Endothelial lipopigment as an indicator of alpha-tocopherol deficiency in two equine neurodegenerative diseases

Current insights into equine neuroaxonal dystrophy/equine degenerative myeloencephalopathy

Equine neuroaxonal dystrophy/equine degenerative myeloencephalopathy (eNAD/EDM) is an inherited neurodegenerative disease associated with vitamin E deficiency in the first year of life. It is the second most common cause of spinal ataxia in horses euthanized for neurologic disease. Equine NAD/EDM is characterized by neurologic signs including a symmetric proprioceptive ataxia (> grade 2/5) and a wide-base stance at rest. There are currently no antemortem tests for eNAD/EDM in any breed. Conclusive diagnosis requires postmortem histologic evaluation of the brainstem and spinal cord at necropsy. Research studies on antemortem biomarkers and genetic testing are ongoing. The development of a genetic test for eNAD/EDM would have widespread impact, even if it were breed specific. Currently, the best approach to eNAD/EDM is to focus on preventing cases by providing pregnant mares and foals with access to pasture. Alternatively, dams' diets can be supplemented with high doses of water-soluble RRR-α-tocopherol during the last trimester of gestation, with continued supplementation of foals through the first two years of life. It is important to measure horses' baseline serum vitamin E levels prior to supplementing. While considered generally safe, oversupplementation of vitamin E is possible and can lead to coagulopathies.

Pathologic Conditions of the Nervous System in Horses

The variety of neurologic diseases which affect horses makes pathologic examination of the nervous system a complex and lengthy process. An understanding of the common causes of neurologic disease, antemortem neurolocalization, and supplementation of the necropsy examination with ancillary testing will help to diagnose a large number of causes of neurologic disease. A general understanding of neuropathology and collaborative relationship with your local pathologists will aid in the definitive diagnosis of neurologic diseases.

Genetics of Equine Neurologic Disease

Neurologic disease in horses can be particularly challenging to diagnose and treat. These diseases can result in economic losses, emotional distress to owners, and injury to the horse or handlers. To date, there are 5 neurologic diseases caused by known genetic mutations and several more are suspected to be heritable: lethal white foal syndrome, lavender foal syndrome, cerebellar abiotrophy, occipitoatlantoaxial malformation, and Friesian hydrocephalus. Genetic testing allows owners, breeders, and veterinarians to make informed decisions when selecting dams and sires for breeding or deciding the treatment or prognosis of a neurologic animal.

Equine degenerative myeloencephalopathy in Lusitano horses

BACKGROUND

Equine degenerative myeloencephalopathy (EDM) is a neurodegenerative disorder that has been previously associated with low vitamin E concentrations.

OBJECTIVE

To describe the clinical, electrophysiologic, and pathologic features of EDM in a group of related Lusitano horses.

ANIMALS

Fifteen Lusitano horses.

PROCEDURES

Neurologic examinations were conducted, and serum vitamin E concentrations were measured. Three neurologically abnormal horses were further evaluated by ophthalmologic examination, electroretinography, electroencephalography, muscle and nerve biopsies, and post-mortem examination.

RESULTS

Six horses appeared neurologically normal, 6 were neurologically abnormal, and 3 had equivocal gait abnormalities. Abnormal horses demonstrated ataxia and paresis. An inconsistent menace response was noted in 4 neurologically abnormal horses and in 1 horse with equivocal findings. All horses had low serum vitamin E concentrations (<1.5 ppm). Ophthalmologic examinations, electroretinograms, electroencephalograms, and muscle and peripheral nerve biopsies were unremarkable in 3 neurologically abnormal horses. At necropsy, major neuropathological findings in these horses were bilaterally symmetric, severe, neuro axonal degeneration in the gracilis, cuneatus medialis, cuneatus lateralis, and thoracicus nuclei and bilaterally symmetric axonal loss and demyelination mainly in the dorsolateral and ventromedial tracts of the spinal cord. A diagnosis of EDM was made based on these findings. Pedigree analysis identified 2 sires among the affected horses.

CONCLUSIONS AND CLINICAL RELEVANCE

Equine degenerative myeloencephalopathy is a neurodegenerative disorder that causes ataxia and, in severe cases, paresis, in young Lusitano horses. The disease appears to have a genetic basis, and although vitamin E deficiency is a common finding, low serum vitamin E concentrations also may occur in apparently unaffected related individuals.

Vitamin E deficiency and risk of equine motor neuron disease

BACKGROUND

Equine motor neuron disease (EMND) is a spontaneous neurologic disorder of adult horses which results from the degeneration of motor neurons in the spinal cord and brain stem. Clinical manifestations, pathological findings, and epidemiologic attributes resemble those of human motor neuron disease (MND). As in MND the etiology of the disease is not known. We evaluated the predisposition role of vitamin E deficiency on the risk of EMND.

METHODS

Eleven horses at risk of EMND were identified and enrolled in a field trial at different times. The horses were maintained on a diet deficient in vitamin E and monitored periodically for levels of antioxidants--alpha-tocopherols, vitamins A, C, beta-carotene, glutathione peroxidase (GSH-Px), and erythrocytic superoxide dismutase (SOD1). In addition to the self-control another parallel control group was included. Survival analysis was used to assess the probability of developing EMND past a specific period of time.

RESULTS

There was large variability in the levels of vitamins A and C, beta-carotene, GSH-Px, and SOD1. Plasma vitamin E levels dropped significantly over time. Ten horses developed EMND within 44 months of enrollment. The median time to develop EMND was 38.5 months. None of the controls developed EMND.

CONCLUSION

The study elucidated the role of vitamin E deficiency on the risk of EMND. Reproducing this disease in a natural animal model for the first time will enable us to carry out studies to test specific hypotheses regarding the mechanism by which the disease occurs.

Evaluation of the risk of motor neuron disease in horses fed a diet low in vitamin E and high in copper and iron

OBJECTIVE

To determine whether equine motor neuron disease (EMND) could be induced in adult horses fed a diet low in vitamin E and high in copper and iron.

ANIMALS

59 healthy adult horses.

PROCEDURE

Horses in the experimental group (n = 8) were confined to a dirt lot and fed a concentrate low in vitamin E and high in iron and copper in addition to free-choice grass hay that had been stored for 1 year. Control horses (n = 51) were fed a concentrate containing National Research Council-recommended amounts of copper, iron, and vitamin E. The hay fed to control horses was the same as that fed to experimental horses, but it had not been subjected to prolonged storage. Control horses had seasonal access to pasture, whereas experimental horses had no access to pasture. Horses that developed clinical signs of EMND were euthanatized along with an age-matched control horse to determine differences in hepatic concentrations of vitamin E, vitamin A, copper, iron, and selenium.

RESULTS

4 experimental horses developed clinical signs of EMND. Plasma concentrations of vitamin E decreased in all 8 experimental horses. There were no significant changes in plasma concentrations of vitamin A, selenium, and copper or serum concentrations of ferritin. There were no significant differences in those analytes between experimental horses with EMND and experimental horses that did not develop EMND. No control horses developed EMND.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that lack of access to pasture, dietary deficiency of vitamin E, or excessive dietary copper are likely risk factors for EMND.

Neurodegenerative diseases in domestic animals: A comparative review

Neurodegenerative diseases are characterised by selective damage to specific neurons in the nervous system. Interest in such diseases in humans has resulted in considerable progress in the molecular understanding of these disorders in recent decades. Numerous neurodegenerative diseases have also been described in domestic animals but relatively little molecular work has been reported. In the present review, we have classified neurodegenerative disease according to neuroanatomical criteria. We have established two large groups, based on whether the neuronal cell body or its axon was primarily affected. Conditions such as motor neuron diseases, cerebellar degenerations and neuroaxonal dystrophies are discussed in terms of their clinical and neuropathological features. In the most studied disorders, we also present what is known about underlying pathomechanisms, and compare them with their human counterparts. The purpose of this review is to re-kindle interest in this group of diseases and to encourage veterinary researchers to investigate molecular mechanisms by taking advantage of current diagnostic tools.

Ocular manifestations of equine motor neuron disease

The characteristics of the ocular manifestations of equine motor neuron disease (EMND) are described. Forty-two horses with histories, clinical signs and necropsies compatible with EMND were the subjects of this study. Ophthalmoscopic lesions that varied in severity were found in 40 of 42 horses and appeared as a distinct pigmented reticulated pattern at the tapetal-nontapetal junction or throughout the fundus, depending upon severity. The pattern colours ranged from yellow brown to black. Areas of hyperreflectivity formed mosaic patterns in the tapetal fundus. ERG B-wave amplitudes were usually at least 50% reduced and many animals showed extinguished amplitudes. None of the horses had apparent visual impairment. Histopathologically, all 42 horses had retinal pigment epithelial (RPE) congestion with ceroid-lipofuscin. Retinal degeneration was variable even within the eyes. Thin layer chromatography (TLC) analysis of the RPE and neural retina identified both green and orange emitting fluorescent compounds not found in normal horses. All unsupplemented horses had plasma vitamin E levels <1.0 microg/ml. The potential significance of this report is the pathognomonic role the ocular manifestations exhibit in helping to diagnose equine motor neuron disease.

Quantitative assessment of motor neuron loss in equine motor neuron disease (EMND)

The mean number of motor neurons was assessed in the C7 spinal cord segment of 5 EMND and 5 control horses. Mean number per section in EMND horses was reduced significantly (P<0.001). The mean neuronal loss was estimated at 31%. Each of the 5 affected horses had a mean neuronal count below the 95% confidence interval for control horses. The statistically significant difference between the 2 groups was consistent in the cranial, middle and caudal thirds of the C7 segment (P<0.001). The results of regression analysis indicated an association between neuronal reduction in EMND horses and the duration of the disease when adjusted for age (P<0.001). This is the first quantitation of the neurodegenerative loss in EMND and it provides a clearer explanation for residual deficits in horses that survive EMND.

Association between plasma vitamin E concentration and the risk of equine motor neuron disease

Equine motor neuron disease (EMND) is a neurodegenerative disorder of the somatic lower motor neurons that results in a syndrome of diffuse neuromuscular disease in the adult horse. The aetiology of this disorder is unknown, although prior studies have suggested that a deficiency in the lipid antioxidant vitamin E (alpha-tocopherol) contributes to the development of EMND. This paper describes a case-control study designed to investigate the association between plasma vitamin E levels and the risk of EMND for horses. Signalment, plasma vitamin E levels at the time of referral, and information relative to dietary and management practices were collected from 53 horses diagnosed with EMND and 69 controls. The mean plasma vitamin E concentration in EMND cases was significantly lower than that of control horses. After controlling for other risk factors of EMND, there was a statistically significant association between plasma vitamin E levels and EMND, with the likelihood of the disease increasing as the vitamin E concentration decreased. These findings support the reported role of vitamin E deficiency as one of the risk factors for EMND.

Equine motor neuron disease

This article reviews the subject of equine motor neuron disease, a neurodegenerative disease of horses. The authors discuss various topics, including epidemiology, pathophysiology, clinical signs, laboratory findings, diagnosis, and treatment.

Equine degenerative myeloencephalopathy

EDM is a neurologic disease of young horses characterized by the insidious development of symmetric ataxia. Decreased or absent cutaneous trunci reflex or slap test responses are considered clinical signs that increase the index of suspicion for this disease. In addition, concurrent predisposing factors, such as familial history, inadequate access to green pasture, and possible exposure to wood preservatives or insecticides, provide further supporting evidence for a clinical diagnosis. Vitamin E deficiency and a hereditary predisposition currently are considered the most significant factors in the pathogenesis of this disease. Histopathologically the lesions of EDM are those of neuraxonal dystrophy, characterized by prominent axonal and dendritic swelling, mild glial proliferation, and neuronal depletion and atrophy with lipofuscin-like pigment accumulation. Animals predisposed to EDM or with a clinical diagnosis of EDM should receive oral alpha-tocopherol acetate supplementation. Improvement in clinical signs may be seen following long-term treatment, but in general, the prognosis for complete recovery is poor.