Conceptual and methodological issues relating to pain assessment in mammals: The development and utilisation of pain facial expression scales

•There are conceptual and methodological problems for animal pain assessment.•Facial expression scales are valid, reliable pain recognition and evaluation tools.•Consistent and careful development of facial expression scales is required.•Facial expression scales need feasibility testing to aid use in clinical practice.•Guidelines are provided for development and use of facial expression scales. Effective management of pain is critical to the improvement of animal welfare. For this to happen, pain must be recognised and assessed in a variety of contexts. Pain is a complex phenomenon, making reliable, valid, and feasible measurement challenging. The use of facial expressions as a technique to assess pain in non-verbal human patients has been widely utilised for many years. More recently this technique has been developed for use in a number of non-human species: rodents, rabbits, ferrets, cats, sheep, pigs and horses. Facial expression scoring has been demonstrated to provide an effective means of identifying animal pain and in assessing its severity, overcoming some of the limitations of other measures for pain assessment in animals. However, there remain limitations and challenges to the use of facial expression as a welfare assessment tool which must be investigated. This paper reviews current facial expression pain scales (“Grimace Scales”), discussing the general conceptual and methodological issues faced when assessing pain, and highlighting the advantages of using facial expression scales over other pain assessment methods. We provide guidance on how facial expression scales should be developed so as to be valid and reliable, but we also provide guidance on how they should be used in clinical practice.