Mouth Pain in Horses: Physiological Foundations, Behavioural Indices, Welfare Implications, and a Suggested Solution
Abstract
:Simple Summary
Abstract
1. Introduction
2. General Features of Pain
2.1. Major Features of Pain Generation and Experience
2.2. Cerebrocortical Involvement in the Conscious Experience of Pain in Mammals
2.3. Terrestrial Mammals Can Have Numerous Other Subjective Experiences in Addition to Pain
2.4. Adjunct Physiological Responses to Pain-Inducing Stimuli
2.5. General Behavioural Responses to Pain
“There is good evidence that facial expression can be a useful, valid and reliable tool for recognising and evaluating pain in humans and other animals. Both the sensory and emotional components of pain have been demonstrated to affect facial expression, which thus gives a true representation of the affective state of the animal. Many of the mammalian species studied to date have similar facial expression responses to pain.”[15]
2.6. Summing up
3. Sensations Generated within the Mouth
3.1. Tissues Supplied by the Trigeminal Nerve
3.2. Tissues Supplied by the Facial Nerve
4. Mouth Pain in Horses
4.1. Bit-Induced Nociceptor Stimulation and Pain
4.1.1. The Interdental Space
- (1).
- The established relationship between tension (T, units N), mass (m, units kg) and gravitational acceleration (g = 9.8 metres/sec2), which is “T = mg” or “T = 9.8 m” [109].
- (2).
- (3).
- An estimated area of bit–gum contact on the interdental space (CAbg) of 0.387 cm2 [2], which is equivalent to a 6.22 × 6.22 mm square.
4.1.2. The Tongue
4.1.3. The Commissures of the Lips and the Buccal Mucosa
4.2. General Comments and Summing up
5. Behavioural Indices of Mouth Pain in Horses
6. Welfare Implications of Bit-Induced Mouth Pain in Horses
6.1. Respiratory Consequences of an Open Mouth and Relocation of the Tongue above or behind the Bit
6.2. Respiratory Impacts of Low Jowl Angles Maintained by the Firm Application of Rein Tension
6.3. Pain-Related Conflict Behaviours and Summing up
7. A Suggested Solution
- What specific competitive events have been tested for control and safety using bit-free bridles?It appears that no data for comparing the responses of horses ridden bitted or bit-free have been collected, although opportunities for doing so are available with dressage. Bit-free dressage, excluding Grand Prix, is supported in the Netherlands [184], and two online American Western dressage associations give riders the choice of competing using bitted or bit-free bridles [185,186]. In addition, it would be worthwhile to set up other studies that compare the safety and control of horses ridden bitted and bit-free in other competitive events, including, but not limited to, flatraces, steeplechases, and harness races. However, the requirement that bits must be worn during such racing events means that off-track trials would probably be needed.
- Have horses trained from the outset to be ridden bit-free been tested in competition for their ease of control and whether or not they can be ridden safely?Not yet, but sufficient time has passed since bit-free riding became more popular for there to be many young horses that would be eligible for such testing.
- Can horses trained from the outset to be ridden with bitted bridles be successfully transitioned to being ridden safely bit-free?Yes, many years of successful transitioning of horses for recreational riding attest to this, as do a number of published studies (Table 2). However, additional studies would be beneficial.
- If so, what proportion of horses are not able to be transitioned to bit-free riding?Evidence is lacking on the proportion of horses that cannot be so transitioned, but anecdotal reports from horse owners or riders familiar with bitted bridle use suggest that a large majority of horses do respond well when ridden bit-free [11]
- In the horses that can be transitioned, how long does it take?
- Are there particular bit-free bridles that improve the success rate of making this transition?No data are yet available. It would be anticipated that there would be greater benefits with bit-free bridles that do not cause pain.
- What are the detailed specifications of the most effective bit-free bridles?Design provisions should not be aimed specifically at causing pain, nor should equipment permit pain to be inflicted, even when misused.
- Are there particular trainers who are more successful with transitioning horses to bit-free riding?This would almost certainly be the case, but none have yet been compared so none have been documented.
- If so, what training of trainers is necessary to increase the number who are successful?The content of training programmes would at least partly depend on the outcomes of the foregoing trials. Presumably such training would involve adequate study of available literature combined with knowledgeable hands-on supervision and involvement. Equestrian organisations could consider sponsoring and encouraging the conduct of training workshops and other such activities in teaching establishments.
- Can riders be trained so that they can successfully transition their own horses to bit-free riding?The answer is yes, but only if they are well informed, willing to take advice, and are well motivated.
- What proportion of horses can never successfully make this transition?As yet, there are apparently no anecdotal or other reports of complete failure to make the transition. The foregoing trials would be expected to provide specific information about success or failure rates, and possible reasons for any failures.
- What is the relative performance of horses ridden bitted and bit-free in different competitive events?Although not yet determined rigorously, anecdotal reports suggest that improvements in performance can be expected. This needs to be investigated in carefully designed studies in which the athletic performance of bitted and bit-free horses is compared during time-based competitions, such as in the cross-country and show jumping elements of three-day-events. This also applies to flatraces, steeplechases, and harness races, but, as noted above, the requirement that bits be worn in such events will probably mean that off-track trials will need to be arranged.
Funding
Acknowledgments
Conflicts of Interest
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Attribute | Description |
---|---|
Purpose | Pain is understood to have evolutionary survival value. |
Detection | Pain sensations depend on activation of a discrete set of receptors (nociceptors) by noxious stimuli. |
Perception | Further processing via nerve pathways to the brain and within the brain enables the noxious stimuli to be consciously perceived as pain. |
Character | Pain perception varies according to site, duration and intensity of stimulation and can be modified by previous experience, emotional state and innate individual differences. |
Definition | Pain is defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or is describable in terms of such damage. |
Variation | The pain-detection threshold is fairly uniform in mammals, whereas pain tolerance may be more species-specific and subject to modification. |
Indicative Pain-Related Behaviours in Ridden Bitted Horses |
Mouth: resists bridling; fussing with the bit, persistent jaw movements, chewing; crossing the jaw; slightly open or gaping mouth; teeth grinding, holding the bit between the teeth; tongue persistently moving or protruding from the mouth, tongue placed above the bit or retracted behind it; excessive salivation or drooling. Head-neck: sudden evasive movements due to abrupt increases in rein tension; side-to-side or up-down head shaking, jawline above horizontal; head tilted, stiff necked; rein-induced low jowl-angle, neck arched, nasal plane at or behind the vertical; reaches forward so rider uses longer rein. Pain face: identifiable nostril flare, lip positions, ear positions, eye white visibility and facial muscle tension. Body movement/gait: stiff or choppy stride, hair trigger responses, crabbing; difficult to control, hesitant to move forward, difficult to stop, side-stepping from straight-line motion; bucking; rearing; tail swishing. Refs: [6,28,83,90,96,97,115,133,140,141,146,147,148,149,150,151,152,153,154,155,156,157,158]; plus YouTube archive videos a |
Bitted to Bit-Free Changes in Ridden Horse Behaviour |
Mouth: all bit-related mouth behaviours absent; quiet, closed mouth, tongue inside mouth and appropriately placed; little or no teeth grinding; no drooling. Head-neck: head shaking absent; lower head-neck position and wider jowl angle; head, neck and spinal column properly aligned longitudinally. Pain face: no indications of mouth-related pain in healthy animals. Body movement/gait: calm, relaxed and cooperative demeanour; engaged, lively, energised and exhibits vitality of fitness; head freedom supports balanced, aligned and smooth rhythm of motion; tail movement in synchrony with spinal movement. Refs: [1,4,9,11,147,148,149,150,151]; plus YouTube archive videos a |
Behaviours of Bit-Free Horses at Rest or When Running Free |
As expected, domesticated horses wearing loosely-but-snugly fitted bit-free bridles do not display any of the bit-related behaviours noted above while standing at rest or engaging in exercise ranging from walking to galloping; nor do horses wearing halters while standing in stalls or moving freely in turnout paddocks. Likewise, neither do wild, free-roaming horses when standing alert or when walking, trotting, cantering and galloping during roundups. Refs: [159,160]; YouTube archive videos of bit-free domesticated horses, and of ~150 free-roaming, wild Brumbies (Australia), Camargue horses (France), Kaimanawa horses (New Zealand) and Mustangs (USA) a |
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Mellor, D.J. Mouth Pain in Horses: Physiological Foundations, Behavioural Indices, Welfare Implications, and a Suggested Solution. Animals 2020, 10, 572. https://doi.org/10.3390/ani10040572
Mellor DJ. Mouth Pain in Horses: Physiological Foundations, Behavioural Indices, Welfare Implications, and a Suggested Solution. Animals. 2020; 10(4):572. https://doi.org/10.3390/ani10040572
Chicago/Turabian StyleMellor, David J. 2020. "Mouth Pain in Horses: Physiological Foundations, Behavioural Indices, Welfare Implications, and a Suggested Solution" Animals 10, no. 4: 572. https://doi.org/10.3390/ani10040572