathlessness seemed much more likely to occur when physiological problems significantly reduced maximal athletic performance.
They noted that the maximum oxygen consumption of Thoroughbred and Standardbred racehorses at peak exertion was about 40 times greater than values at rest.
“This is far greater than the 6 to 8-fold increase observed in endurance-trained human athletes and the 10-fold increase seen in some other mammals.”
The nasopharyngeal airflow required to meet these peak oxygen demands in Thoroughbreds is about 1800 to 2000 litres per minute, which is 25 to 27 times greater than resting values.
Achieving such high airflows posed a significant physiological challenge, they said, made harder when narrowing of the nasopharynx, larynx and/or trachea increased flow resistance.
Studies have found that, in exercising horses, the nasopharynx, larynx and trachea contributed about 95% to airflow resistance when inhaling and about 45% to it during expiration.
Head-neck position important
The head-neck position, or head carriage, was also an important determinant of airflow resistance, often expressed as jowl angle.
When galloping, a horse may extend its head and neck out towards jowl angles of 120–130 degrees, which straightens and widens the nasopharynx and reduces nasopharyngeal airflow resistance.
In contrast, when tightly reined in during exercise, the horse’s nasal bone may be nearly vertical to the ground, increasing the angulation of the nasopharyngeal airway, decreasing its cross-sectional area and markedly impeding airflow at that point.
“In the extreme example of the hyperflexion of the Rollkur position the cross-sectional area of the laryngeal opening is also significantly reduced. As expected, both such reductions disproportionately increase inspiratory airflow resistance.
“Various studies provide evidence that maintaining low jowl angles by rein tension can also induce or exacerbate a range of dynamic upper respiratory tract disorders during strenuous exercise.”
Mellor and Beausoleil noted that bits were introduced around 2300 BC.
“The bit is a potential source of considerable discomfort for the horse, as the named tissues of the mouth are extremely sensitive to mechanical stimulation. This statement is supported by clear behavioural evidence that horses find bits aversive, as well as by convincing skeletal evidence of long-term bit-inflicted mouth injuries that would induce pain.” Such features were absent or had a much lower prevalence in wild horses.
Experienced equestrians were well aware of mouth sensitivity, they said.
“Even today, however, the desire to exert control over horses overcomes concerns about the harm that can be done by using some bits that have been designed to apply sufficient pressure to cause significant pain-inducing soft tissue injury.”
Use of such bits was increasingly regarded as inhumane, abusive and, if tested in some jurisdictions, would likely be illegal, they said.
“Of course, even simple bits that may be used gently, for example, the snaffle bit, when used inexpertly or with malicious intent may cause significant injury and pain.”
The pair traversed the function of a closed mouth in exercising horses.
They said an airtight lip-seal was part of the mechanism that contributed to generating and maintaining negative pressure in the oropharynx [mid-throat].
After swallowing, this negative pressure held the soft palate firmly against the immobile root of the tongue, helping to maintain a more open airway.
“In addition, it would prevent soft palatal instability and an associated increase in airflow turbulence that would otherwise occur during forceful expiratory excursions at high respiratory minute volumes. The respiratory benefits of these effects in the exercising horse are obvious.”
However, the presence of a bit often broke the airtight lip-seal and could dissipate the negative pressure in the mouth, destabilising the soft palate and leading to increasing airflow resistance as well as negative inspiratory pressure.
Rein use that markedly reduced the jowl angle worsened these effects by decreasing the cross-sectional area of the nasopharynx near the larynx.
Mellor and Beausoleil are deputy directors of the Animal Welfare Science and Bioethics Centre at Massey University.
Equine Welfare during Exercise: An Evaluation of Breathing, Breathlessness and Bridles
David J. Mellor and Ngaio J. Beausoleil.
Animals 2017, 7(6), 41; doi:10.3390/ani7060041