The grimace scale: a useful tool for assessing pain in laboratory animals

Abstract

Accurately and promptly assessing pain in experimental animals is extremely important to avoid unnecessary suffering of the animals and to enhance the reproducibility of experiments. This is a key concern for veterinarians, animal caretakers, and researchers from the perspectives of veterinary care and animal welfare. Various methods including ethology, immunohistochemistry, electrophysiology, and molecular biology are used for pain assessment. However, the grimace scale, which was developed by taking cues from interpreting pain through facial expressions of non-verbal infants, has become recognized as a very simple and practical method for objectively evaluating pain levels by scoring changes in an animal's expressions. This method, which was first implemented with mice approximately 10 years ago, is now being applied to various experimental animals and is widely used in research settings. This review focuses on the usability of the grimace scale from the "cage-side" perspective, aiming to make it a more user-friendly tool for those involved in animal experiments. Differences in facial expressions in response to pain in various animals, examples of applying the grimace scale, current automated analytical methods, and future prospects are discussed.

Keywords: animal welfare; facial expressions; grimace scale; laboratory animal; pain assessment.

Fig. 1.

Differences in facial expressions due to pain in various animals. Differences in facial expressions due to pain in various animals were categorized into three levels of pain intensity: none (0), moderate (1), and clearly present (2). Commonly observed behavior in animals when experiencing pain includes squinting of the eyes. (A) In mice, the nose and cheeks bulge, the ears do not face forward, and there is an increased gap between them. The whiskers either curve backward along the cheeks or stand up forward. (B) In rats, the nose and cheeks become flatter, the ears take on a pointed shape, and the gap widens. The whiskers become stiff and angle along the face. (C) In rabbits, the cheeks become flatter, the nostrils change from a “U” to a “V” shape, the ears fold inwards to a cylindrical shape, and the whiskers stand away from the cheeks and point downwards. (D) In cats, the snout widens into a horizontal elliptical shape, the ears become pointed, the whiskers remain straight and forward, and the head tilts below or in front of the shoulder line.