Article Commentary: Acquired Equine Motor Neuron Disease

Abstract

Equine motor neuron disease (EMND) is an acquired neuromuscular disease of horses that was first recognized in 1985 and reported in a preliminary report often horses in 1 990.3 The disease is characterized pathologically by degeneration of motor neurons in the ventral horns of the spinal cord and in selected brain stem nuclei. Death of motor neurons results in secondary axonal degeneration and denervation atrophy of skeletal muscle. Clinical signs of diffuse neuromuscular disease are seen in EMND and include marked weight loss due to muscle atrophy despite a normal appetite, generalized weakness with a short-strided gait, a characteristic stance with low head carriage and feet positioned well under the body, frequent shifting of weight from limb to limb, excessive recumbency, excessive sweating, especially following exercise, and muscle fasciculations. Involvement of neurons in the spinal ganglia is thought to be the basis of the hyperesthesia noted in some cases. Clinicopathologic features include mild to moderately increased serum levels of creatine kinase and/or aspartate aminotransferase and frequent increase in cerebrospinal fluid protein concentrations. Denervation potentials, primarily positive sharp waves, can be detected with concentric needle e l ec t r~myography .~ .~ .~ .~~ The clinical and pathologic features of the equine disorder closely resemble those of the motor neuron diseases of human beings, of which the most important is amyotrophic lateral sclerosis (ALS; “Lou Gehrig’s disease”). 1,23,14.18.22 This name is applied to a group of fatal, degenerative motor neuron disorders that are characterized by progressive degeneration of spinal and selected brain stem motor neurons ,~~2~8~14~18~22 with only mild involvement of sensory n e ~ r o n s . ~ J ~ The pathology of the human disorder differs from that of EMND, however, in that extensive degeneration of pyramidal tracts is seen in human beings with classical ALS. 1,2,8~14~Ig,22 Only mild degeneration of pyramidal tracts is present in horses with EMND. However, the pyramidal tracts are poorly developed, and less extensive, in ungulates as compared to human beings.I0 Alternatively, EMND may be more analogous to progressive muscular atrophy, a variant of ALS in which pyramidal tracts are spared. I Histopathologic and ultrastructural lesions in neurons of horses with EMND3 closely resemble those seen in ALS. 1,2,8,14,18320322 Immunocytochemical studies have shown that the swollen neurons in EMND contain dense aggregates of neur~filaments,~ a finding similar to those in ALS.17 This equine disorder is distinctly different, clinically and pathologically, from previously described diseases of the equine neuraxis. Equine degenerative myeloencephalopathy and cervical stenotic myelopathy are diseases of spinal cord white matter and result in clinical evidence of spastic paresis and ataxia. Protozoal encephalomyelitis may result in focal or multifocal degeneration of motor neurons and degeneration of the white matter of the brain and spinal cord, but the resulting lower motor neuron weakness is localized and frequently asymmetric. Rabies virus polioencephalomyelitis may initially produce signs suggestive of lower motor neuron disease. However, the rapid, diffuse spread of this lesion will be accompanied by other neurologic ~ i g n s . ~ J ~ The motor neuron degeneration seen in EMND is pathologically similar to that seen in a presumed inherited motor neuron disease of zebras (Equus burchelli),12 but EMND is clearly a sporadic acquired disorder affecting horses of all ages and breed^.^,^,^ Quarter Horses are overrepresented in the various breeds affected, and Thoroughbreds are also at increased risk. The risk of EMND increases with age, with a peak at 16 years of age.I6 At the time of this writing, 67 cases of EMND have been confirmed by pathologic and/or clinical and clinicopathologic findings, and many more are suspected. Most of the 67 recognized cases of EMND have occurred in the northeastern USA, the area from which most cases seen at the New York State College of Veterinary Medicine are drawn. However, cases have been confirmed in Ohio (four), Indiana (two), West Virginia (one), Maryland (one), Kentucky (one), Alaska (two), California (one), Oregon (one), Nebraska (one), Tennessee (one), Florida (one), and Ontario, Canada (one). Confirmation has relied on recognition of typical clinical and clinicopathologic findings and on frozen section histochemistry applied to muscle samples mailed by overnight carrier to appropriate laboratories. Additional cases are suspected in Great Britain.” At Cor-