Facial expressions in stressed horses

Background

To investigate how pain is affected by stress, this project examined how stress in healthy individuals can be measured, and how stress and fatigue affect the measurement of pain. Two methods were used to induce stress in the horses in the study: loading into transport and isolation from other horses.

The horses' facial expressions were recorded on video and analyzed using the EquiFACS (Facial Action Coding System). This system is based on documentation of the horse's facial expressions. A pilot study has previously shown a tendency for the occurrence of a “stress face,” which includes wide-open eyes, flared nostrils, multiple movements of the muzzle, and a change in the frequency of blinking.

The hope with this project was to be able to correctly identify pain even in environments where emotional states are frequently present, such as at animal hospitals, in horses in motion, or when someone is riding the horse.

Implementation and results

Twenty-eight horses were subjected to stress by being loaded into horse transporters. Ten of these horses were also subjected to social isolation. A heart rate monitor continuously recorded the horses' heart rates, and the horses' facial expressions were recorded during the experiment.

The frequency and duration of each facial expression was then determined using the EquiFACS system (see above). Heart rate increased during the experiment, confirming that the interventions were stressful.

The following facial features were observed when the horses were stressed:

• visible whites of the eyes
• flared nostrils
• wide-open eyes
• raising of the inner eyebrows
• visible tongue
• increased ear movements
• increased blink rate.

The facial measures were successfully used to train a computer program to distinguish between stressed and calm horses, with an accuracy of 74%. Most of the facial features identified are consistent with previous observations in stressed horses, such as dilated nostrils, multiple movements of the muzzle, wide-open eyes, visible tongue, and ear movements.

The clear differences between stress and pain shown in this study are of great value to equine practitioners in general. Is the horse stressed, or is it actually in pain? To ensure the welfare of horses, caregivers must be able to respond to the signs that horses actually show. We therefore plan to develop online courses for private individuals to enable them to detect illness or discomfort at an early stage.