Researchers Evaluate Horses as Model of Human Depression

"Horses' and humans' basic body systems and physiology are quite similar, so in many cases research conducted in one species could be translated to the other," wrote Stacey Oke, DVM, MSc, in a recent article published in The Horse.

Some of the ailments horses and humans have in common include uveitis, cancer, and respiratory disease, among others. And according to recent study results from a team of French scientists, depression could be the next disorder studied in horses and translated to humans.

"Ethological approaches focused on animals' spontaneous behavior in their home environments might prove useful, especially if the environmental conditions offered to animals share features with environmental conditions known to induce depression in humans," explained lead researcher Carole Fureix, PhD, an equine behavior scientist at the University of Rennes 1 in France.

Stress at work and interpersonal stressors, for example, has been shown to cause a variety of problems for humans, including depression, Fureix said.

"Domestic horses may encounter social and spatial restriction, but also share with humans the characteristic of working on a daily basis and have 'interpersonal' interactions with other working horses or working with a 'boss' who is the human who manages or rides it," she added.

Keeping that in mind, Fureix and colleagues evaluated 59 lesson horses at three riding schools between January and June 2007. Fifteen mares and 44 geldings aged 5 to 20 years worked four to 12 hours each week in riding lessons and were stalled the remainder of the time. French Saddlebreds were the most prevalent breed represented in the study, making up 68% of the population.

The research team put the horses through several tests, including:

  • Observing the horses in their stalls and noting if/when the animal displayed a "withdrawn" posture (which Fureix described as "bouts of immobility when horses displayed an unusual posture, characterized by horses standing with eyes open, stretched neck, and similar height between neck and back"; "withdrawn" horses also exhibited no ear and/or head movement and a fixed gaze with no eye movement for spans of time; see image below);
  • Studying the animals' response to tactile stimulation (by applying force to areas on the horse's body with a filament to induce a muscle twitching response, which is typically used to remove flies from the body);
  • Evaluating the horses' response to a person suddenly appearing at their stall doors and assigning scores ranging from "very friendly" to "indifferent" (where the horse showed no evidence of directed attention towards the person and no change in behavior or gaze) to "very aggressive";
  • Examining horses' response to a novel object (a metallic cage decorated with long red fluorescent ribbons) in a familiar environment (the arena where the horses work); and
  • Measuring the animals' cortisol levels after a day's work and after a day's rest.

They team then compared the data collected with information obtained by evaluating four nonworking horses living in natural conditions (essentially living in a herd situation and not receiving regularly exercise).

Fureix et al. found that 24% of the 59 horses evaluated adopted a withdrawn stance at least once during the observation in the stalls. Interestingly, all the horses that displayed the withdrawn stance were French Saddlebreds, and 33% of the mares included in the study appeared withdrawn.

Withdrawn horses were less responsive to tactile stimulation than nonwithdrawn animals, the team found, and were less reactive to humans approaching their stall suddenly. The researchers also found withdrawn horses more reactive to the novel object as compared to their nonwithdrawn counterparts, and the former group of horses had lower blood plasma cortisol concentrations than the latter group.

So what does it all mean?

Fureix explained, "(Withdrawn) animals, displaying an atypical posture and characterized by their unusual gaze, head, and ears fixity, were more indifferent than the others to environmental stimuli in their home environment. However, they reacted more emotionally in other, more challenging situations. Finally, these 'depressed' horses exhibit lower plasma cortisol levels.

"All of these characteristics present strong similarities with some aspect of the depressive states of other nonhuman species and human ... models," she said.

Not only could the information obtained in this pilot study provide researchers with a new model with which to study depression in humans, it could also help equine veterinarians, behaviorists, and horse owners better assess equine welfare based on ethologic responses indicative of depression.

The study, "Towards and Ethological Animal Model of Depression? A Study in Horses," appeared in June in the open access journal PLoS One. The entire study can be viewed online.

Different Postures

The horse on the left exhibits the "withdrawn" or depressed posture as described in the study. The horse in the middle is in a standing, observing posture while the horse on the right is resting, as evidenced by the closed eyes and resting hoof.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.