Humans, other primates, and even dogs share a common feature:  We express ourselves, including our physical pain, through our faces.  Horses, maybe not so much.  At least not in a way researchers have quantified until recently.

To investigate, researchers in Italy, Germany and the United Kingdom worked together to develop and test a standardized pain scale based on equine facial expressions.  Michela Minero, DVM, PhD, ECVBM-CA, published the results in March.

The Question

Facial action coding systems are used for nonverbal humans – as well as mice, rats, and rabbits – to identify pain response.  Minero and her team wondered if the same systems could work on horses.

To create and validate an equine facial pain scale (called the “horse grimace scale,” or HGS) and investigate postoperative pain experienced by horses, the researchers enlisted a facial recognition specialist and lay evaluators to grade high-definition images of horse facial expressions before and after castration (gelding) surgery.

More than 240,000 horses are castrated in Europe every year, but less than 40% of them receive pain relievers after surgery, Minero said, adding that a better understanding of whether or not horses experience pain post-castration could help improve patient welfare.

The Horses

The team evaluated 40 stallions and colts of different breeds, aged 1-5 years old, before and after the horses underwent routine surgical castration with a closed technique under general anesthesia.

Researchers divided the horses into two mixed-age and breed groups.  The first group (19 horses) received a single injection of flunixin meglumine (a nonsteroidal anti-inflammatory sold under the trade name Banamine) immediately before anesthesia.

The second group (21 horses) also received flunixin meglumin injections prior to anesthesia and again six hours post-operatively.

A third group of control six horses included those that underwent non-invasive indolent diagnostic procedures under general anesthesia.

None of the study horses had pre-existing pain-related conditions.

The Images

In conjunction with their procedures, the study horses were hospitalized for five days in an observation stall equipped with high-definition video cameras.  On the days prior to surgery, researchers collected 30-minute videos of each horse to create a baseline.  Videos of the horses were also taken eight hours post-operatively.  Researchers then extracted still images from the videos, zooming and cropping so that the photographs only included the heads and necks.

The Expert

A treatment-blind observer experienced in assessing facial expression in other species (But not with horses) evaluated the study’s still images to create a six-point HGS, which includes the following pain-related facial expressions:

Pain-related facial expressions include:

Stiffly backward ears
Orbital (eye) tightening
Tension above the eye area
Strained chewing muscles
Muscles strained
Pronounced chin

The Evaluation and Results

Once the facial expression expert defined a HGS, five treatment blind participants individually used the scale to evaluate and score images of horses in the study.  These also were not people experienced with horses.

The research team trained the evaluators to use the HGS.  The photo set included 60 randomly selected preoperative still images and 60 randomly selected postoperative still images.

The evaluators found no difference in horses before surgery.  However, the evaluators correctly identified 73% of postsurgical images as postsurgical.

Additionally, horses in the group that received flunixin meglumin injections after their surgeries consistently scored lower on the HGS than those that in the non-treatment group, indicating pain relief from the analgesic.