Use of the horse grimace scale to identify and quantify pain due to dental disorders in horses

Abstract

Dental disorders can cause discomfort and chronic pain, affecting the athletic performance and welfare of the horses. However, dental disorders may not be manifested with recognizable clinical signs and may, therefore, lead to late diagnosis or care. The present study aimed to evaluate the effect of dental disorders on equine welfare by analyzing the equine facial expression using the Horse Grimace Scale (HGS). Six different facial characteristics (ears held stiffly backwards, tension above the eye area, orbital tightening, prominently strained chewing muscle, mouth strained with pronounced chin, and strained nostrils with flattening of the profile) were evaluated in 33 adult horses, both males and females, that were regularly involved in sports or working activities. The animals had not received dental treatment for minimum 6 months and were evaluated using the HGS immediately before diagnosis and 15 days after undergoing odontological treatment. Initially, pain was scored in direct physical presence (in loco) by the investigator. At the same time, the horses’ faces were photographed for further analysis by three evaluators trained in the use of horse grimace scale, and unaware of the study and condition of each animal. Additionally, four other equine veterinarians, not trained in the use of HGS, evaluated the same photographs subjectively, scoring 0 for no pain, 1 for mild pain, 2 for moderate pain, and 3 for severe pain, based on their professional experience. A significant agreement in the pain scores (p < 0.05) (Kappa test) was observed among all the evaluators who used the HGS (direct physical examination and from horses’ photographs), with a strong and significant intra-class correlation coefficient (0.86; p < 0.05). However, there was no agreement among the veterinarian evaluators who used the subjective scale. In comparison with that before treatment, the pain score significantly decreased after dental treatment, as evaluated by the trained veterinarians (p < 0.05). While two of the subjective evaluators identified an improvement, the other two did not. In conclusion, dental disorders result in discomfort or pain and modify the facial expression of the horses. The HGS is reliable for the identification and quantification of pain associated with dental disorders either by face-to-face evaluation or by evaluation of photographs. However, these images are not suitable for subjective evaluation.