Although a grimace is widely recognized as a response to pain, the same facial expression was not found to be a reliable indicator of lameness in racehorses, according to new research from Australia.
The University of Melbourne study was based on 38 Thoroughbreds—12 that had been referred to the clinic for lameness or poor performance, and 26 in training with a licensed racehorse trainer who considered them fit for training and racing.
The horses were evaluated by veterinarians during brief “jog-up” exams (similar to pre-race checks) and assigned a score of 0 (no lameness) to 5 (non-weightbearing). In addition, the horses wore body-mounted inertial sensors that detected gait asymmetries associated with lameness. This combined data were used to determine if the horse would be considered “fit to race” in an actual racetrack setting.
Two independent observers used still images extracted from the jog-up videos to score each horse’s facial expression based on 22 signs of pain.
“Several scales that assess facial expression, including the horse grimace scale (HGS) and the facial expressions in ridden horses (FEReq), have been previously found to detect orthopedic pain or differentiate between lame and non-lame ridden horses, respectively,” says lead author Katrina Anderson, MVSc. “What was not known was whether facial expressions were associated with asymmetry or subtle lameness in racehorses during brief lameness assessments mimicking a pre-race examination.”
When the data were compared, the researchers determined that only three of the 22 facial parameters —exposed whites of the eye and two areas of tension around the eye—were associated with a level of lameness that would render the horse unfit to race. In fact, horses showing two specific “pain” parameters—tension around the mouth and tension in the lips—were actually less likely to be lame.
“We hypothesized that any indicator of tension may be associated with lameness,” says Anderson. “However, in this case we found that evidence of strain and tension around the mouth and lips were more likely to be observed in our ‘fit to race’ horses. This finding was unexpected.”
Although tension in a horse’s face can be an important clue when assessing a horse’s comfort level, she says that a single facial expression isn’t likely to be a reliable indicator of pain or subtle lameness in a racehorse.
“The important takeaway from this study is that people working with horses should be familiar with each individual horse’s baseline so that they may be better placed to identify a change in expression or behavior over time,” says Anderson. “This is likely more useful than assessing a horse at one point in time.”