The effect of tongue-tie application on stress responses in resting horses

Abstract

Tongue-ties (TT) are commonly applied to both Standardbred and Thoroughbred racehorses to increase control, by preventing them from getting their tongue over the bit, and as a conservative treatment for equine respiratory conditions, principally dorsal displacement of the soft palate. This study investigated responses to TT application in horses, at rest, using both behavioural (head-tossing, ear position, gaping and lip licking) and physiological (salivary cortisol concentrations, eye surface temperature and heart rate) indices. Twelve Standardbred horses (six of which were naïve to TT) were used in a randomised crossover design. The study comprised 3 phases; Phase 1 (Baseline), Phase 2 (Treatment), and Phase 3 (Recovery). At phase 2, tongue tie application (TTA) was performed using a rubber band that was looped around the tongue and secured to the mandible for 20 minutes. The control treatment (C) incorporated 30 secs of tongue manipulation, at the start of the 20 min, however no TT was applied. Behaviours (head- tossing, ear position, mouth gaping and lip-licking) and heart rate (HR) were recorded for the duration of the study and analysed in ten minute intervals. Salivary samples were taken at the end of each phase for subsequent cortisol assays and infrared thermography images were taken of each eye at 5-minute intervals. Statistical analyses were performed in SPSS using linear mixed effects models and repeated measures general linear models, to determine differences between treatments and within treatments, over time. Compared to control, there was more head-tossing/shaking (p<0.001), gaping (p<0.001) and backwards ear position (p<0.001) and less forward ear position (p<0.001) during TTA, in Phase 2. Horses with previous experience of TT showed more head-tossing (p=0.040) and gaping (p=0.030) than naïve horses. Lip-licking was more frequent after TTA treatment than control, during Phase 3 (p<0.001). Salivary cortisol concentrations increased after TTA (1846.1pg/mL ± 478.3pg/mL vs 1253.6pg/mL ± 491.6pg/mL, p=0.047). Mean HR, and mean right and left eye temperature did not differ significantly between treatments in any phase (all p> 0.05). The findings of this study suggest the application of a tongue-tie causes changes to both behavioural and physiological parameters suggestive of a stress-related response. Further research is needed that will enable racing and sport horse regulatory bodies to make informed decisions about the appropriate use of tongue-ties in horses.