Study finds dental discomfort can be identified in a horse’s face using the horse grimace scale

Animal behaviourists have come to appreciate that horses can display a range of subtle facial expressions that indicate mood and pain. Vets routinely assess pain in conditions such as colic or laminitis by reference to pulse rate, sweating, posture or restlessness, but even low-grade discomfort can affect the expression on a horse’s face. So can facial expressions identify dental pain in horses?

A team at the veterinary school in Utrecht developed a system called the equine Utrecht University scale for the facial expression of pain (EQUUS-FAP), which showed how horses with colic can be scored, while behavioural scientists at the universities of Portsmouth and Brighton developed the equine facial action coding system (EquiFACS) to map and catalogue the complete range of possible facial expressions.

Vets and behaviourists in Germany and Italy have been working on a scoring system they call the horse grimace scale, to narrow down the changes in expression that specifically indicate pain. Studies showed that this was accurate in revealing pain after castration.

Different breeds and even individuals may be more stoical and less reactive to pain but, nonetheless, these facial expressions appear to be reliable.

Signs of dental pain in horses

A Brazilian team used the horse grimace scale to assess possible dental pain in horses used in rodeo work or cattle ranching. None of the horses had any particular dental problems, as far as the owners or riders were aware, and none had received dental treatment for at least six months.

The scientists scored the horses using the established grimace scale, with its six key indicators: ears held stiffly backwards, tension above the eye area, tightening around the eye, prominently strained chewing muscle, mouth strained with pronounced chin and strained nostrils with flattening of the profile.

The scores were made by trained assessors looking at the horses in the flesh, and photographs were taken for other trained assessors to score the horses. Photos were also given to equine vets – who were not trained in the use of the scale – to score the pictures for signs of pain using their professional experience alone.

The horses, 33 in total, then underwent full dental and oral examination, under sedation. Any sharp tooth edges, ulcers and hooks or other abnormalities were treated.

Two weeks later, the assessment process was repeated by all groups. None of the vets or scientists knew which horses had been found to have dental problems.

Settling scores

There was surprising correlation between high scores for pain on the horse grimace scale and significant dental disorders found in the dental exams.

All the trained assessors using the six key indicators – those who saw and scored the horses in their stables, and those who looked at the photographs – could tell which horses were in pain. But the horse vets who were not trained in the use of the established scale could only guess, based on overall appearance, and were only randomly correct in line with general probability.

This research reveals that dental problems and associated pain are common yet are frequently overlooked or unappreciated by owners and riders. Trained assessors can detect dental pain in horses by reference to facial expressions. Yet even experienced horse vets cannot accurately determine pain just by looking at the faces of their patients, unless they have been trained in the use of an objective scale.

Further reading…

● Facial expressions: Applied Animal Behaviour Science 225, 104970-76

Ref Horse & Hound; 7 May 2020