Facial pain expression assessment in lame horses in locomotion using the Equine Utrecht University scale for facial assessment of pain (EQUUS-FAP)

Abstract

The Equine Utrecht University Scale for Facial Assessment of Pain (EQUUS-FAP) was developed for box-rested horses and validated for acute colic pain, acute and post-operative head-related pain, pain after orthopedic surgery and acute orthopedic trauma. Although it has been validated on live observations of boxrested horses, it has not been validated on videos and photos of box-rested horses, though other studies have done so with successful results for horses after castration or with acute laminitis. It has also not been validated on ridden horses in locomotion, though previous studies have focused on pain expression in ridden horses. EQUUS-FAP has also been used before on trotted horses with live observations, though this has not yet been validated. In this study, the goal was to test the reliability and validity of the EQUUS-FAP scale (modified for video and photo coding) in determining pain scores of trotting lame horses from videos and photos, and to find out whether a correlation exists between pain face scores and an asymmetry index found from Qualisys gait analysis. It was hypothesized that repeatability would be better for pain scoring from photos than videos, and it was not expected there would be a greater difference between baseline and induced lameness scores for photos than videos. It was also hypothesized that there would be a positive correlation between pain face scores and the asymmetry index. Lameness was induced in eight horses using two previously validated models (special modified shoe and LPS injection), and pain scores for each horse were quantified after randomization of videos and photos during baseline and after induction of lameness by two blinded observers, an equine veterinary specialist in anesthesia and a master’s student in Biology, both of whom were trained to use the scale. Results from using the modified EQUUS-FAP pain scale for video and photo scoring indicated acceptable reproducibility (intra- and inter-observer), though a significant increase in pain-related expressions during guided trot after induction of lameness compared to baseline was not found; consequently, no correlation was found between pain scores and Qualisys asymmetry scores. It was concluded that this study does not support the clinical application of this pain scale using video and photo coding for horses in locomotion with lameness. Several explanations were speculated, including effects of other states such as stress, fatigue and fear on pain scores, the scale missing subtle elements of pain expression for moving horses, and the quality of videos and photos not being adequate to detect more subtle signs of pain. Future studies will have to confirm whether facial parameters could be used to assess pain status in lame moving horses, or whether gait and body parameters, for example, should be included.