Upper respiratory tract (URT) obstruction is a common cause of poor performance in racehorses and a major cause of lost earnings and wastage from training. Bits have often been incriminated as a cause of upper respiratory tract obstruction in horses but there are no scientific studies confirming or disproving this hypothesis. Dynamic laryngeal collapse (DLC) associated with poll flexion is a form of URT collapse that is only seen when horses exercise on the bit. It is characterised by bilaterally symmetric vocal fold collapse, with some degree of concurrent arytenoid cartilage collapse that can result in moderate to severe inspiratory obstruction [1]. The disorder quickly resolves when horses are ridden off the bit and exercise with a more extended head and neck carriage.
Recently, researchers in Norway have investigated whether use of a propriety bitless bridle (Dr Cook’sTM Bitless Bridle), instead of a conventional snaffle bit bridle, would reduce the severity of DLC in affected horses measured objectively using inspiratory tracheal pressures [2]. Nine Norwegian Swedish Coldblooded trotters previously diagnosed with DLC were exercised on two consecutive days using a standardised high‐speed treadmill protocol with either a conventional bridle with a snaffle bit, or Dr Cook’sTM Bitless Bridle. Head and neck position, rein tension, inspiratory tracheal pressure measurements, and laryngeal videoendoscopy recordings were obtained. A heart rate greater than 200 bpm, and similar degrees of poll flexion/head height, had to be achieved in both bridles for the individual horse’s data to be included for comparison. Data from 7 horses met these criteria. The change in mean inspiratory tracheal pressure between free and flexion phases in the bitless bridle (‐15.2 ±12.3 cmH2O) was significantly greater (p < 0.001) than in the snaffle bit bridle (9.8±7.9cmH2O). Mean inspiratory pressure during the free phase was significantly more negative (p<0.001) with the snaffle bit bridle (‐32.3±6.3cmH2O) compared with the bitless bridle (28.5±6.9cmH2O). Mean pressures in flexion phase (snaffle bridle ‐42.1±10.8cmH2O, bitless bridle 43.7±15.6cmH2O) were not significantly different between bridles (p=0.2).
Although only using a small number of horses, this study could not provide any clear evidence that the effect of a snaffle bit in a horse’s mouth influences the development or severity of DLC. Instead, head and neck angles induced by rein tension seems to be the key event in provoking DLC in susceptible horses.
References
1. Strand, E., Hanche-Olsen, S., Grønvold, A.M.R. and Mellum, C.N. (2004) Dynamic bilateral arytenoid and vocal fold collapse associated with head flexion in 5 Norwegian Coldblooded Trotter racehorses. Equine Veterinary Education 16, 242-250.
2. Fretheim‐Kelly, Z., Fjordbakk, C.T., Fintl, C., Krontveit, R. and Strand, E. (2020), A bitless bridle does not limit or prevent Dynamic Laryngeal Collapse. Equine Vet J. doi:10.1111/evj.13287