Horses with shivers often have difficulty walking backward or lifting a hind limb because of fixed, hyperflexed abducted hind limbs or rigid hind-limb extension. This neuromuscular disorder is also accompanied by trembling, or “shivering,” of the tail and thigh muscles—hence, the moniker “shivers.” Affected horses tend to be able to move forward at walk, trot, and canter without problem until the syndrome progresses to hind-limb hyperflexion at the walk. Notable signs begin by 7 years of age.
Study on Source of Shivers in Horses
In a recent study, equine practitioners from Michigan State University, Colorado State University, and the University of California, Davis, examined histopathology samples of brain tissue from affected horses to try to identify the condition’s source.
Previous study results identified selective degeneration of Purkinje cell (PC) axon projections in the lateral deep cerebellar nuclei. This is significant because the cerebellum is critical for “coordination of locomotion through integration of sensory information from the spinal cord and feedback from the motor cortex, basal ganglia, and thalamus.” Transmission of this information enables the PC to send efferent inhibitory outputs to motor pathways. “Walking backward, compared with walking forward, is more specifically influenced by cerebellar activities because the cerebellum exerts anticipatory postural adjustments in the absence of the ability to visually monitor steps,” the authors noted.
While this study investigated a cerebellar cause of shivers, other signs of cerebellar dysfunction, such as proprioceptive deficits, truncal sway, and intention tremor, are not considered clinical signs of shivers.
Study Population
In the current study, the research team hypothesized that horses with shivers have a distinct pattern of gene expression within the cerebellar white matter and compared differences in gene expression between affected and control horses. The study involved horses taller than 16.3 hands, including three Belgian drafts, one Thoroughbred, and one Appendix Quarter Horse, ages 7-13. All experienced shivers signs, such as hyperflexion of both hind limbs when walking backward and reluctance to lift a hind limb. They had had normal gaits at walk and trot. Other neurologic examination findings were normal. The four control horses were taller than 16.2 hands and included three Thoroughbreds and one Irish Sport horse. The control horses had other issues, such as persistent lameness, a sinus tumor, or chronic back pain, but no evidence of neurologic disease. The control horses did not resist lifting a hind limb and moved normally both forward and backward. All nine horses had been donated and were euthanized humanely to obtain, process, and analyze brain tissue.
Research Findings
The authors reported that “within the lateral cerebellar hemispheres, both spatial gene and protein expression differ significantly between shivers and control horses.” The lateral cerebellar hemispheres are critical to “motor planning and timing and onset and coordination of movements.” Differentially expressed genes (DEG) occurred in high numbers in cerebellar axons of shivers horses compared to control horses. The researchers found that PC soma resists injury; PC axonal damage is not associated with ataxia or tremors, and shivers horses do not typically display these clinical signs. A neuroinflammatory cascade results in neurodegeneration and motor dysfunction in shivers horses by suppressing inhibitory GABA (gamma aminobutyric acid) receptor activities to decrease the number of Purkinje cells within cerebellar white matter. Axonal injury also causes reduced expression of neurofilaments that affect the growth and stability of myelinated axons and enable optimal conduction velocity of electrical impulses along axons.
The study authors identified “decreased expression of proteins involved in myelinating axons in shivers horses” and concluded that “Our finding of altered expression of neurofilaments, myelin, and ion pumps and channels in the cerebellum of horses with shivers implicates an abnormality in axons in horses with shivers that could impact the cerebellum’s ability to coordinate movement.”
Further, shivers horses’ brain tissue has increased expression of calcium-dependent phospholipid binding protein (annexin) that is associated with inflammation and neurodegeneration. A number of other proteins related to heat shock, oxidative stress, calcium buffering or signaling, and actin had increased expression in the shivers horses in this study. Reduced expression of other proteins is associated with impaired motor coordination. Some of these are gender-specific, consistent with the fact that male horses are three times more likely to develop shivers than females.
In conclusion, the authors reported that Purkinje cell axonopathy is a cause of shivers in horses.
Reference
Valberg SJ, Willams ZJ, Henry ML, Finno CJ. Cerebellar axonopathy in Shivers horses identified by spatial transcriptomic and proteomic analyses. Journal of Veterinary Internal Medicine May 2023; DOI: 10.1111/jvim.16784
Related Reading
- Is That Horse Ataxic or Just Lame?
- Disease Du Jour: Equine Neurologic Exam
- Disease Du Jour: Equine Degenerative Myeloencephalopathy (EDM)
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