Assessing facial expressions to detect pain may improve animal welfare

New research has led to a practical pain assessment tool for cats.

The so-called grimace scale uses facial features combined with head and ear position to determine the relative level of pain a particular cat is experiencing.

Dr. Daniel Pang of the University of Calgary’s Faculty of Veterinary Medicine, with colleagues at the University of Montreal, including lead author Marina Evangelista, published the new scale in Scientific Reports in late 2019.

The tool may be especially useful for cat owners and veterinarians since cats tend to be stoic and often hide their pain well. This work adds to a growing list of similar grimace scales developed for a range of species. So far, researchers have developed species-specific grimace scales for pigs, horses, sheep, ferrets, laboratory rodents and rabbits.

Before the cat research came along, researchers at the University of Guelph assessed the facial expressions of piglets to develop a pig-specific grimace scale. Dr. Abbie Viscardi, together with her supervisor, Dr. Patricia Turner videotaped hundreds of piglets for their 2017 study published in Frontiers in Veterinary Science.

The piglets underwent standard surgical castration procedures, which is commonly done on pig farms. The researchers assessing facial expressions for 15 minutes of every hour for the eight hours after castration. This study was particularly labour intensive — more than 600 piglet faces were assessed by two observers who were blinded to the treatment group to validate the scale. Collectively, they viewed 45 hours of video.

The piglet pain scale they developed focused on three key factors. The first is ear position — piglets in pain fold their ears back, whereas those not in pain have them perked forward. The second factor was a combination of cheek tightening and a nose bulge. Piglets in pain tighten their cheeks so that the nose bulges to the side. The third is orbital tightening, a fancy way of saying squinting their eyes. Piglets in pain squint their eyes compared to those that are not in pain.

When all three are seen together, piglets were deemed to be in moderate-to-severe pain.

The horse scale also considers ear position, tension around the eyes and cheek muscles. In addition, the scale incorporates strain in the mouth, chin and nostrils. The horse scale was developed using standard castration surgery in stallions in a 2014 study that was published in PLOS One. Further work is necessary to know if it applies well to horses that are in pain for other reasons such as colic surgery, lameness and wounds.

Researchers used spontaneous disease rather than surgery to develop and validate the sheep grimace scale in a 2016 study published in Applied Animal Behaviour Science.

Sheep that developed mastitis (inflammation of the udder) and foot rot, which causes lameness, were assessed for changes to their facial expression. Similar to the other scales, the sheep one includes ear position, cheek eye and nostril tension. People trained to use the scale were able to accurately identify animals that were affected by mastitis or foot rot, as compared to their clinical diagnosis.

The use of these objective pain assessment tools could be a great additional method for producers to maintain high levels of animal welfare. Future research could validate their use for on-farm, practical use.

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