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Detecting Pain in our Horses

Facial expressions are commonly used to assess pain and other emotional states in humans, but can the same be said for horses?  Researchers have been trying to streamline an effective and accurate way to detect pain in horses.  Grimace scales have been designed by monitoring the appearance of facial changes from individual or a combination of muscles of the face.  These grimace scales are species-specific, and are said to be more advantageous than other methods generally used to detect pain in animals.  The Horse Grimace Scale (HGS) can be performed quickly, it is easy to learn how to properly apply it, and because it is not necessary to touch a painful area on a horse it is safer for the observer to use the grimace scale.  The HGS offers a practical method of pain detection for horses, and may be useful as a part of an early warning system for horse owners and managers.

When using the Horse Grimace Scale there are six areas of the equine head and face that must be observed closely.  These include the ears, eyes, cheek, chin/muzzle, and the nostrils.  Pain in these areas is described as “stiffly backwards ears, orbital tightening, tension above the eye area, prominent strained chewing muscles, mouth strained and pronounced chin, and strained nostrils and flattening of the profile”.  Each area is scored on a 0-2 scale with 0 signifying not present, 1 signifying moderately present, and 2 signifying obviously present.  When each area is scored and then totaled it creates a final pain intensity score.  In a recent study it was reported that it was more challenging to properly score tension above the eye and strained chewing muscles in horses with darker coats, and when only observing the front-view of a horse it was challenging to accurately score prominent chewing muscles as well as chin prominence/mouth strain.  For a better idea of what each score would look like here are images from the same study for comparison.

Another research group thought it could be better to simplify this process and instead of numerical scores have observers simply recognize and score a pain face as “yes or no”.  These researchers still focused on the five key areas: ears, eyes, nostrils, muzzle, and facial muscles.  A “pain face” was described as having one or more of the following:
  • Lowered ears with concomitant outward rotation - this causes the distance between the ears to increase at the base.
  • Contraction of the muscle above the eye (specifically the levator anguli oculi medialis).
  • Tense stare.
  • Nostrils dilated in the medio-lateral direction (from the midline to the outside).
  • Edged shape of the muzzle with lips pressed together and flattened chin.
  • Tension of the facial muscles.

These can all be observed in the “pain face” image below.

And now compare that to a “non-pain” or “relaxed” face.

It is important to note that there are many physical and behavioral components that should be taken into consideration when evaluating pain in horses as well.  Pain can be expressed through the exhibition of general non-specific indicators such as decrease in normal activity, lowered head carriage, fixed stare, rigid stance, and reluctance to move.  These features combined with the results of the studies conducted above may offer a reliable and effective method to assess pain in horses.  Further evaluation is needed for more exacting results, however, this is still a positive step in the right direction for equine welfare.  

For more information on the research studies reviewed for this blog article please see the links below.  And as always, Dr. Kyla and Dr. Maya are available to answer any of your questions.

An Equine Pain Face by Karina B Gleerup, Björn Forkman, Casper Lindegaard and Pia Andersen

A partial video of the above research can be found here:

Development of the Horse Grimace Scale (HGS) as a Pain Assessment Tool in Horses Undergoing Routine Castration by Emanuela Dalla Costa, Michela Minero, Dirk Lebelt, Diana Stucke, Elisabetta Canali, and Matthew C. Leach

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