Shivers, a progressive neuromuscular movement disorder in a horse’s hind limbs, baffles equine veterinarians and horse owners alike. While rare, the condition is not curable. Historically, however, regular exercise has been one of the best ways to keep horses comfortable as the disease progresses.

Long History of Questions With Few Answers

Shivers, a chronic neuromuscular condition in horses, dates back centuries, but its causes remain a mystery. Clinical signs can vary, but many horses exhibit hyperflexion when raising their hind limbs up and away from the body. Horses can present with tremors in the leg, causing the limb to shake uncontrollably. Some horses will demonstrate hyperextension when they position the hind legs so far backward its stance isn’t in the normal resting position.

Age, gender, height, and breed can impact a horse’s likelihood of developing shivers, but most horses with shivers show clinical signs before the age of 10. It can affect both genders, but geldings are three times more likely to be diagnosed with the condition. Shivers in ponies is rare, but horses over 16.3 hands in height are more susceptible.

Developing a Common Definition for Shivers

The veterinary community had lacked a consensus around the true definition of shivers until Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, published the first of several research studies about the disease.

Valberg designed an online survey for 300 owners of affected horses to establish a working definition of the disease. After she assessed the responses and 70 accompanying videos of the horses, Valberg and her team noted an essential distinction between shivers and other neuromuscular conditions.

“We sought to define clinical signs that a horse presents as having shivers and to differentiate that from other movement disorders,” Valberg says. “Standing hyperflexion is much more common than shivers. It presents in horses with no abnormalities when walking forward or backward.” However, when owners lift the hind feet, they will often hyperflex the limb and hold it in that way for an extended period. “By the definition that we came up with, shivers horses always have difficulty backing up, but walking forward is normal until late progression of the disease,” she adds.

Valberg’s paper sheds light on how to define shivers based on whether a horse demonstrated hyperextension or hyperflexion when backing up. It is vital to establish an agreed-upon disease definition, because in at least 50% of shivers cases the condition worsens over time, whereas standing hyperflexion does not, she says.

The consensus around a working definition of shivers proved to be an essential step, but understanding the root cause of the syndrome remained a mystery.

Probing the Equine Brain

Horses with shivers demonstrate clinical signs such as muscle trembling in the hind legs and acute discomfort while being shod. Studying muscle control of the legs and its connection to the cerebellum (the structure at the back of the brain that’s involved in motor coordination) opened a critical door for researchers grasping the disease’s root cause.

One study by Valberg’s team and Anibal Armien, DVM, MS, PhD, Dipl. ACVP, equine neuropathologist at the University of Minnesota’s Veterinary Diagnostic Laboratory, in Saint Paul, aimed to further understand the cause of shivers.

In this study, the team matched four Warmbloods and one Thoroughbred with shivers with three control horses, two Warmbloods and one Thoroughbred. The control horses were donated due to moderate chronic osteoarthritis causing lameness and making them unsuitable for riding.

The researchers performed a complete neurologic examination on each horse and took videos of each horse walking to assess forward and backward movements as well as manual lifting of the hind legs. They recorded any presence of hyperflexion or hyperextension and scored gait patterns as normal, mildly abnormal, moderately abnormal, or very abnormal.

They then euthanized each horse due to the severity of their disease and collected tissue from the brain, spinal cord, and nerves.

Over two years, Armien examined the brain, spinal cord, and peripheral nerves in these euthanized horses. “Dr. Armien looked at every part of the peripheral nerves, all the way up the spinal cord, taking sections of it and then very carefully going through the entire brain,” says Valberg. “He did immunohistochemical staining to look at what types of cells showed abnormalities and narrowed it down to the Purkinje cells in the cerebellum.”

Abnormalities in the Purkinje cells proved to be an essential breakthrough because these cells regulate brain signals coordinating movement in the body. Shivers horses revealed lesions at the distal (lower) end of the cells’ axons, interfering with their ability to regulate signals passing from the brain to the spinal cord and the muscles. The researchers also saw 80 times more degeneration in the Purkinje cell axons in shivers horses than in control horses. Another study of protein and gene expression in the cerebellum also confirmed the role of Purkinje cells in the disease.

Future Research Possibilities

Horse owners want answers about diseases like shivers, and answers come from research, says Valberg. However, the path to research begins with financial support and funding new ideas. To increase understanding of the disease and equine welfare, horse owners, veterinarians, and researchers should work together to create possibilities for new research.