Texas Equine Veterinary Association

reciprocal excitatory and inhibitory central signals to motor units that supply ipsilateral limb muscles. Some movement disorders such as stringhalt and shivers or shivering describe a specific pattern of abnormal hindlimb movement induced by walking backward or forward. A more scientific classification system for horses is desirable but difficult to devise currently due to the rare nature of movement disorders, the lack of diagnostic biomarkers, clear causes, and overlap between syndromes. In human and small animal medicine, the term dyskinesia is used to characterize intermittent involuntary self- limiting episodes of jerky movements that impair normal voluntary movements. Cervical vertebral stenotic myelopathy (CVSM) is a common cause of ataxia in horses secondary to spinal cord compression. Early articles describing this problem indicate genetic predisposition as a known risk factor. Further studies have shown the problem is a developmental abnormality, which might have genetic predisposition and environmental influences. CVSM is characterized by ataxia and weakness, caused by narrowing of the cervical vertebral canal and compression of the spinal cord often in combination with malalignment and malformation of the cervical vertebrae. Stenosis of the vertebral canal, anywhere from the first cervical vertebral body (C1) to the first thoracic vertebral body (T1), is the most important abnormality in CVSM. An unusual neurologic condition of horses termed "segmental myelitis" was first observed by Rooney in Kentucky in 1964. Rooney renamed the syndrome "focal encephalitis-myelitis" because of brain involvement and Prickett, Rooney and others reported on 44 cases at the annual meeting of the American Association of Equine Practitioners in 1968 and on 52 cases in 1970. In 1974, protozoa were first seen in association with characteristic lesions, and the disease was given its current name, equine protozoal myeloencephalitis (EPM) by Mayhew and colleagues, who reported on 45 cases at the AAEP meeting in 1976. Over the years, a better understanding of EPM etiology and epidemiology has been obtained. However, EPM pathogenesis remains uncertain. Equine protozoal myeloencephalitis is an infectious neurologic disease of horses in North, Central, and South America. The disease is caused by the coccidian parasite S neurona and less frequently by the related pathogen N. hughesi. Horses are infected by ingesting food or water that has been contaminated with feces from an infected opossum. The modes of transmission remain 65

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