Where We Stand With Shivers

Shivers is seen as gait abnormalities when horses back up.

Photo: Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR

Shivers is a chronic neuromuscular disease that causes gait abnormalities in affected horses, evident as they are backing up. Other signs include trembling of the tail when the horse holds it erect, trembling of the thigh muscles, and a flexed and trembling hind limb. 

Most horses with shivers begin to show signs before 5 years of age, and most cases (74% of horses in a recent study) become progressively more severe. While shivers can affect horses of any sex, geldings are three times more likely to be diagnosed with the disorder than mares. Horses taller than 16.3 hands are also more susceptible than shorter horses. The syndrome affects several breeds, including draft horses and Warmbloods, and occasionally lighter breeds, including harness horses, Quarter Horses, and Thoroughbreds. 

Diagnosing shivers is straightforward when signs are clear; however, milder cases are more difficult to pinpoint. If your horse exhibits muscle quivering, difficulty backing up, discomfort while being shod behind, or other signs of shivers, have him evaluated by your veterinarian to rule out other painful conditions and possible causes of lameness, such as stringhalt and upward fixation of the patella. 

As disease progresses, gradual and progressive hind limb muscle atrophy (wasting) can occur. In the aforementioned study, 11 of 19 horses with shivers (58%) exhibited hindquarter weakness. Severely affected animals might not lie down. These sleep-deprived horses often develop wounds on the fronts of their fetlocks from dozing while standing. Advanced cases may adopt an abnormal base-wide stance behind. Veterinarians have noted excessive sweating in some cases.

Research Insights

What causes shivers remains unknown. However, results from our extensive study published in February 2015 revealed that horses with shivers have brain damage in a small area of the cerebellum, which is largely devoted to regulating balance and muscular activity. Horses with shivers often move forward normally and can even reach high levels of athletic competition, but they have difficulty with less natural gaits such as backing up. The study results suggest that a cerebellar pathway—separate from spinal circuits involved in faster natural forward gaits—regulates these slow, learned movements, providing a possible explanation for the unique deficits. Furthermore, the muscles of affected horses show a significant change in fast-twitch type IIX muscle fiber similar to that seen in horses in intensive training. While normal horses acquire these fiber changes through exercise, horses with shivers lack the “off-switch” the cerebellum normally provides for muscle contraction, causing the muscles to be constantly active.

Because shivers is breed-related, it might have a genetic basis. But scientists have not yet identified a specific genetic pattern, and there is no genetic test.

Treatment Options

There is currently no effective treatment for shivers. Occasionally the signs might improve, especially with turnout and exercise, but they could also regress in painful or stressful situations and when stalled. Some nutritionists recommend feeding horses with shivers a high-fat, low-carbohydrate feed, but researchers have not seen that this dietary change makes a clear impact.  

Providing adequate levels of vitamin E is important. Your veterinarian should check the vitamin E levels in your horse’s serum. If they are below normal limits, then you might need to provide supplemental vitamin E. Ensuring your horse is not deficient might slow progression, but it does not appear to improve the clinical signs of shivers. Massage and acupuncture may be helpful in keeping a horse comfortable and competitive. 

Affected horses can become tight behind and take longer to get back in shape if laid up. As such, keeping affected horses in work with constant turnout seems to help relieve signs.

Take-Home Message

Have your veterinarian perform a complete evaluation of any horse with suspected shivers. With a thorough lameness exam, he or she can rule out any other abnormalities that could be causing the clinical signs. The exam should involve backing horses up and lifting the hind limbs in a flexed position. Taking videos of the horse walking forward and backing up on a regular basis can help you track changes in severity.

If your horse has shivers you can participate in my research by visiting https://cvm.msu.edu/research/faculty-research/valberg-laboratory/information-on-shivers.

About the Author

Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR

Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, is the Mary Anne McPhail Dressage Chair in Equine Sports Medicine and a Professor of Large Animal Clinical Sciences at Michigan State University. She is a leading researcher on the subject of tying-up and the genetic basis for equine neuromuscular disorders.

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