A bitless bridle does not appear to limit or prevent dynamic laryngeal collapse in horses predisposed to the condition, the findings of fresh research suggest.
However, the treadmill study, in which the effects of a bitless bridle and a snaffle bit on upper airway performance were assessed, did raise important questions around conventional bits.
The bit appeared to cause some obstruction when the horse had free head carriage when compared with the bitless bridle.
The significance of this is unknown, the researchers say, but it does support the idea that the bit causes abnormal airway dynamics.
“Other types of upper respiratory tract disorders such as nasopharyngeal collapse may be more affected by the action of bits and as such should be investigated in future studies in breeds predisposed to these disorders,” the study team reported in the Equine Veterinary Journal.
Upper respiratory tract obstruction is a common cause of poor performance in racehorses and bits are often blamed. However, no studies to date have confirmed or refuted this.
Zoe Fretheim‐Kelly and her colleagues at the Norwegian University of Life Sciences set out to discover whether using Dr Cook’s™ Bitless Bridle, when used instead of a conventional snaffle bit bridle, would reduce the severity of dynamic laryngeal collapse in horses prone to the problem.
The study involved nine Norwegian Swedish Coldblooded trotters, all racing fit and aged between 4 and 7. They comprised four mares, four stallions and one gelding.
The horses were exercised on two consecutive days using a standardised high‐speed treadmill protocol, randomly wearing a conventional bridle with a snaffle bit on one day and Dr Cook’s™ Bitless Bridle on the other.
The exercise protocol consisted of one minute of exercise with free extended head carriage, followed by a minute in poll flexion, achieved by gathering and driving the horse “into the bit”.
This was followed by the horse running alternating phases of one minute of free head carriage, and one minute of poll flexion until the horse showed signs of fatigue, failed to engage the bit, or completed four phases in total.
Inspiratory tracheal pressures were monitored to provide an objective indication of whether upper respiratory obstruction occurred. The head and neck position, rein tension and laryngeal video endoscopy recordings were also monitored and recorded.
To be included, each horse’s performance had to meet certain criteria, which included reaching a heart rate of at least 200 beats per minute. This ultimately excluded two horses.
Fretheim‐Kelly and her fellow researchers said the study could not provide any clear evidence that the effect of a snaffle bit in a horse’s mouth influenced the development or severity of dynamic laryngeal collapse.
Instead, head and neck angles induced by rein tension seemed to be the key event in provoking the problem in susceptible horses.
Nasopharyngeal collapse and dorsal displacement of the soft palate were not observed in any of the included horses, regardless of bridle type or whether tension was applied to the reins.
As previously reported, poll flexion was found to increase peak inspiratory pressures in most horses when compared to inspiratory pressures when exercised with a free head.
The hypothesis goes that the bit plays a role in dynamic laryngeal collapse by interfering with the function of the tongue.
However, since the findings show that the bit has no major effect on the development or severity of the condition, a change in bit type is unlikely to bring relief to horses affected by the performance-limiting disorder, the researchers suggested.
“Other types of upper respiratory tract disorders such as nasopharyngeal collapse may be more affected by the action of bits and as such should be investigated in future studies in breeds predisposed to these disorders,” they said.
They concluded: “The use of a jointed snaffle bit does not appear to be the inciting event in the development of dynamic laryngeal collapse in Norwegian Swedish Coldblooded Trotters”.
Nor does it appear to affect the severity of obstruction when the condition is present, they added.
Therefore, a change in bit or bridle type will likely not provide improvement of airway function during racing in clinical cases affected with the disorder.
The full study team comprised Fretheim‐Kelly, Cathrine Fjordbakk, Constanze Fintl and Eric Strand, all with the Norwegian University of Life Sciences; and Randi Krontveit, with the Norwegian Medicines Agency.
A bitless bridle does not limit or prevent dynamic laryngeal collapse
Zoe Fretheim‐Kelly, Cathrine T. Fjordbakk, Constanze Fintl, Randi Krontveit, Eric Strand.
Equine Veterinary Journal, 25 May, 2020, https://doi.org/10.1111/evj.13287