The Horse Grimace Pain Scale with images and explanations for each of the 6 facial action units (FAUs). Each FAU is scored according to whether it is not present (score of 0), moderately present (score of 1) and obliviously present (score of 2).

The Horse Grimace Pain Scale with images and explanations for each of the 6 facial action units (FAUs). Each FAU is scored according to whether it is not present (score of 0), moderately present (score of 1) and obliviously present (score of 2).

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The assessment of pain is critical for the welfare of horses, in particular when pain is induced by common management procedures such as castration. Existing pain assessment methods have several limitations, which reduce the applicability in everyday life. Assessment of facial expression changes, as a novel means of pain scoring, may offer numerous...

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... A number of procedures have also been developed to assess pain responses in other large animal models of appendicular osteoarthritis. 232,238-241 Significant advances in facial recognition technology with the application of AI and DML has also emerged as useful methodology in animal pain models, although it may be more suitable for acute rather than chronic painful conditions.[242][243][244][245][246] The mouse continues to be a popular animal model for such IVD studies (see supplemental information) and a multitude of genetically modified mice have been developed to ask specific questions relevant to disc pathobiology (see supplemental information). ...
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Animal models have been invaluable in the identification of molecular events occurring in and contributing to intervertebral disc (IVD) degeneration and important therapeutic targets have been identified. Some outstanding animal models (murine, ovine, chondrodystrophoid canine) have been identified with their own strengths and weaknesses. The llama/alpaca, horse and kangaroo have emerged as new large species for IVD studies, and only time will tell if they will surpass the utility of existing models. The complexity of IVD degeneration poses difficulties in the selection of the most appropriate molecular target of many potential candidates, to focus on in the formulation of strategies to effect disc repair and regeneration. It may well be that many therapeutic objectives should be targeted simultaneously to effect a favorable outcome in human IVD degeneration. Use of animal models in isolation will not allow resolution of this complex issue and a paradigm shift and adoption of new methodologies is required to provide the next step forward in the determination of an effective repairative strategy for the IVD. AI has improved the accuracy and assessment of spinal imaging supporting clinical diagnostics and research efforts to better understand IVD degeneration and its treatment. Implementation of AI in the evaluation of histology data has improved the usefulness of a popular murine IVD model and could also be used in an ovine histopathological grading scheme that has been used to quantify degenerative IVD changes and stem cell mediated regeneration. These models are also attractive candidates for the evaluation of novel anti‐oxidant compounds that counter inflammatory conditions in degenerate IVDs and promote IVD regeneration. Some of these compounds also have pain‐relieving properties. AI has facilitated development of facial recognition pain assessment in animal IVD models offering the possibility of correlating the potential pain alleviating properties of some of these compounds with IVD regeneration. Animal models of IVD deneration have yielded invaluable information on the pathobiology of this degenerative condition and identified prospective therapeutic targets.The complexity of the degenerative changes and multiple therapeutic targets identified by these models suggests artificial intelligence methodology may be required to unravel these complexities and provide a rationale way forward in the development of effective repair strategies.
... Our results suggest that EDA can be used to assess sympathetic stimuli continuously and objectively in horses and to develop tools to assess their welfare. Many of the currently available assessments of pain, stress, and welfare are at least somewhat subjective, including facial expressions [51][52][53] and body behaviors [54] whose sensitivity and evaluation can be affected by short observation periods and a lack of experience of the evaluator [55] Importantly, inter-observer agreement for facial expressions was lower than agreement on behaviors [56]. Continuous, objective monitoring may be useful in horses recovering from medical care, adjusting to new environments, or under stressful conditions, especially when personnel time is limited. ...
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The continuous monitoring of stress, pain, and discomfort is key to providing a good quality of life for horses. The available tools based on observation are subjective and do not allow continuous monitoring. Given the link between emotions and sympathetic autonomic arousal, heart rate and heart rate variability are widely used for the non-invasive assessment of stress and pain in humans and horses. However, recent advances in pain and stress monitoring are increasingly using electrodermal activity (EDA), as it is a more sensitive and specific measure of sympathetic arousal than heart rate variability. In this study, for the first time, we have collected EDA signals from horses and tested the feasibility of the technique for the assessment of sympathetic arousal. Fifteen horses (six geldings, nine mares, aged 13.11 ± 5.4 years) underwent a long-lasting stimulus (Feeding test) and a short-lasting stimulus (umbrella Startle test) to elicit sympathetic arousal. The protocol was approved by the University of Connecticut. We found that EDA was sensitive to both stimuli. Our results show that EDA can capture sympathetic activation in horses and is a promising tool for non-invasive continuous monitoring of stress, pain, and discomfort in horses.
... In addition, it can be used by farmers and veterinarians to understand about facial expression changes of goats during pain suffering. This can be achieved by changes in facial expressions because these changes are involuntary response to pain by animals 33 and leads to the higher sensitivity in pain evaluation 3 . ...
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No study is undertaken to establish a face grimace scale for goats. Therefore, the present study was designed to measure facial expressions, peripheral temperatures and cortisol hormone of goats affected with foot rot disease. Forty goats (20 healthy and 20 infected) were used in this study. Two images were captured from each animal to detect facial grimace scale of goats. Eye, ear and nasal temperatures were measured using infrared thermal camera. In addition, cortisol hormone was obtained from blood serum. Results revealed that there was a significant drop (P<0.01) in eye, ear and nasal temperatures of infected goats compared to healthy animals. In addition, cortisol hormone was significantly (P<0.01) higher in infected animals at day 1 and day 7. The changes in facial expressions were significantly (P<0.01) different than healthy animals, except for the cheek (masseter) muscle. The total pain score was significantly (P<0.01) higher in day 1 and day 7, compared to healthy animals. Sensitivity of facial grimace scale was 0.75 and 1-specificity was 0.35. It was concluded that using changes in facial expressions in goats is a good tool for detecting pain.
... In 2010, Langford et al. (21) introduced a facial expression score to assess pain in mice by comparing the facial expressions of painless and painful animals. Since then, similar comparable "grimace scales" or "facial expression scores" were developed and reported for various species, such as rats (22), rabbits (23), ferrets (24), sheep (25), horses (26), pigs (27), cattle (28), and cats (29). In most of these studies, scientists produced frames out of videos pre and post painful experiences in animals. ...
... Therefore, pain assessment in farm animals is especially critical. Several studies report evidence that facial expressions are valid and reliable for evaluating pain in farm or large domestic animals (25,26). ...
... The ability and reliability to score a respective FAU varied considerably (Figure 2). In horses, the evaluation of "ear position" seemed easy, but 21% of observers noted "not able to score" for tension above the eye, strained mouth, and pronounced chin (26,46). The frequency of appearance of the FAUs also had an influence. ...
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Introduction Facial expression scoring has proven to be useful for pain evaluation in humans. In the last decade, equivalent scales have been developed for various animal species, including large domestic animals. The research question of this systematic review was as follows: is facial expression scoring (intervention) a valid method to evaluate pain (the outcome) in large domestic animals (population)? Method We searched two databases for relevant articles using the search string: “grimace scale” OR “facial expression” AND animal OR “farm animal” NOT “mouse” NOT “rat” NOT “laboratory animal.” The risk of bias was estimated by adapting the Quality Assessment of Diagnostic Accuracy Studies (QUADAS) checklist. Results The search strategy extracted 30 articles, with the major share on equids and a considerable number on cows, pigs, and sheep. Most studies evaluated facial action units (FAUs), including the eye region, the orbital region, the cheek or the chewing muscles, the lips, the mouth, and the position of the ears. Interobserver reliability was tested in 21 studies. Overall FAU reliability was substantial, but there were differences for individual FAUs. The position of the ear had almost perfect interobserver reliability (interclass coefficient (ICC): 0.73–0.97). Validity was tested in five studies with the reported accuracy values ranging from 68.2 to 80.0%. Discussion This systematic review revealed that facial expression scores provide an easy method for learning and reliable test results to identify whether an animal is in pain or distress. Many studies lack a reference standard and a true control group. Further research is warranted to evaluate the test accuracy of facial expression scoring as a live pen side test.
... A fixed stare, dilated nostrils and clenched jaw have also been associated with apathy [162]. In addition, a dull demeanour, reduced alertness often accompanied by self-isolation and lowered head carriage have been described with non-specific and abdominal pain [169,170], features often observed in combination with a 'pain face' grimace [171,172]. ...
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A detailed understanding of what is usual for a species under optimal conditions is critical for identifying and interpreting different features of body function that have known impacts on animal welfare and its assessment. When applying the Five Domains Model to assess animal welfare, the key starting point is therefore to acquire extensive species-specific knowledge relevant to each of the four physical/functional Domains of the Model. These Domains, 1 to 4, address areas where objective information is evaluated and collated. They are: (1) Nutrition; (2) Physical environment; (3) Health; and (4) Behavioural interactions. It is on the basis of this detailed knowledge that cautious inferences can then be made about welfare-relevant mental experiences animals may have, aligned with Domain 5, Mental State. However, this review is focused entirely on the first four Domains in order to provide a novel holistic framework to collate the multidisciplinary knowledge of horses required for undertaking comprehensive welfare assessments. Thus, inferring the potential mental experiences aligned with Domain 5, the final step in model-based welfare assessments, is not considered here. Finally, providing extensive information on free-roaming horses enables a better understanding of the impacts of human interventions on the welfare of horses in both free-roaming and domestic situations.
... Based on the study of FAU and their relation to pain, researchers in veterinary medicine have developed grimace scales to score the diverse facial expressions associated with pain in various animal species (158). Langford et al. (11) were among the first to put such scales into practice using the Mouse Grimace Scale. ...
... Facial expressions have been studied using these FAU to determine degrees of pain in diverse research protocols. The FAU have been shown to be simple, non-invasive, real-time or retrospective tools for recognizing pain in rodents (11,147,158,161,163,170). ...
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One of the most controversial aspects of the use of animals in science is the production of pain. Pain is a central ethical concern. The activation of neural pathways involved in the pain response has physiological, endocrine, and behavioral consequences, that can affect both the health and welfare of the animals, as well as the validity of research. The strategy to prevent these consequences requires understanding of the nociception process, pain itself, and how assessment can be performed using validated, non-invasive methods. The study of facial expressions related to pain has undergone considerable study with the finding that certain movements of the facial muscles (called facial action units) are associated with the presence and intensity of pain. This review, focused on rodents, discusses the neurobiology of facial expressions, clinical applications, and current research designed to better understand pain and the nociceptive pathway as a strategy for implementing refinement in biomedical research.
... When the state is not controlled, the only available option for establishing ground truth is by human annotators. This may introduce bias and error, depending on the annotators' expertise (veterinarians, behavior specialists, laymen), the number of annotators and agreement between them, and also on whether specific measurement are instruments used (e.g., validated grimace scales (Dalla Costa et al., 2014)). Table 1 includes a classification of the works reviewed here according the data annotation strategies discussed above. ...
... The idea of dividing the face into regions, or parts, is especially relevant for works on pain assessment that are based on species-specific grimace scales. Such scales typically divide the animal face into at least three parts, including ears, eyes and nose/mouth/nostrils (e.g., Dalla Costa et al., 2014). One example is the work of Lu et al. (2017), providing a multilevel pipeline for assessment of pain level in sheep, based on the sheep facial expression pain scale (SPFES (McLennan et al., 2016)), according to which the sheep face is divided into regions of eyes, ears and nose. ...
... Another example of a parts-based approach is provided in Lencioni et al. (2021) in the context of horse pain. Based on the horse grimace scale (Dalla Costa et al., 2014), this work also focuses on three regions of the horse face: ears, eyes, and mouth and nostrils, training three separate pain classifier models based on convolutional neural networks (CNNs) for each of the regions. The outputs of these models are then fused using a fully connected network for an overall pain classification. ...
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Advances in animal motion tracking and pose recognition have been a game changer in the study of animal behavior. Recently, an increasing number of works go ‘deeper’ than tracking, and address automated recognition of animals’ internal states such as emotions and pain with the aim of improving animal welfare, making this a timely moment for a systematization of the field. This paper provides a comprehensive survey of computer vision-based research on recognition of pain and emotional states in animals, addressing both facial and bodily behavior analysis. We summarize the efforts that have been presented so far within this topic—classifying them across different dimensions, highlight challenges and research gaps, and provide best practice recommendations for advancing the field, and some future directions for research.
... Recently, evaluation of facial expressions in animals have been incorporated into multidimensional pain instruments. For example, grimace scales with facial expressions considered to be associated with pain have been developed for domesticated species [17][18][19][20] . Although audience and directed attention are shown to affect the production of facial expressions in non-human primates and dogs, facial cues may potentially indicate emotional experiences determining generalized patterns as well as accommodating individual variation [21][22][23][24] . ...
... Cats in painful conditions are shown to present a slightly narrowed eye aperture and reduced distance between the cheeks, mouth and nose region" 20,26 . Horses experiencing pain, however, can present squeezing 19 and widening of both eyes 47 . The eye area in cynomolgus macaques experiencing pain was described as "flinching of the facial muscles around the eyes and/or contraction of the skin at the back-top of the head" 48 . ...
... It is also possible that the less acute nature of postoperative pain 65 , and residual effects of anaesthetics resulted in less obvious pain expression over time. However, assessment and description of pain facial features were successfully achieved in animals undergoing surgeries such as castration and ovariohysterectomy 19,26 . ...
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Changes in facial expression provide cues for assessing emotional states in mammals and may provide non-verbal signals of pain. This study uses geometric morphometrics (GMM) to explore the facial shape variation in female Japanese macaques who underwent experimental laparotomy. Face image samples were collected from video footage of fourteen macaques before surgery and 1, 3, and 7 days after the procedure. Image samples in the pre-surgical condition were considered pain-free, and facial expressions emerging after surgery were investigated as potential indicators of pain. Landmarks for shape analysis were selected based on the underlying facial musculature and their corresponding facial action units and then annotated in 324 pre-surgical and 750 post-surgical images. The expression of pain is likely to vary between individuals. Tightly closed eyelids or squeezed eyes and lip tension were the most commonly observed facial changes on day 1 after surgery (p < 0.01974). A good overall inter-rater reliability [ICC = 0.99 (95% CI 0.75–1.0)] was observed with the method. The study emphasizes the importance of individualized assessment and provides a better understanding of facial cues to pain for captive macaque care.
... These behavioural indicators can be overt and dangerous for the human [1], commonly leading to punitive action on the part of the handler and adding to the horse's distress [2], but they are also present at a much more subtle level. Expressions of pain are now well-recognised in the 'pain ethogram' and concept of the 'equine pain face' [3][4][5][6]. In particular, the equine pain face has been researched in the context of castration surgery [4], laminitis [5] and pain induced by tourniquet on the front leg and capsaicin applied topically [6]. ...
... Expressions of pain are now well-recognised in the 'pain ethogram' and concept of the 'equine pain face' [3][4][5][6]. In particular, the equine pain face has been researched in the context of castration surgery [4], laminitis [5] and pain induced by tourniquet on the front leg and capsaicin applied topically [6]. The inclusion of behavioural indicators alongside physiological markers is recognised as an important factor in the recognition of pain [7]. ...
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One of the key welfare concerns for horses in the United Kingdom is lack of recognition of fear in horses. This study aimed to gain an understanding of how well horse care givers recognise fear and/or anxiety in horses by interviewing equine behaviourists (who interact with large numbers of horse care givers and talk to them about this topic routinely). The experiences of Animal Behaviour and Training Council (ABTC)-registered equine behaviourists working with horse caregivers were examined, including the ability of clients to recognise fear and/or anxiety in horses, how clients respond when discussing fear as the reason for their horse’s behaviour, and what explanations the participants use to explain fear and anxiety. Semi-structured interviews were conducted with nine participants and analysed using thematic analysis before being written up to reflect the discussion points. When asked how well horse caregivers recognise fear and/or anxiety in horses, three key response themes emerged: caregivers are extremely poor at recognizing fear and anxiety in horses; some clients do recognise behavioural signs indicating fear and/or anxiety but only the overt signs (e.g., rearing, running away) rather than the more subtle signs (e.g., tension in face, subtle avoidance behaviours such as a hesitant gait); and fear and/or anxiety behaviour is often misinterpreted or mislabelled. These key themes recurred throughout several other interview questions. This study has provided initial insights into the lack of recognition of fear and anxiety of horses by their caregivers in the United Kingdom as well as tried and tested approaches to conversations to change this. Such synthesis of experience and techniques across the equine behaviour sector, together with the information gained regarding perception of equine caregivers, could be a valuable approach to improve the effectiveness of behaviour consultations and welfare initiatives.
... compromised welfare can exert effects on species-specific behavioral patterns. As another valuable parameter, facial expression patterns reflecting the experience of pain have been reported across species boundaries (33)(34)(35)(36)(37)(38)(39). While early descriptions by Darwin already suggested parallels in facial expressions reflecting emotions in different animal species and humans, Langford et al. (33) were the first group to systematically study facial expressions as a measure of pain. ...
... The grimace scale for mice comprises five action units (AUs): orbital tightening, nose bulge, cheek bulge, ear position, and whisker change (33). Subsequently, grimace scales have been developed and assessed in different species including rats (34)(35)(36)(37)(38)40). ...
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Several studies suggested an informative value of behavioral and grimace scale parameters for the detection of pain. However, the robustness and reliability of the parameters as well as the current extent of implementation are still largely unknown. In this study, we aimed to systematically analyze the current evidence-base of grimace scale, burrowing, and nest building for the assessment of post-surgical pain in mice and rats. The following platforms were searched for relevant articles: PubMed, Embase via Ovid, and Web of Science. Only full peer-reviewed studies that describe the grimace scale, burrowing, and/or nest building as pain parameters in the post-surgical phase in mice and/or rats were included. Information about the study design, animal characteristics, intervention characteristics, and outcome measures was extracted from identified publications. In total, 74 papers were included in this review. The majority of studies have been conducted in young adult C57BL/6J mice and Sprague Dawley and Wistar rats. While there is an apparent lack of information about young animals, some studies that analyzed the grimace scale in aged rats were identified. The majority of studies focused on laparotomy-associated pain. Only limited information is available about other types of surgical interventions. While an impact of surgery and an influence of analgesia were rather consistently reported in studies focusing on grimace scales, the number of studies that assessed respective effects was rather low for nest building and burrowing. Moreover, controversial findings were evident for the impact of analgesics on post-surgical nest building activity. Regarding analgesia, a monotherapeutic approach was identified in the vast majority of studies with non-steroidal anti-inflammatory (NSAID) drugs and opioids being most commonly used. In conclusion, most evidence exists for grimace scales, which were more frequently used to assess post-surgical pain in rodents than the other behavioral parameters. However, our findings also point to relevant knowledge gaps concerning the post-surgical application in different strains, age levels, and following different surgical procedures. Future efforts are also necessary to directly compare the sensitivity and robustness of different readout parameters applied for the assessment of nest building and burrowing activities.