Neuropathic pain linked to dangerous equine behavior

A tenet of good horsemanship has long held that if a horse chronically misbehaves, your first step must be to determine whether physical pain is the cause. A study from Colorado State University not only validates this approach but identifies a specific type of neuropathic pain that can lead to behaviors so dangerous that euthanasia may be the kindest option.

The Colorado researchers reviewed the cases of 14 horses referred to the University veterinary clinic for “severe performance limitations resulting in euthanasia.” The horses were generally well-behaved and easy to handle around the barn, but became extremely dangerous under saddle—bucking, rearing and refusing to go forward, and engaging in other intractable behaviors. After repeated and extensive diagnostic workups failed to identify an underlying cause, and multiple therapies did not change the behavior, the owners opted to have the horses euthanatized.

MIcroscopic image of clusters of neurons around a horse's spine

Postmortem examinations revealed moderate to severe inflammation in dorsal root ganglia—clusters of neurons in
areas of the spine—among a group of horses that had been euthanatized because of dangerous behavior. (Getty Images/image of human spinal ganglion cells under the microscope.

“This is such a hard decision for all of these owners,” says Melinda Story, DVM. “Generally, I find that the owners get to this point because of their belief that it is truly in the best interest of the horse. They—and I—believe they are acting out of selfless love. They do not want the horse to suffer, and the horse does not look like he is suffering when just standing in the barn. But when engaged, the horse quickly changes to a protective behavior—ears back while walking out of the stall, and progressing to truly dangerous behavior. A critical factor in these horses is the lack of response, or short-lived response, to therapy after extensive diagnostics to define the problem and attempts to treat the problem.”

As part of the diagnostic workup, the study horses were examined by an equine surgeon and a sports medicine and rehabilitation specialist. Using each horse’s case history, a myofascial examination (hands-on examination of the skin and muscles), chiropractic, lameness, and neurologic evaluations, and diagnostic imaging, the clinician made a tentative diagnosis of neuropathic pain related to the spine before euthanasia was performed.

During postmortem examinations of the study horses, researchers examined all structures and tissues surrounding the spine as well as collecting tissue samples for histo–pathologic examination. They found that all the horses had moderate to severe ganglionitis—inflammation in clusters of neurons known as dorsal root ganglia—in areas of the spine.

“The ganglia are very complex structures and are being intensely evaluated in many research models as well as humans and horses,” says Story. “The dorsal root ganglion is a structure within the dorsal nerve root. It lies protected within the intervertebral foramina [openings within the vertebrae].”

There is no way to see the ganglia in a living horse, Story continues: “Imaging the axial skeleton in the horse is very difficult in general. MRI is the imaging of choice in human medicine but is not possible in the axial skeleton of the horse.”

Currently, no specific treatments or therapies are available for ganglionitis in horses. “We are early in the understanding of this in horses,” says Story, who adds that not every horse misbehaving due to pain has such a dire prognosis. “There are certainly many cases where the source of pain can be diagnosed, resolved and the behavior improves. These horses [with ganglionitis] are quite different in that there is essentially no/short-lived response to any/all therapies.”

Story says that owners and trainers are the best advocates for the horses whose misbehaviors may stem from physical pain. “The most important thing for me to relay to the owners [with this study] is to believe in themselves and their horse. Owners/riders know the horses better than anyone.  All too often they are told that the horse is fine, and their worries are pushed aside. The owner is the advocate for the horse. They need to keep pushing to find answers if they truly think something is wrong with the horse.”

Reference: “Dangerous behavior and intractable axial skeletal pain in performance horses: A possible role for ganglioneuritis (14 Cases; 2014–2019),” Frontiers in Veterinary Science, December 2021.

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