Equine Motor Neuron Disease

CHAPTER 133 Equine Motor Neuron Disease



Thomas J. Divers, Hussni O. Mohammed


Equine motor neuron disease (EMND) is a neurodegenerative disorder of the somatic lower motor neurons of horses. The disease was first described in 11 horses in 1990. Results of epidemiologic, laboratory, pathology, and experimental studies strongly support vitamin E deficiency as the primary risk factor for EMND. Pathologic changes are generally limited to the lower motor neuron system. Laboratory and pathologic findings are supportive of an oxidative disorder affecting ventral horn somatic motor neurons, especially those supplying muscles with highly oxidative type 1 fibers. The lipopigment deposition seen in the retinal pigment epithelium of affected horses is a result of light-generated oxidative damage. The increased lipopigment in motor neurons and central nervous system (CNS) capillary endothelial cells also reflects increased lipid peroxidation in the CNS.


Horses with EMND have almost always been housed at the same location for at least 18 months. Most horses affected in the United States have had little or no pasture access, which helps explain the vitamin E deficiency. The disease seems more prevalent in the northeastern United States, but it has been reported in most states and many countries of the world. Based on the frequency of horses with EMND referred to Cornell University, the incidence of the disease appeared to begin declining in 1999, shortly after the link between EMND and vitamin E deficiency was established. Although similar epidemiologic findings have been reported in Europe and most horses with EMND on that continent have limited or no access to pasture, a surprising number of cases include owners who report that the horse has had adequate access to pasture. Interestingly, and unexplained thus so far, the pastured horses with EMND all had abnormally low plasma vitamin E concentrations.



CLINICAL FINDINGS


Horses with acquired EMND are 2 years of age or older. Clinical signs generally seen are acute onset of trembling, a short-strided gait, almost constant shifting of weight in the pelvic limbs when standing, and excessive recumbency. Muscle wasting is noticeable, and, when queried, owners often remark that weight loss was apparent before the onset of trembling. Funduscopic examination reveals brown streaking discoloration (Figure 133-1) in nearly 40% of affected horses, although visual impairment is rarely reported. The tailhead is often carried in an elevated position, and the head and neck are carried in an abnormally low position. In a smaller percentage of horses, trembling and constant shifting of weight may either not occur or not be observed, and the predominant clinical sign may be muscle wasting. Although these clinical signs are characteristic for EMND, other neuromuscular disorders, especially certain chronic myopathies and equine dysautonomia, may have a similar clinical appearance.


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Figure 133-1 Photograph of the fundus of a horse with equine motor neuron disease. The streaking pattern represents abnormal ceroid-lipofuscin accumulation in the retinal pigment epithelium, a change that is caused by vitamin E deficiency.

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May 28, 2016 | Posted by in EQUINE MEDICINE | Comments Off on Equine Motor Neuron Disease

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