Four pain scales put to the test in horses with mild joint pain

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File image. Photo by Nikita Voloshyn

Four pain scales were put to the test in horses with joint-related pain to learn more about how they perform in cases involving orthopaedic discomfort.

The scales, developed as tools to improve pain assessment in horses, are based on behaviors and/or facial expressions, in which the observer allocates a score based on the character of the behavior or expression.

“Little is known about behaviors and facial expressions at rest in horses with orthopedic pain since pain is mainly assessed by lameness evaluation during movement,” Katrina Ask and her colleagues at the Swedish University of Agricultural Sciences reported in the journal Animals.

The aim of their study was to investigate the relationship between the behaviors and expressions noted in the pain scales and actual orthopedic pain.

The research involved six mares and two geldings, all of whom underwent clinical assessments before the experiment.

Each was walked, trotted and lunged to detect any pre-existing movement asymmetry using an objective system involving optical motion capture cameras. They were also assessed subjectively by suitably qualified observers.

The horses were each confined to a stable for the experiment.

Each was assessed simultaneously by three experienced observers on four pain scales — the Horse Grimace Scale, the Equine Utrecht University Scale of Facial Assessment of Pain, the Equine Pain Scale, and the Composite Pain Scale.

The Horse Grimace Scale was one of the four pain scales used for the study by Swedish researchers.
The Horse Grimace Scale was one of the four pain scales used for the study by Swedish researchers.

Orthopedic pain was then induced temporarily in the horses by injecting lipopolysaccharides into one hindlimb joint, causing an inflammatory reaction. The hindlimb used on each horse was the one assessed as having the most asymmetry by earlier testing and observation.

The resulting mild to moderate pain was assessed in five ways — by using the four existing equine pain scales, and by measuring the degree of movement asymmetry during motion.

These pain assessments were carried out at least three times for each horse, until each horse had returned to movement asymmetry similar to that of the baseline measurement.

For each pain assessment, the three evaluators stood outside the box stall and independently assessed the horses, using the four pain scales simultaneously.

Measures such as body temperature and heart rate were also taken, as required by some of the pain-scoring systems.

The study team found that behaviors and facial expressions commonly co-occurred and were strongly associated with movement asymmetry.

Posture-related scale items were the strongest predictors of movement asymmetry.

Body behaviors and facial expressions

“Display of facial expressions at rest varied between horses but, when present, were strongly associated with movement asymmetry,” the researchers reported.

However, the reliability of facial expression items was lower than the reliability of behavioral items.

Overall, the results indicated that pain scales for orthopedic pain assessment would benefit from including posture, head position, location in the box stall, focus, interactive behavior, and facial expressions, they said.

“This could improve orthopedic pain detection in horses during rest with mild lameness.”

The results obtained suggested that pain scales combining facial expressions with body behaviors should be used when performing direct pain assessment of horses with mild orthopedic pain at rest, they said.

Discussing their findings, the researchers said the scale item most strongly associated with movement asymmetry when comparing all of them after computer modeling was actually the body temperature (taken rectally as part of the Composite Pain Score assessment). It was followed closely followed by posture and heart rate — another measure required under the Composite Pain Score.

However, the authors noted that the lipopolysaccharides injected can produce a fever, and a fever can in turn increase the heart rate.

Scale items occurring together varied greatly, they said, but, in general, body behaviors and facial expressions were seen together in horses experiencing pain. Facial expressions were positively associated with movement asymmetry.

“Lame horses can be expected to modify their pain at rest, by simply avoiding situations that may increase pain intensity, for example, loading of the painful limb. This may result in other behaviors occurring less frequently during pain.”

The patterns seen in their analysis may confirm this theory, since facial expressions were rarely seen when only posture-behaviors were present.

The authors noted that, of the 37 scale items evaluated in the study, several were included in more than one pain scale, but were weighted differently in the scale design.

“The results of this study indicate that posture should be weighted higher than other behaviors for mild orthopedic pain at rest.

“It is however important to emphasize that only hindlimb lameness was induced in this study and that posture-related changes may show higher or lower associations with movement asymmetry if the lameness is located in the front limb.

“Whether the localization of the lameness should be included in an orthopedic pain scale cannot be determined from this study.

The study team comprised Ask, Marie Rhodin, Lena-Mari Tamminen, Elin Hernlund and Pia Haubro Andersen, all with the Swedish University of Agricultural Sciences.

Ask, K.; Rhodin, M.; Tamminen, L.-M.; Hernlund, E.; Haubro Andersen, P. Identification of Body Behaviors and Facial Expressions Associated with Induced Orthopedic Pain in Four Equine Pain Scales. Animals 2020, 10, 2155.

The study, published under a Creative Commons License, can be read here

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