Development of a facial expressions of pain scale in horses undergoing routine castration

The objective of this study was to develop and validate a standardized pain scale based on facial expressions of horses (Horse Grimace Scale HGS). Forty stallions of different breeds aged 1÷5 years underwent routine surgical castration with closed technique under general anaesthesia. They were assigned to one of two treatment groups balanced for breed. A (n= 19) received one single injection of Flunixin-Meglumine immediately before anesthesia. B (n= 21) received Flunixin-Meglumine before anesthesia and six hours after the surgery. As a control group (C) horses (N=6) needing noninvasive indolent diagnostic procedures under general anesthesia were recruited. None of the subjects had had pre-existing painful conditions. All horses were hospitalized in an clinic for 5 days in an observation box with two HD video-cameras positioned on the top opposite sides of the box. Thirty-minute videos were recorded on the day prior to surgery (baseline) and at 8 hours after anesthesia. From each video high quality images of the horses’ head were extracted. These images were compared to identify changes in facial expressions (see Langford et al, 2010) by a treatment-blind observer experienced in assessing facial expressions in other species. Six facial action units were defined (stiffly backwards ears, orbital tightening, tension above the eye area, strained chewing muscles, mouth strained and pronounced chin, strained nostrils and flattening of the profile). 60 pre and 60 post treatment images (randomly selected) were then scored in a random sequence using the action units by five treatment-blind participants. The mean HGS was analysed using a GLM for repeated measures with time (pre-vs-post treatment) as the within-subjects factor and treatment as the between subjects factor. 73,3% of images were correctly assessed as true post-surgery or true basal. Inter-class correlation coefficient among observers was 0,91 showing high inter-observer reliability. A significant effect of time, treatment and time*treatment were found (P<0,001). A and B significantly differed from C (P<0,01). LP: A new approach to assess pain based on a standardized scale of facial expressions was developed. It has the advantages of being not time consuming, easily trainable and applicable.