A team of researchers from Italy, Germany and England successfully developed and validated a standardised scale to recognise pain in horses based on their facial expressions. Called a Horse Grimace Scale, the scale is easy to teach and will assist in pain detection in horses. It will have a positive impact on horse welfare, particularly when they undergo routine painful procedures, such as castration.

For this open-access study, a group of 40 stallions of different breeds aged 1-5 years underwent surgical castration with closed technique under general anaesthesia. An additional six horses undergoing non-invasive surgeries were used as the anaesthesia control group. High-definition cameras were used the day prior to surgery and eight hours post-operatively to capture images of the horse’s expressions. High-quality images of the horses’ faces were extracted from the videos and then scored by five treatment-blind observers.

The researchers identified subtle changes in facial expressions that indicate pain to develop the horse grimace scale. It comprises six facial action units (see image below), including stiffly backwards ears, orbital tightening, tension above the eye area, prominent strained chewing muscles, mouth strained and pronounced chin, strained nostrils and flattening of the profile.

Changes in behaviour and pain scores were assessed and it was concluded that only horses undergoing castration showed significant changes at eight hours post- compared to pre-operatively. Explorative behaviours and alertness were also reduced.

Horse Grimace Pain Scale

Facial expressions are commonly used to assess pain and other emotional states in humans. Standardised methods the assess facial expressions describe the changes to the surface appearance of the face, resulting from individual or combination muscle actions, referred to as ‘action units’. The use of the HGS is able to assist in the same way with horses. Its use post-operatively for scoring pain will have distinct advantages over manual behaviour analysis, which can be complex and time-consuming.

Excerpt horse grimace scale

According to Dr Emanuela Dalla Costa and her colleagues, “Annually, it is estimated that 240,000 horses are castrated in Europe and, despite the severity of pain associated with routine castration in horses being contentious, the findings of previous studies have demonstrated this procedure is very likely to be associated with some degree of pain.” The HGS could also be further developed to identify facial action units associated with other states, such as fear and anxiety, so that we are able to differentiate pain from other states, she explains.

Read the full open access.

Compiled with information courtesy: Michela Minero, Emanuela Dalla Costa, Dirk Lebelt, Diana Stucke, Elisabetta Canali and Matthew Leach, Università degli Studi di Milano, Milan, Italy, Haveland Equine Hospital, Brielow, Germany, Newcastle University, NewCastle, United Kingdom.