Two recently published papers have taken further steps to define the muscle disorder, “shivers” (difficulty lifting up the hindlimbs combined with difficulty walking backward, hyperflexion of the hindlimb often occurs when backing up, often no difficulty moving forwards) that is seen in several different breeds of horses. In the first paper, Dr. Stephanie Valberg and others looked at what type of horses this disease most commonly affects.
Three hundred and five surveys were received as a result of a Web-based survey and the results of this survey, along with 70 videos, led the group to conclude that shivering is a chronic, often gradually progressive movement disorder that usually begins before seven years of age and has a higher prevalence in tall male horses. The second paper, with Dr. Valberg again leading the group, looked at the type of movement that was characteristic of horses with shivers as compared to horses with stringhalt (another motion disorder of the hindlimbs). Their conclusion was that shivering affects horses most significantly when these horses are asked to back, showing either extreme flexion or extension of the hindlimbs, with some cases also having the involvement of the forelimbs, leading to 3 distinct variations of movement in this disorder. While shivering can look similar to stringhalt, analyzing the gaits at a walk easily differentiated shivers from stringhalt.
In an as-yet unpublished study, Dr. Valberg was able to use electrodes attached to the skin over the flexor and extensor muscles of the hindlimbs to determine the sequence these muscles were firing in during backing. In horses with shivers, these muscles were not firing in the correct sequence, and “this is exactly what the cerebellum (region of the brain that coordinates voluntary motor movement) does; it controls the timing of locomotion,” said Dr. Valberg. A closer examination of the cerebellum of affected horses showed 80 times more degeneration than that of a normal horse, a significant difference that has led them to believe that this is the definitive cause of shivers.
Scientific Articles: Valberg, SJ et al. “Epidemiology of shivering (shivers) in horses.” Equine Vet J 2014 May 6.
Valberg, SJ et al. “Posture and movement characteristics of forward and backward walking in horses with shivering and acquired bilateral stringhalt.” Equine Vet J 2014 Mar 10.
Companion article: Barakat, Christine “The Mystery of Shivers” Equus Jan. 2015, issue 448, pp 39-45.