Using facial expressions of pain in animals

Dr Matthew Leach, from Newcastle University, has received funding from the NC3Rs to validate the use of facial expression for pain assessment in rodents, rabbits and non-human primates.

Research details

Principal Investigator: Matthew Leach, Lecturer in Animal Science
Organisation: Newcastle University
Award: £247,800, in 2012, over 36 months
Title: The assessment of pain using facial expressions in laboratory mice, rats, rabbits and macaques

Read more about Dr Leach's research

Case study

There is a need for new methods to assess pain in animals

Many research animals undergo painful procedures. In the UK, it is common practice to give analgesia after surgical procedures. The effective alleviation of pain depends on the ability to reliably assess its severity and duration. Traditional methods of pain recognition and assessment that monitor gross behaviour or clinical signs (for example, weight loss) are time consuming to conduct and limited by the fact that they are not specific to pain. They also tend to focus on the animal’s physical reaction to pain (the sensory component of pain), rather than how it makes it ‘feel’ (the emotional component), which is arguably the most critical from an animal welfare perspective.

The grimace scale uses facial expressions for pain assessment 

Facial expressions are considered by some to be the ‘gold standard’ method for pain assessment in non-verbal humans, such as newborn babies and people with severe cognitive impairments. Researchers at McGill University (Canada) have identified specific facial expressions in rodents that relate to pain intensity in nociceptive tests. Termed the grimace scale, the facial expressions provide a potential new approach for assessing pain in animals. In 2011, Dr Matthew Leach, Newcastle University, was awarded NC3Rs funding to investigate this further, focusing on rodents, rabbits and macaques. Facial expressions, such as narrowing of the eyes or fattening of the cheeks, could be easier to use than behavioural indices of post-surgical pain such as back arching, belly pressing and flank twitching, since all of the indicators are focused in one small area - the face. Evidence from Dr Leach’s previous work suggests that humans have a tendency to focus on an animal’s face when assessing pain.

Validating the grimace scale in mice following surgical and other procedures

Vasectomy is carried out as a routine procedure in most facilities that produce genetically modified mice. Using a combination of high-speed cameras, high-definition video footage, and manual and automated behavioural scoring systems, Dr Leach has validated the Mouse Grimace Scale (MGS) for the assessment of pain following vasectomy via the scrotum in CD1 mice[1]. This demonstrated that the MGS provides a quick and reliable ‘cage-side’ means of assessing (or ‘scoring’) the severity and duration of pain associated with vasectomy with minimal training required. It takes only one hour to complete the scoring of 18 animals pre- and post-operatively using the MGS compared with 18 hours using behavioural indicators. Importantly, the scale is also sensitive enough to assess the effectiveness of two routinely used analgesics, meloxicam and bupivacaine.

Dr Leach is currently evaluating the MGS for assessing chronic pain in two mouse cancer models – a bladder tumour model in C3H mice, and Ewing’s Sarcoma model in Rag-2 immunodeficient mice induced by intravenous implantation, with the aim of developing improved humane endpoints.

A grimace scale for rabbits

Dr Leach has developed the Rabbit Grimace Scale (RbtGS). The scale was produced using data from a study commissioned by the Swedish Board of Agriculture, comparing clamp tattooing of the ear, a procedure commonly used to identify rabbits farmed for meat production in Europe, with and without local anaesthesia. Like the rodent grimace scales, the RbtGS is based on changes in a number of ‘facial action units’, such as orbital tightening, and bulging or flattening of the cheeks and nose. Changes in these action units reliably indicate acute pain following tattooing without anaesthesia and correlate with physiological signs of stress, such as increased heart rate. The RbtGS is now being validated for scoring pain following ovariohysterectomy.

Using facial expressions to assess pain in non-human primates

There is currently no objective method of assessing post-operative pain in non-human primates. Dr Leach is working on a grimace scale for rhesus macaques using an already established research tool for analysing facial movement – the Macaque Facial Action Coding System. This is analogous to a human system used to describe and understand facial expressions and their role in communication.

Expanding the use of the grimace scale

There have been two publications to date. Dr Leach has given over 20 lectures on his work, including at the annual congresses of the American Association of Laboratory Animal Science, Australian and New Zealand Laboratory Animal Associatio,n and Chinese Veterinary Medicine Association. New collaborations have been established to develop grimace scales for mice used as models of pancreatic cancer, neurotrauma and multiple sclerosis with scientists at Cancer Research UK, Queen Mary, University London and the Babraham Institute respectively. In addition, collaborations are underway to develop grimace scales for other species including lambs (Massey University, New Zealand) and horses (University of Milan, Italy).

This case study was published in a review of our research portfolio in November 2013.


[1] Data was collected from mice undergoing vasectomy for routine genetic modification programmes and not specifically for the purpose of this study.