Welfare issues arising from bit use need to be addressed, say researchers. © Mike Bain 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.
Pressure changes
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.