Equine Facial Action Coding System for pain assessment in horses

Horses are very expressive animals and generally it is easy to recognize signs of pain, often signs specific to anatomical location or organ involvement, when present. Occasionally you run into a mildly depressed horse without specific signs of colic that leads you to a hunt for the source of depression with pain being one presumptive cause. One exception often discussed is assessing pain from gastric ulcers. In this study an attempt is made to fine facial characteristic changes associated with pain which may help with differencing other causes of symptoms. It could also be useful in assessing treatment response.
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Equine Facial Action Coding System for determination of pain-related facial responses in videos of horses
PLoS One. 2020 Nov 3;15(11):e0231608.
Authors
Maheen Rashid 1 , Alina Silventoinen 2 , Karina Bech Gleerup 3 , Pia Haubro Andersen 4
Affiliations
1 Dept. Computer Science, University of California Davis, Davis, California, United States of America.
2 Dept. Anatomy, Physiology and Biochemistry, Swedish University of Agricultural Sciences, Uppsala, Sweden.
3 Dept. Clinical Sciences, University of Copenhagen, Taastrup, Denmark.
4 Dept. Clinical Sciences, Swedish University of Agricultural Sciences, Uppsala, Sweden.

Abstract

During the last decade, a number of pain assessment tools based on facial expressions have been developed for horses. While all tools focus on moveable facial muscles related to the ears, eyes, nostrils, lips, and chin, results are difficult to compare due to differences in the research conditions, descriptions and methodologies. We used a Facial Action Coding System (FACS) modified for horses (EquiFACS) to code and analyse video recordings of acute short-term experimental pain (n = 6) and clinical cases expected to be in pain or without pain (n = 21). Statistical methods for analyses were a frequency based method adapted from human FACS approaches, and a novel method based on co-occurrence of facial actions in time slots of varying lengths. We describe for the first time changes in facial expressions using EquiFACS in video of horses with pain. The ear rotator (EAD104), nostril dilation (AD38) and lower face behaviours, particularly chin raiser (AU17), were found to be important pain indicators. The inner brow raiser (AU101) and eye white increase (AD1) had less consistent results across experimental and clinical data. Frequency statistics identified AUs, EADs and ADs that corresponded well to anatomical regions and facial expressions identified by previous horse pain research. The co-occurrence based method additionally identified lower face behaviors that were pain specific, but not frequent, and showed better generalization between experimental and clinical data. In particular, chewing (AD81) was found to be indicative of pain. Lastly, we identified increased frequency of half blink (AU47) as a new indicator of pain in the horses of this study.

To help clarify some of the terms used in the abstract above this was taken from the discussion section of the article:
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Using the HFI method on the experimental data, the two most prevalent AUs in painful horse were the chin raiser (AU17) and nostril dilator (AD38) (Table 1). These two AUs seem to have equivalents in the Horse Grimace Scale [10] as the configurations “mouth strained and pronounced chin” and “strained nostrils and flattening of the profile”; in the Equine Utrecht University Scale for Facial Assessment of Pain (EQUUS-FAP) scale [8] as the configuration (regarding nostrils) “A bit more opened” or “Obviously more opened, nostril flaring” and “Corners mouth/ Lifted a bit” or “Obviously lifted”; and in the Pain Face [11] as the configuration “Edged shape of the muzzle with lips pressed together” and “Nostril dilated in the medio-lateral direction”. This shows that facial expressions of pain as described by EquiFACS occur in the same anatomical regions as described in previous descriptions, such as Pain Face and Horse Grimace Scale.

The third most prevalent AU of the painful horse face was the half blink (AU47), which is defined as a reduction of the eye opening by the eyelids, but without complete closure of the eye [14]. The increased rate of half blinks has—to our knowledge—not been documented before as an indication of pain, probably because it is only possible to appreciate this activity from close inspection of video. The action takes place in less than half a second [14]. Decreased eye blink rate has recently been described as a non-invasive measure of stress in horses [24] and the ethogram of the Pain Face contains evidence of increased blinking during pain [11]. The Horse Grimace Scale [10] contains “Orbital Tightening” as a feature with the following description: “The eyelid is partially or completely closed”. The description does not specify the duration of the closure of the eyelid, and may correspond to any of eye closure (AU143), half blink (AU47), or blink (AU145). Since EquiFACS uses temporal information during annotation, the type of eye closure can be determined unambiguously.