Novel pain assessment developed for horses

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Results from a study by European researchers provide a new approach to assessing pain in horses on the basis of a standardized scale of facial expressions called the Horse Grimace Scale.

The six researchers tout its benefits as including accuracy and ease of training potential evaluators to use it.

A horse before castration (left) and eight hours after the procedure. (Photos courtesy of Animal Welfare Indicators)

To begin, they evaluated 40 stallions and colts of various breeds between 1 and 5 years of age that underwent routine surgical castration under general anesthesia. One group received the nonsteroidal anti-inflammatory drug flunixin meglumine immediately before anesthesia, and the other group received flunixin prior to anesthesia and six hours after surgery. A third group, of control horses, underwent noninvasive, indolent diagnostic procedures under general anesthesia.

Then, using images collected from high-definition videocameras set up in the horses’ observation stalls, two of the veterinary researchers, Drs. Michela Minero and Matthew C. Leach, determined six signs of pain in horses: stiffly backward ears, orbital (eye) tightening, tension above the eye area, strained chewing muscles, strained mouth and pronounced chin, and strained nostrils with flattening of the profile.

Finally, once the Horse Grimace Scale was defined, five participants—not experienced with horses—were trained to use the scale to evaluate and score images of the study horses before and after surgery.

The evaluators found no difference in HGS scores of horses before surgery. However, the evaluators correctly identified 73.3 percent of images obtained after surgery as post-surgical. Additionally, horses that received flunixin after surgery consistently scored lower on the HGS than horses that did not, indicating pain relief from analgesic administration after surgery.

The researchers, whose findings were published March 19 in the open-access journal PLoS One, explained in the abstract of their study that the assessment of pain is critical for the welfare of horses, and the HGS “potentially offers an effective and reliable method of assessing pain following routine castration in horses.”

The drawing in the middle (b) shows the position of two high-definition cameras in a stall. Pictures on the left (a) and right (c) show frames grabbed from camera one and camera two, respectively, that monitored a horse before and after castration.

Existing pain assessment methods have several limitations that reduce their applicability in everyday life. The HGS may offer numerous advantages and overcome some of these limitations. According to the study, the benefits of the Horse Grimace Scale are as follows:

  • It is not time-consuming to perform.
  • Observers can easily and rapidly be trained to use it.
  • It uses the potential tendency to focus on the face when scoring pain.
  • It can be used to effectively assess a range of painful conditions, from mild to severe pain.
  • It can increase the safety of the observer when assessing pain in large animals, as grimace scales do not require the observer to approach the subject and palpate the painful area for the assessment.

Further, the study adds, the HGS can be applied in association with other behavior-based methods to enhance the assessment of pain in horses and could be implemented in practice by owners and stable managers as an effective on-farm early warning system.

The researchers are associated with the Animal Welfare Indicators project, which is financed by the European Union and is aimed at developing animal welfare assessment protocols, including pain assessment protocols for sheep, goats, horses, donkeys, and turkeys.