Shivers is a chronic neuromuscular syndrome in horses that causes shaking and cramping of the hind limbs. The disease is especially concerning for performance horses that require fine motor control. Shivers has no known origin, cure, or treatment. These are the mysteries that Dr. Stephanie Valberg is trying to solve—and she’s well on her way. 

Shivers is nothing new–it has been recognized for centuries. But there was no consensus on what shivers actually was until 2015, when Dr. Stephanie Valberg, MSU Mary Anne McPhail Dressage Chair in Equine Sports Medicine, and her team published a paper describing specific types of shivers: shivers-hyperflexion, shivers-forward hyperflexion, and shivers-hyperextension. The clinical signs vary in type and severity, but most commonly can be observed while the horse is backing up, or in advanced cases when the horse moves forward. The horse can develop painful cramps and uncontrolled shaking, which can inhibit movement. 

After defining the clinical signs of the disorder, Dr. Valberg began working with Dr. Anibal Armein, a neuropathologist from the University of Minnesota’s College of Veterinary Medicine, to determine whether shivers might be due to cerebellar Purkinje cell axonal degeneration, meaning the breaking down of specific nerve fibers located in a part of the brain called the cerebellum. The cerebellum is the portion of the brain that controls motor function.  

This spring, Dr. Valberg performed research to characterize electromyographic (EMG) hind limb muscle activity patterns in horses with shivering, and then correlate this with gait abnormalities and cerebellar degeneration. She partnered with neurophysiologists Dr. Juergen Konzak and Dr. Joshua Aman to test the muscle recruitment patterns of a variety of horses with and without shivers.

At MSU, Dr. Aman used surface EMG to evaluate specific muscles that the horses use for their gaits. The team walked horses forward and backward to evoke some of the problems related to shivers. The goal was to capture that muscle activity and correlate it with where the abnormalities are in the brain.

This research was conducted May 12-14 at the MSU McPhail Equine Performance Center. Six draft horses–four of which were the control group–had a small amount of blood drawn for DNA testing. Valberg’s team shaved small patches of hair off of each horse’s hind limbs and attached four sets of electrodes. The team then fastened a sensor to the horses’ hind feet. Data from the sensors and electrodes fed into a monitor harness and was sent to a computer system.

Dr. Valberg and Dr. Aman collected data during four key movements: when the horses lifted their hind limbs, walked backward, walked forward, and trotted. Each set of electrodes measured a different muscle. The data showed that the horses with shivers lost some degree of temporal precision in muscle firing. While in healthy horses, flexor muscles were active when extensor muscles were silent and vice versa, both flexor and extensor muscles fired at the same time in shivers horses, especially when they were walking backward. 

“What this means is that for the first time, we know why a limb freezes and trembles in shivers horses. It is caused by abnormal firing of flexor and extensor muscles,” said Dr. Valberg. “We now know that the abnormal shivers gait occurs because the part of the brain that controls motor movement patterns (the cerebellum) has chronic degenerative changes. Knowing where in the brain the disease originates and how this lesion could cause clinical signs will help us focus our research into the cause of shivers. Treatments need to be developed based on the underlying disease process. We also plan to develop this EMG technique as a potential diagnostic test for shivers. Right now, there is no diagnostic test. It would be terrific if we could measure recruitment patterns to be able to more accurately diagnose shivers in horses.”

Horses displaying classic shivers gait abnormalities were found to have cerebellar Purkinje cell axonal degeneration. Because the cerebellum regulates the pattern and timing of flexor and extensor muscle recruitment, the results support the thesis that a focal abnormality in the cerebellum causes shivers. This knowledge will help the team focus their research to develop treatments based on the underlying disease process in Purkinje cells.

Now Valberg’s team is researching the possibility that shivers may be inherited. The team’s next step is to determine which genes are dysregulated in the cerebellum and whether shivers is inherited. Dr. Valberg’s team is currently looking to obtain DNA from horses with shivers and from Warmblood and Thoroughbred geldings 16.3 h or taller that do not have shivers. Information about submitting a video and hair sample for the team to use in an upcoming genetic study can be found on Dr. Valberg’s website.

“We suspect, but do not yet know, if shivers is inherited,” Dr. Valberg said. “You may inherit a genetic mutation that makes you susceptible to a degenerative disease, and then if the right environment meets the right genetic susceptibility, the disease develops in those horses. We know that height and gender have a big impact on whether horses show signs of shivers. If they are under 16.3 hands tall or a female, they are much less likely to show signs of the disease. So you could inherit the genetic mutation, but it may not express itself if you are a female and short. But if you are a male and tall, it may become obvious. This really complicates finding the genetic basis for the disease.”

Shivers can significantly lower a horse’s quality of life and be a difficult condition for owners to support. When owners of a horse with shivers feel the horse has reached the end of a comfortable life, Dr. Valberg is encouraging owners to consider donating their horse to the shivers study. These donated horses are vital for research to determine a root cause and a treatment or cure. Donated horses will be humanely euthanized and their brains immediately frozen to preserve the cerebellum’s gene expression patterns. The research will focus on the molecular pathways that become dysregulated in the cerebellums of shivers horses.

“When we understand the way cellular function is disrupted in shivers horses, we can target specific pathways for specific and more effective treatment,” said Dr. Valberg.

Owners that wish to donate their horse with shivers may submit a video of their horse walking forward and backward while exhibiting the classic signs of shivers. Once screened and approved by Dr. Valberg’s team, the horse can be donated to MSU. Please email Dr. Valberg for more information.