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.