Existing pain assessment tools for horses based on facial expressions : a review

This review considers validity, reliability and clinical applicability of existing pain assessment tools based on facial expressions: Horse Grimace Scale (HGS), Equine Pain Face, EQUUS-FAP. The HGS was shown to be a potentially valid tool for assessing castration related pain: time and treatment-blind observers scored significantly increased HGS from pre to post-surgery and scores were unaffected by anesthesia alone. Inter-observer reliability was good after a short training of assessors (Dalla Costa et al. 2014. PLoSONE 9(3): e92281). The Equine Pain Face was observed after the induction of two noxious stimuli. Compared to basal and control, the number of pain face features after stimuli was significantly higher. Inter-observer reliability was not assessed (Gleerup et al., 2014. Veterinary Anesthesia and Analgesia, 42, 103–114). Both HGS and Equine Pain Face were developed using pictures. EQUUS-FAP was directly scored by non-treatment blind observers to evaluate acute colic pain. EQUUS-FAP is a composite scale comprehensive of behavioural and facial features. It showed high inter-observer reliability and different values were reported between control and colic patients and between conservatively and surgically treated horses. (VanLoon and VanDierendonck 2015. The Veterinary Journal 206: 356–364) All considered studies reported similar changes in the facial features of horses experiencing different painful conditions. After short training, new assessors learned to reliably recognize horses in pain. Scoring facial features is a promising tool to assess pain, however, further validation studies would clarify whether it is applicable to different clinical conditions and environments.